corrosion resistance of nickel and nickel- containing

CORROSION RESISTANCE OF

NICKEL AND NICKEL-CONTAINING ALLOYS

IN CAUSTIC SODA AND OTHER ALKALIES

(CEB-2)

A PRACTICAL GUIDE TO THE USE OF NICKEL-CONTAINING ALLOYS

NO 281

Distributed byNICKEL

INSTITUTEProduced byINCO

CORROSION RESISTANCE OF NICKEL AND NICKEL-CONTAINING ALLOYS IN CAUSTIC SODA AND OTHER ALKALIES (CEB-2)

A PRACTICAL GUIDE TO THE USE OF NICKEL-CONTAINING ALLOYSNO 281

Originally, this handbook was published in 1973 by INCO, The International Nickel Company, Inc. Today this company is part of Vale S.A.The Nickel Institute republished the handbook in 2020. Despite the age of this publication the information herein is considered to be generally valid.Material presented in the handbook has been prepared for the general information of the reader and should not be used or relied on for specific applications without first securing competent advice.The Nickel Institute, the American Iron and Steel Institute, their members, staff and consultants do not represent or warrant its suitability for any general or specific use and assume no liability or responsibility of any kind in connection with the information herein.

Nickel Institute

[email protected]

Table of Contents

Page

PART I. INTRODUCTION 3

PART II. CORROSION BY CAUSTIC SODA........................... 4 A. Nickel .................................... _ . . . . . . . . . . . . . . . . . 4

1. Effect of Concentration, Temperature and Carbon Content ...... _ . . . . . . 4 2. Effect of Velocity ............... _ ... __ ..................... _ 6 3. Effect of Aeration .•......................................... 6 4. Effect of System Thermal Gradients .............................. 7 5. Effect of Impurities .,. _ ................. _ ............ _ . . . . . . . 7 6. Effect of Stress ............................................. 8 7. Effect of Dissimilar Metal Contact .. _ ...... _ ...... _ ............. '. 8 8. Cathodic Protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

B. Nickel-Chromium Alloys (Alloy 600) .. _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 C. Nickel·Copper Alloys (Alloy 400, Alloy K-500) ....................... _. 10 D. Copper-Nickel Alloys .... _ ...... _ ............. ___ ............ _ ., 11

Copper-Nickel Alloy CA 706 (90-10) Copper-Nickel Alloy CA 710 (80-20) Copper-Nickel Alloy CA 715 (70-30)

E. Iron-Nickel-Chromium Alloys (Alloy BOO) ......... _ .............. _ . .. 13 f. Austenitic Chromium-Nickel Stainless Steels (AISI 300 Series) . . . . . . . . . . .. 13 G. Iron-Base Nickel·Chromium...copper-MoJybdenum AHoys and Nickel-Base Chro-

mium...copper-Molybdenum Alloys .......... _ ....... __ ............. , 15 {Alloy 825. CARPENTER 20Cb-3, HASTELlOY alloy G and cast ACt CN-7M alloys}

H. Nickel-Base Molybdenum or Molybdenum·Chromium-lron Alloys. . . . . . . . . . . 16 (HASTElLOY alloy C-276, Alloy 625. HASTEllOY alloy B)

I. Cast Irons and Ni-Resists .... _ . _ .... __ .. __ . __ . . . . . . . . . . . . . . . . . . . . 17

PART m. CORROSON BY OTHER ALKAliES.. ............... .... .... 19 A. Caustic Potash (KOH) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 19 B. Ammonia and Ammonium Hydroxide ............................... 20 C. Other Alkaline Solutions of Sodium and Potassium Salts ................ 22

PART IV. INDUSTRIAL APPLICATIONS. ... .... . ............... ...... 24 A. Caustic Soda Manufacture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 24 B. Caustic Potash Manufacture ..................................... 28 C. Caustic Soda Storage and Transportation .......................... ". 28 D. Soap Manufacture ....................... _ ......... _ . . . . . . . . . .. 30 E. Pulp and Paper Industry ...... __ ............... _ .. _ . . . . . . . . . . . ... 32

1_ Digesters _............................................ ... 32 2. liquor Heaters ............................................. 33 3. Black liquor Evaporators .................. _ . . . . . . . . . . . . . . . 34 4. Recausticizing ............................................. 34

f. Aluminum Industry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 35 G. Caustic fusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 35 H. Petroleum Refining ............ _ .............. _ . _ . . . . . . . . . . . . .. 36 I. Caustic DescaJing . _ . _ .................... _ . . . . . . . . . . . . . . . . 37 J_ Reclaiming Caustic for Economy and Pollution Control. . . . . . . . . . . . . . 37

PART V. WELDING ............. __ ..... _ . . . . . . . . . . . . . . . . . . . . . . 38 A. fabrication of Nickel·Clad Equipment .. _ ..................... " 38. B. Repair of Equipment in Caustic Service ......... _ ...... _ . . . . . . . . . . .. 39 References .......... _ ......................................... ' 40 Trademarks ..................................... . . ... Inside back cover

Table I

Nominal Compositions of Nickel Alloys in Use or Corrosion Tested in Caustic Solutions

Composition. %

M;aterial Hi Fe Cr Mo Cu C Si Mn

WROUGHT MATERIALS

Nickel Hickel 200 99.5 0.15 - - 0.05 0.06 0.05 0.25 Hickel20t 99.5 0.15 - - 0.05 0.01 0.05 0.20 DURAH.CKEL· alloy 30t 94.0 0.15 - - 0.15 0.55 0.25 0.25

Nickel-Chromium Alloys .HCOHEL· alloy 600 16.0 7.2 15.8 - 0.10 0.04 0.20 0.20 H.MONtC· alloy 75 71.4 0.5 20.5 - - 0.10 - -

Nickel-Copper Alloys MONU· alloy 400 66.0 1.35 - - ll.5 0.12 0.15 0.90 MON£!., • al'lI1 K.SO!!:. 65.0 leO - - 29.5 0.15 0.15 0.60

Copper-NiCkel Alloys Copper-Nkke:1 allo, CA 706 10.0 1.25 - - 88.0 - - 0.3 Copper·Nickel alloy CA 710 20.0 0.75 - - 78.0 - - 0.4 Coppef·Nlekel alloy cA 715 30.0 0.55 - - 61.0 - - 0.5

Iron·Nickel-Chromium Alloys 'NCOLO"- alloy 800' 32.0 46.0 20.5 - 0.30 O.M 0.35 0.75

Stainless Steels AISI Type :202 5.0 67.0 18.0 - - 0.15ma. 1.0 max 8.1 AISI Type 31)2 9.0 70.5 18.0 - - 0.15 max 0.5 1.5 AI5ITy, •. 304 9.5 70.0 18.0 - - 0J)8 max 0.5 1.5 AISI TYlle 304l 10.0 69.0 18.0 - - 0.03 max 0.5 1.3 AISI tYPe 316 13.0 65.0 17.0 2.0mi .. - O.OS max 0.5 1.7 AISI tJllO 316l 13.0 65.0 17.0 2.0 min - 0.03 max 0.5 1.8 AISI Type 309 13.5 60.5 23.0 - - 0.20 malt 1.0 max 2.0m3X AISI Type 310 20.0 52.0 25.0 - - 0.25mn 1.0 max 2.0ma. AISI Type 330 35.0 41.0 1.5.1) - - 0.25 rna. 1.0 rna. 2.0 mal AISI Type 347 11.0 68.0 18.0 - - 0.08mu 1.0 max 2.0mu AISI Type. 438 - Bal 17.0 - - 0.12 max - -

Iron Base Nicket·Chromium-Copper-MoIybxlenum Alloys c;"'Rn'n:~- Stainless No. 20 <1' 29.0 43.0 20.0 2.0 min 3.0m;n 0.07 max 1.0 0.8 CARPEfC-r£R' Sta,"t~$s No. 2OCb-3 34.0 39.0 20.0 2.5 3.3 0.01 max 0.6 08

Nickel Ba';'e l'bn-Chromium·MoIybdenum Anoys 'HCOl.OY- a!Joy 825 41.8 30.0 21.5 3.0 1.8 0.03 0.35 0.65 HAST£LLO .. " alloy G 45.0 19.5 22.2 6.5 2.0 0.03 0.35 1.3 HAST£t..~OY· :alloy C ~2) ~.O 5.0 15.5 16.0 - 0.08 max 1.0 rna. 1.0 max

HASTELLO .. - alloy C·2l6 54.0 5.0 15.5 16.0 - 0.02 max 0.05max 1.0max

'HC_I:L - alloy 625 60.0 5.0ma. 21.5 9.0 - 0.10 max 0.5 max 0.5 max Nickel Base Molybdenum Alioy

.. ASULLO'\'- alloy B 61.0 5.0 1.0 max 28.0 - 0.05 max - -

CAST MATERIALS

Nickel America .. Casti"e Institute

ACI CZ·l00 95.0 min 1.5 max - - - 1.0 max 2 .. 0 1.5 max Nickel·Ch.romlum·fron Alloy

ACI CY·40 70.0 9.5 15.5 - - 0.3 3.0 max 1.5 max Nickel-Copper Alloys

Hickel·Copper alloy 50S 64.0 2.0 - - 29.0 O.OS 4.0 0.80 ACI "'-35 64.0 3.5 max - - 29.5 (1.35 mall 2:.0 mat 1.5 max

Stainless Steels ACI Cr-8 19.5 66.0 9.5 - - 0.08mllll: 2.0m,n 1.5 mall ACI Cf·8M 19.5 63.0 10.5 2.5 - OJl8 max 1.5ma. 1.5 max ACI CA·tS 1.0 max 83.0 12.8 Q.5max - 0.ISm3l 1.5mu 1.0ma. ACt HA - 87.0 9.0 1.1 - 0,20mu' ).0 rna. O.S

Iron Base Nickel-ChromiUm-CjPper Alloys I wo ...... tn:· Stainless 24.0 48.0 20.0 3.0 1.75 0.1)7 max 3.3 0.6 ACI CN-7M "'0 29.0 44.0 20.0 2.0 mill 3.0 min 0.01 milx 1.0 UiJlnax

Nickel Base 'ron-Chromium·Molybdenum Alloys CHLORIM£T" 3 I 60.0 2.0 18.0 JUt - 0.01ma1l 1.0 1.0 ILLIUM' alloy to 56.0 6.5 22.5 U $.5 0.20 0.65 1.25

Nickel Base Molybdenum Alloy .' cHt.o,nMII!T· 2 63.0 3.0ma. - 32.0 - 0.15 IIIa~ 1.0 1 .. 0

Nic,kel Base Sitleon Alloy "AS.TEL~OY· alloy D 82.0 2.0lllllx 1.0 rna. - 3.0 O.l?max 90 0.5·1.25

Nickel Alloyed Cast Irons NI·Res)st Type 1 15.5 69.0 2.2 - 6.5 2.8 2.0 1.2 HI·Ru;st Type 2 20.0 70.0 2.2 - 05ma. 3.0 max 1.9 1.2 HI-Resist Type 3 30.0 62.0 3.0 - O.Sma. 2.6ma. 1.5 0.6 Ni·Resist Type 4 30.5 55.0 5.0 - 0.5 rna. 2.6 mal 5.5 06 Ni·Resist Type D2 20.0 72.0 2.1 - - 3.0 max 2 . .f 0.9 HI·Resist Type D3 30.0 61.0 3.0 - - 2.6 max 2.2 O.Sma.

(1) An improved version of this alloy. CARPENTER'" stainless',No. 20Cb·3. has replaced CARPENJ£R'" ~ta'nie~s No. 20 (2) An improved version of this alloy. HASTEllOY· alloy C·276. has replaced .. ASTUt:OY· alloy C. (3) Cast Alloy 20 alloys such as DUR,MUO alloy 20. ALOYCO" alloy 20. etc • • See ins;de back cover for registered trademarks.

2

Otber

--

AI 4.5; Ii 0.5

-Ii 0.l5; AI 0.15

-AI 2.8: Ti 0.5

Pb 0.05 max; In 1.0 rna. Pb 0.05 rna.; Zn 1.0 max Pb 0.05 max; In 1.0 max

-N 0.25 max

--------

Cb;-Ta IOxC min --

Cb+Ta 0.6

AI 0.15: Ii 0.9 W 0.5; Cn ,. Ta 2.12 Co 2.5ma<; W 4.0: V 0.4 max Co 2.5 rna.; W 4.0: V OA max Cb- Ta 3.65

Co 2.5 rna.: V 0.2·0A; P 0.025 rna.; S 0.030 max

--

-----------

Co 1.5 max

_.-------

Corrosion Resistance of Nickel and Nickel-Containing

Alloys in Caustic Soda and Other Alkalies

PART I. INTRODUCTION

Caustic soda (sodium hydroxide) is the most widely used and avaBablealkaline material. In the United States almost all of the caustic soda is produced as a co-product in the production of chlorine by the electrolysis of sodium chloride. The electrolytic cells used can be divided into two general types: mercury cells and diapllragm cells. With mercury cells,high purity SOt;(. caustic is produced directly, whereas with diaphragm cells. the caustic concentration produced is within the range of 9 to 15 per cent, and has to be further purified and concentrated before sale. A small amount of caustic soda is produced by the limesoda proce$S which WaS formerly the prime source for this chemical.

Caustic soda is generally marketed in concentrations of 50 percent, 73 per cent or anhydrous. The chemical industry is the largest consumer of caustie soda, followed by the rayon and film industries. the pulp and paper industry and the aluminum industry.

A large number of alloys can be used for handling caustic soda, and selection is based upon such factors as concentration. temperature, im-

3

purities in the caustic, the necessity for product purity. corrosion rate, susceptibility to stresscorrosion cracking (caustic embrittlement) and economics. Caustic soda can be handled in cast iron or steel equipment at low· temperatures. if iron contamination is not detrimental to end use. At elevated temperatures, however, iron and steel are subject to caustic embrittlement and high corrosionrates. Plant and laboratory tests and operating experience over many years have demonstrated that nickel and nickel alloys are the preferred materials for handling caustic solutions in many applications. Nickel can be used for practically an concentrations and temperatures.

In addition to caustic soda. several other important alkalies are discussed in this bulletin. but no attempt has been made to be all-inclusive.

Nominal compositions of alloys referred to in the text are shown in Table L Materials other than nickel-eontaining alloys included in a number of tests are reported for reference purposes.

An corrosion rates are reported as mils penetration per year (mpy). (1 mil = 0.001 inch.)

Fig. 1 - These caustic soda evaporator units are a combination of both solid Nickel 200 and steel clad with Nickel 200. Diaphragm cell liquor feeds into the double-effect evaporator: overflow from a settler tank feeds the single'effect evaporator for conc.entration to 50% caustic soda. The system produces 700 tons of salt and delivers 434 tons of NaOH (100%1 per day.

Ph%qraDh by courtesy ot the Swenson Division Of Whiting Corporation.

PART II. CORROSION BY CAUSTIC SODA

A. Nickel 1. Effect of Concentration, Temperature

and Carbon Content Corrosion test results for nickel in commercial caustic soda solutions were obtained by a number of investigators at different times and locations. Typical test data are shown in Table II and these have been incorporated in the isocorrosion chart, Figure 2. Only at high caustic concentration near the boiling point does the corrosion rate exceed one mil per year. This isocorrosion chart is intended only as a guide; there are specific conditions under which higher or possibly lower corrosion rates can prevaiL These conditions are discussed later.

700 r---..,.-----r---r----..,.---"r"> 371

600

500

u... 4oo .. :; "0 Q; Q

~ 100 f-

200

100

316

]60

'13

<OJ mpy "- 0.1 mpy

38

o '--__ ....l.-_.L---l. __ --''--__ ...L-__ ~ •. 17.8

o 20 40 60 80 100

Fig. 2 -Isocorrosion chart for Nickel 200 and Nickel 201 in sodium hy~roxide.l

Some tests which have been carried out at elevated temperature and pressure in autoclaves indicate satisfactory performance for nickel in caustic soda solutions even above the atmospheric boiling point, as shown in Table IIU

4

Fig. 3 - View of caustic transfer piping from marine storage tank area to terminal where rayon grade 50% caustic soeta is loaded into a barge. Several hundred feet of lightweight. welded Nickel 200 piping in 8-inch and 12-inch sizes are used.

In caustic concentrations above 75 per cent and including molten caustic soda. nickel is second only to silver in resistance to corrosion. When nickel is to be used at temperatures above 316 C (600 F). a low-carbon grade, Nickel 201 (0.02% C max). should be employed to preclude the possibility of graphite precipitation in the grain boundaries and a resultant loss in ductility. Nickel 201 is often used for the construction of tubular evaporators for continuous vacuum concentration of caustic soda from 50 and 73 per cent to anhydrous at temperatures up to 404 C (760 F). with nickel pickup in the finished caustic of only one to two ppm.3

Molten caustic soda has been considered as a heat transfer medium for nuclear energy application. In static tests at Harwell in England, Gregory. et aL.~ concluded that Nickel 201 was a satisfactory container material for molten caustic soda at temperatures up to 580 C (l076 F)_ Some of their data are shown in Table IV.

Table II

Typical Corrosion Test Data for Nickel and High Nickel Alloys in Caustic Soda Solutions

HaOH ;oncentration,

%

D-1

4

4

5-10

14

22

34

30"50

49-51

50

50

50

72-73

72

73

73

13

14

15

60 to nearly anbydrnus

Temperature

C F

3{l 86

30 86

3{l 86

21-32 70-90

88 190

50-60 12{)-140

65 150

81 178

55-75 131-167 31165 av 149

55-61 131-142 31158 a1l 136

60-70 14~158 av65 al/ 149

150 302

116 273

121 282

95-100 203-212

100-120 212-248 avll0 a1l230

104-116 244-251 av 110 av248

130 266

135 271

15~260 302-500

• Less than 0.005 mils per year.

A.e1ation Agitation

Nooe Hone

Hone None

Air agitated Air agitated

Extensive due to filling tank

None None

None due to filling tank

Extensive Mild

None None

None due to lining tank


Moderate by lOOgpm flow from pump

None None


Moderate due to filling tank

None by rocking of tank



Not specified by movement of tank car

Not specified due 10 filling tank

None None

Test Period,

days

27

1&2

1&2

124

90

133

37

16

30

135

393

14

183

119

111

52

126

II trips of 7-9 days

35

2

5

Comments

Test coupons removed, cleaned and dried each day fOf lOdays

Average of tests run at 8 separate laboratories

Average of tests run at 8 separate laboratories

Storage lank

First effect of multiple-effect evaporator

Storage tank coupons immersed 95% of time

Storage tank in which air was bubbled through from bottom

Single-effect evaporator_ Rates are average of 3 tests

Storage tank coupons fully immersed

Storage tank

T ransler piping. at pump discharge

laboratory test on tubing; average of 4 coupons

Storage tank

Storage tank

rest tank_ simulating action of tank car

Storage tank coupons immersed 95% of time

Storage tank coupons lully immersed

Coupons in railroad tank car

Storage tank between evaporator and finishing pots. Ammonia Soda Process

Concentration in caustic evaporator

Corrosion Rate, mils per year

Hicke! 200

0.01

0.05

0.05

0.15

0_02

nil

0.03

0.09

0.02

0.02

0.07

-0.3

OJ

0.13

0.05

0.02

0.3

1.6

3.9

NickelCopper Alloy

(MONEL alloy 40ll)

0.01

0.16

0.21

O.ll

0.05

0_01

-

0.19

0.03

0.02

0.10

-0.7

0.3

0.16

0.04

0.10

0.4

1.7

13.4

NickelChromium

Alloy ltNCONEl

alloy 6nO\

nW

-

-0.05

0.03

0_01

0.03

-

0.02

0.02

0.03

0.25

0.4

0_1

0.14

0.06

0.0\

-

1.3

-

Table llt

laboratory Corrosion Tests in Caustic Solutions at Elevated Temperatures

NaOH Concentration,

%

Temperature

C F

Test Period,

hr Nickel 206

20 110 262 15 nil 40 110 262 15 nil 60 llO 262 15 nil 80 110 262 15 nil

20 115 272 19 nil 40 115 272 19 nil 60 ll5 272 19 nil 80 US 272 19 nil

20 162 355 19 nil 40 162 355 19 nil 60 162 355 19 nil 80 162 355 19 nil

20 149 332 19 40 149 332 19 60 149 332 19 20 132 270 ]9 40 132 270 19 GO 132 270 19 80 132 270 19

20 111 340 19(2 tests} 40 111 340 19 (2 tests) 60 l7l 340 1912 testsl 80 17l 340 19 (2 tests) 20 156 345 20 40 156 345 20 GO 15G 345 20 86 156 345 20 20 127 293 15 40 127 293 15 GO 127 293 15 80 127 293 IS

20 150 334 18 46 150 334 18 GO 152 336 19 20 183 394 15 GO 183 394 15 80 183 394 15

2. Effect of Velocity

Velocity has little effect on the corrosion rate of nickel in caustic at temperatures below 500 C (932 F) but at 540 C (1004 F) and above, increasing velocity may cause a several-fold increase in the rate of attack. Figure 4 shows the results of two-week laboratory experiments by Gregory, et al., in high temperature molten caustic soda under dynamic conditions.

6


MONEL alloy 400

nil 3 1

WORTHITE sIs (solution

quenched)

4 9 1 nil

25,69 36.28 2,38 nil. nil

14 17 33 1

3. Effect of Aeration

HiResist Type 2

94 6 17 28

Cast ACI CN·7M

10 1 12

nil 151 2

Aeration has not been observed to accelerate corrosion in lower concentration caustic soda solutions. However, at high concentrations and temperatures, such as occur when concentrating to anhydrous, precautions should be taken to minimize aeration.

M"lten Coust;c Sodo 720 C (1328 Fl 480

400

>-~ 680 C (1256 Fl E 320

~ 0

oc 240 .----------c

_Q

E (; u •

600 C {I I 12 Fl 580 C {1076 Fl

540 C (l004 F)

635 C P 175 Fl

• 400 C ( 752 FI 500 C I 952 F)

100 200 300 400 500 600

Rotot;on $peed_ rpm

Fig. 4 - Corrosion rate of nickel as a function of rotational speed.'

Table IV

Static Corrosion Rates of Nickel and Nickel Alloys in Molten Caustic Soda


Temperature

460e 500e 58fl e 680e Alloy (750 f) (932 f) (1076 f) (1256 f)

Hickel20t 0.9 1.3 2.5 37.8 HASTELLOY a!toy e 100.5 HASTELt.OY aUoy D 0.7 2.2 9.9 MONEl. alloy 400 1.8 5.1 17.6 INCONEL aUoy 600 U 2.4 5.1 66.4 OURANICKEI. alloy 30t 1.7 3.2 10.4 40.7 NIMONIC all01 75 1.1 14.3 20.8 47.6

(pitted)

• Gained weight. Swollen outside surface largely oxide-heavily cor· roded.

4. Effect of System Thermal Gradients In molten caustic soda at temperatures above about 550 C (1022 F), nickel is subject to thermal gradient mass transfer.:;· 4;. 7 In this type of attack, nickel is dissolved in caustic at a high temperature surface and is precipitated at a low temperature surface in a circulating system. Gregory, et at, concluded that the corrosion rate of nickel in molten caustic soda could be ten times as great under dynamic conditions as it was under static conditions because of the solubilitytemperature relationship.:;

The mass transfer effect can be inhibited but not prevented by maintaining a hydrogen-con-

7

taining atmosphere in the vicinity where corrosion is occurring. Forestieri and Lad found that, as a result of the presence of chromite ion (CrO:I- 1 ), mass transfer and cOITosion were essentially eliminated for 50 hours by one per cent addition of 325-mesh chromium powder in a test loop operating at a fluid velocity of 15 fps and 816 C (i500 F) with a temperature difference of either 11 C (20 F) or 22 C (40 F) .>1.9 However, a small mass transfer deposit was obtained after 250 hours, indicating that a single chromium addition would not protect a nickel system indefinitely.

5. Effect of Impurities Chlorates in caustic can increase corrosion rates as indicated in the later section on caustic soda manufacture (page 27). Small amounts of sodium chlorate are produced in electrolytic diaphragm cells. The effect of the chlorate on corrosion rate is not critical unless the chlorate is decomposed. and thermal decomposition does not occur below a temperature of 260 to 290 C (500 to 554 F). If it is intended to operate nickel equipment at or above this temperature range. four alternatives are available:

a. Use "rayon grade" caustic which has a specification of 5 ppm maximum chlorate content.

b. Use caustic produced by electrolytic mercury cells or by the lime-soda process, or,

c. Use anhydrous caustic; there are no chlorates in the anhydrous grade .

d. Add reducing agents as discussed in the section on caustic soda manufacture (page 27) .

The presence of oxidizable sulfur compounds in caustic soda tends to increase the corrosion rate of nickel at elevated temperatures. This is noted particularly with hydrogen sulfide, mercaptans, or sodium sulfide, and to a much lesser extent with partially oxidized compounds such as thiosulfates and sulfites.

The effect of the addition of oxidizable sulfur compounds to caustic soda on the corrosion rate of nickel has been studied in the laboratory with the results shown in Table V. Test 1 was made during the evaporation of a commercial caustic soda solution under 28 inches of vacuum. Sulfur content of the original caustic. calculated as per

Table V

Effect of Oxidizable Sulfur Compounds on Corrosion of Nickel 200 in Caustic Soda

Temperature: 130 C c::: 5 C (266 F c::: 9 Fl.

Corrosion Test Rate, No. Corrosive mils per year

Commercial Sodium Hydroxide being concentrated from 50 to 75% NaOH (Sulfur content at start. calculated as H,S. 0.009%} 1.7

2 75% C.P.· Sodium Hydroxide 0.6

3 75% C.P. Sodium Hydroxide plus 0.75% Sodium Sulfide 22.8

4 75% C.P. Sodium Hydroxide plus 0.75% Sodium Th~wlf~e ~9

5 75% C.P. Sodium Hydroxide plus 0.75% Sodium Sulfite 5.2

6 75% C.P. Sodium Hydroxide plus 0.75% Sodium Su!!ate 0.6

• Chemically pure.

cent H;!S in dry caustic, was 0.009 per cent. Test 2 was made in chemically pure caustic soda. Tests 3 through 6 were made in chemically pure caustic to which the various sulfur compounds had been added.

It has been found that the detrimental effect of oxidizable sulfur compounds in caustic can be counteracted by the addition of sufficient sodium peroxide to form sulfates. An excess of peroxide does not seem to be detrimental. as shown in Table VI which compares the resistance of nickel, iron, and copper to fused caustic soda with and without an addition of 5c;. sodium peroxide. lo

In each test, 5 grams of the substance were fused for four hours in a laboratory crucible of the given metal and analyzed for metal pickup.

6. Effect of Stress Experience has indicated that Nickel 200 is not subject to stress-corrosion cracking in pure caustic solutions. However, it is subject to stress-corrosion cracking by mercury, and there have been a few cases of cracking of nickel when "upsets" occur in producing plants that utilize mercury cells.

In addition, cracking along precipitated grain boundary jTraphite in Nickel 200 has occurred after caustic soda exposure above 316 C (600 F). As indicated previously. a low-carbon grade

8

(Nickel 201) will circumvent this problem. Applied or residual stresses apparently do not

significantly affect the genera! corrosion rate of nickeL 11

7. Effect of Dissimilar Metal Contact Galvanic corrosion can occur in caustic soda solutions if different materials of construction are electrically connected. Whether this effect is academic or critical depends upon the specific conditions that exist in a partiCUlar installation. For instance, the data in Table VII illustrate that gray cast iron corrodes from about one and one-half

Table VI

laboratory Tests in fused Caustic Soda with and without Addition of 5% Sodium Peroxide

Temperature Metal Pickup. grams

COHosive

Caustic Soda

Caustic Soda with 5% Sodium Peroxide

• SI rangly attacked

C

350 360 400 450 500 550 600

350 400 450

f Nickel Imn

662 .4 680 .01·.02 752 Irace·.02 .426 842 .01·.02 .2·.3 932 .005·.015 .2·.3

1022 .4·.43 1112 .i3·.3

662 .0024 .024 752 .0135 .025 842 .OBI .Il

Table VII

Galvanic Corrosion of Gray Cast Iron

Conditions: Corrodent: 5% sodium hydroxide.

Temperature: 43 C (ll 0 F).

Flow: 16 feet per minute.

Aeration: Saturated WIth air.

Cathode to anode area ratio 2: 1.

Copper

trace .013 .03

Corrosion Rate of Corros ion Rate of Gray Cast Iron. Cathodic Material, mils per year mils per year

In In Cathodic Galvanic Galvanic Material Insulated Couple Insulated Couple

Nickel 200 l.l 1.5 Nickel 200 0.6 ]6 0 <0.1 MONEL alloy 400 0) 2.1 <01 <0.1 MONEL alloy 400 0.6 17 0 0

0.75 1.72 Average Average

• Slight welgh1 gain

to three times its normal rate when connected to Nickel 200 or MONEL alloy 400, under the given set of conditions. However, the normal rate for cast it-on in 5~-;' caustic is so low that these higher corrosion rates are usually tolerable.

At higher caustic concentrations, temperatures, and with large cathode to anode r;:ttios. galvanic corrosion becomes more pronounced. The galvanic current curves shown in Figure 5 are from tests made above and close to the upper tube sheets of operating caustic evaporators. The general conclusions to be drawn from these tests are that in concentrated caustic soda solutions. significant galvanic corrosion may occur on cast iron or steel when in contact with nickel or copper. In the construction of caustic evaporators. it is desirable, if not actuaHy necessary, to use nickel or nickel-clad steel tube sheets in conjunction with nickel tubes.

70r---~----'----r----.----r----'----r---.

u. q

~ b.O

~ 5.0

t Si 4.0 c

~

{; 3.0 " o

" ~ 1.0 .(

GoJvonrc Current Flow Between C05-t 1£on end Copper

A"'?oe: c.,;~ !"on Ar~" . C 3QS -;:: ;:. C,~d~ode: Ccpoe r A~e'1 0.*9"4 ;-:: =.

JS

7.0,-----,..-----,----,..-----.--__ -,-____ ,--__ -,-__ --,

~

.~

0

" ?

U " "D

" .(

3.0

2.0

1.0

0 Q S

Golvonic Cv ... enf flow Between Cad Iron ond Niclel

A"\Cd:~: C~S~ h,)~ Af"-e:2 =-.C c.J~a ~ J c. C,+ode: N;d:e~ Are~ ...:. il.24; 5.;)::.

Fig. 5 -- Current measurements between cast <ron, copper and nickel specimens set up above and near the upper tube sheet of an evaporator concentrating caustic soda from 47 to 60 per cent. NaOH under 26 in. vacuum using steam at a pressure of 75 Ib per sq in.

9

8. Cathodic Protection In the continuous production of anhydrous caustic, experience has shown that cathodic protection can be applied successfully to nickel evaporating equipment. In one such case, a cathode current density of about 1 ampere per square foot of exposed nickel surface provided satisfactory protection. In other less corrosive applications, as in storage of 75~(· caustic, current densities as low as 0.01 ampere per square foot have been reported effective. Nickel 200 anodes are used in these applications_ Laboratory tests in 75% caustic soda at 120 C (250 F) and also in fused anhydrous caustic soda at 480 C (900 F) have shown that \",ith equal areas of nickel for anode and c.athode, and with an applied anode current density of 10 amps per square foot, the corrosion rate of the nickel anode does not exceed that of uncoupled· nickel. A pure technical grade of sodium hydroxide was u~ed in these tests, which contained less than 0.04 per cent of heavy metal impurities. The result.c; are shown in Table VIn.

Table Viti

laboratory Tests of Nickel 200 Anodes and Cathodes in 75 Per Cent and Fused Caustic Soda

Temperature: 120C (250F) for 75% caustic. 480 C (900 F) for fused caustic.

Duration of Tests: 18·21 hr.

Volume of Solution Used: 1 liter.

Anode Current Density: 10 amp per sq ft.

Area of Specimens: 0.066-0.087 sq ft.

Average Corrosion Rate. mils lIer year

Nickel 200 Anode Nickel 200 Cathode NickeI200-Uncollpletl

75% Caustic

0.8 0.2 1.0

fused Caustic

11.3 0.9

11.2

Note that the cathodic nickel surface benefited from cathodic protection, while the corrosion rate of the nickel anode was not increased.

B. Nickel-Chromium Alloys Nickel-chromium alloys, such as INCONEL alloy 600, are approximately equi\'alent to nickel in

corrosion resistance in caustic soda, as shown in Tables II, IV and XL.

Alloy 600 is commonly used in equipment for the production of anhydrous caustic when sulfurbearing fuels are used for heating because it is more resistant to sulfidation than nickel.

There have been a few instances of stresscorrosion cracking of Alloy 600 in some strongly alkaline environments. A review of these serviee failures has indicated that they usually occurred in concentrated caustic solutions at high temperatures, 190 to 450 C (374 to 842 F). In seven-day laboratory tests, caustic concentration, temperature, and the presence of air were shown to be important variables, as shown in Tables IX and XIV. No stress-corrosion cracki ng occurred if the Alloy 600 U-bend specimens were stress-relieved at 900 C (1650 F) for one hour or 769 C (1400 F) for four hours after bending.

Table IX

Stress·Corrosion Cracking of INCONEl Alloy 600 U-Bend Specimens in Caustic Solutions

Seven-Day Tests

Temperature Over-pressure.

Caustic CORcentratiolt, weight %

C f 150 psi Caustic 10 50 90

200 390 Air NaOH OK OK 250 480 Air NaOH stress-cracked 300 570 Air NaO" OK stress·cracked stress·cracked

200 390 Argon NaO" OK OK 250 480 Argon NaOH OK 300 570 Argon NaOH OK OK OK

200 390 Air KOH OK slight inter· granular penetration

250 480 Air KOH stress·cracked 300 570 Air KOH OK OK stress-cracked

Note: Testing performed in autoclaves under static conditions without replenishment of air or argon.

C. Nickel-Copper Alloys Nickel-copper anoys, such as MONEL alloy 400, are practically as resistant to caustic soda as nickel, as shown in Table II.

The corrosion rate of Alloy 400 is higher than nickel at caustic soda concentrations above 75 per cent when concentrating to anhydrous. It is also

10

Fig. 6 - This barge has eight tanks with a capacity of 34,000 barrels .. The tan~s are used to carry fuel oil or as· ph~lt. and a specli;ll 54.00·barret tank fabricated of INCONEl alloy 600-clad steel is used to carry 73 010 caustic soda, am· monia·basefertilizers, or jet fuels.

higher than nickel at temperatures above the atmospheric boiling point, as shown in Table III. However, it should be noted that even in those cases where AHoy 400 is inferior to nickel, the corrosion rates are still quite low.

There have been a few reports of stress-corrosion cracking of cold-worked and stressed Alloy 400 in caustic soda. However, the eXact conditions under which most of these failures occurred are not known. It is known that some of the reported failures associated with mercury cell caustic were caused by intergranular attaek by mercury and subsequent loss of ductility.

Laboratory tests have shown that Alloys 400 and K-500 can be susceptible to stress-corrosion cracking unde.r extreme exposure conditions. that is, high stresses in combination with high temperatures and concentrated caustic soda can cause cracking. Table X shows the results observed with tensile loaded specimens tested at 300 C (570 F) in condensing steam after being coated with either potassium or sodium hydroxide. Under these exposure conditions, Alloy 400, which had been cold-worked or cold-worked and stress-relieved prior to testing, was susceptible to stress-corrosion cracking~ cold-worked material that had been annealed at 850 C (1560 F) or 950 C (1740 F) prior to testing was resistant. As with Alloy 400. Alloy K-500 cracked when cold-

Table X

Stress-Corrosion Tests on MONEL Alloy 400 and MONEl Alloy K-500

Alloy Heat Treatment

MONEL alloy 4(1(1 None-as cold-drawn 850 C (1562 fltIti hr/W.O.

MONEL alloy 40(1 Stress relieved 540 C 11004 fl/ ¥z hr

MONEL alloy 40(1 Works anneal· 950 C U742 fll Y2 hr

MONEL alloy K·S(I(I None-as COld-drawn 870 C U598 f)f5 min/W.O. 580 C (1076 fl/S hf/fC· 810 C US98 A/S min/W.O. + 580 C {1016 fl/16 hr/FC·

MONEL alloy K·500 None-as cold·drawn 870 C (1598 fl/5 min/W.O. 580 C n076 AIS hrl FC· 870 C (1598 fl/5 min/W.Q. + saocno76 flfl6 hr/fe·

• Fumace-coaled at about 10 C (18 Fl/nr to 480 C (896 Fl then "if.-coa!ed to room lef'lperature.

NO = Not Determined o == Specimen fractured 1 == Coarse 'cracks visible to naked eye 2 == Fine cracks visible to naked eye 3 == Deep cracks visible under microscope

worked and was resistant in the annealed condition. However. the thermal-hardening treatment at 580 C (1076 F) rendered the alloy very susceptible to cracking.

The practical interpretation of these data is difficult because threshold values of stress, caustic soda concentration, and temperature at which stress-corrosion cracking will occur have not been established. With these limitations in mind, it would appear prudent to stress-relieve AHoy 400 in the range of 538 to 566 C (1000 to 1050 F) or anneal it ill the range of 760 to 816 C (1400 to 1500 F) for one to three hours when it is to be used in higher strength caustic at elevated temperatures.

D. Copper-Nickel Alloys The corrosion resistance of copper-nickel alloys in can.stic soda solutions is dependent upon the nickel content of the alloy, as illustrated in Figure 7. There are a limited amount of additional data which are shown in Table XI.

11

Yield Applie II Strength, Stress, ton/ S1l in. tonI sq in.

43.8 33.1 12.8 16.3

24.0 20.1

11.4 8.3

52.5 33.1 21.2 10.3 65.5 37.2

44.9 37.2

53.2 33.1 NO 10.3 NO 37.2

NO 37.2

Type and Degree of Cracking

NaO\( KO\(

IIG 41'11 5 5

OIG OIG

5 5

3IG+TG 5 5 5 4TG 5

OrG OIG

4NI 5 OIG

DIG

4 = Shallow cracks visible under microscope 5 = No cracks

TG = Transgranular cracks IG == Intergranula. cracks I'll = Type of cracking nQt jdentified-cracks very

short.

>-0-E

'" 12 o a:: 5 8 .;;;

l? ~ 4

20 40 60

Per Cent Nidel in Copper.Nickel Alloys

100

Fig. 7 - Results of corrosion tests of copper-nickel alloys in 50% caustic soda evaporator.

Copper-nickel alloy CA 715 (70% Cu-30% Ni)

possesses excellent resistance to dilute concentrations of caustic soda at low temperatures and appears to have useful resistance to caustic soda solutions of up to 73 per cent at the boiling point. However, this resistance does not extend to fused caustic. Alloy CA 715 has been used successfully as evaporator tubes for concentrating to 50 per cent where copper pickup by the caustic could be tolerated.

Copper-nickel alloys CA 706 (90(~ Cu-lO('~ Ni)

Table XI

Corrosion of Copper-Nickel Alloys by Caustic Soda Solutions

Nominal

HaOM Alloy

Composition Concen- Copper- Temperature tration, Hickel Wt% Wt%

% Alloy Cu Hi C F

5 - 60 40 15-20 I 59-68 11 60 40 Hot-Exact temperature

unknown

5 70 30 15·20 .1. 59-68 11 70 30 Hot-Exact temperature

unknown 50 70 30 95 203 50 CA 715 70 30 65 149 73 70 30 105 221

60-75 70 30 150-175 302·347 60-1Im 70 30 150·260 302·500

100 70 30 400·410 752·770

5 80 20 15·20 59·68 60·75 80 20 150-175 302·347

CA 710 60·100 80 20 150·260 302·500

100 80 20 400·410 752·770

50 90 10 95 203 CA 706

73 90 10 105 221

,. Less than 0.1 mit per year.

and CA 710 (80% Cu-20% Ni) have useful resistance to caustic soda solutions but their application is limited to lower concentrations and temperatures than AHoy CA 715. Because of the limited data available it is difficult to define limits for these two alloys.

While corrosion of the copper-nickel alloys by caustic solutions may be aggravated by the presence of sulfur compounds, Alloy CA 715 is able to resist attack under some conditions, as shown in Table XII. No data appear to be available on the susceptibility of these alloys to stress-corrosion cracking in caustic soda solutions.

Test Corrosion Dura- Rate, tion, mils per days year Comments

21 Nil· laboratory test in glass bottle. 25 0.5 Diaphragm cell liquor·

coupons in distributor box to settlers.

21 Nil laboratory test in glass bottle. 25 4.3 Diaphragm cell liquor·

coupons in distributor box to settlers. 67 0.8 Velocity 1.8 ftl sec. Salt saturated. 30 Nil In storage tank.

118 1.2 l/Z 4.4 In evaporator concentrating from 60-75%. 2 21 In evaporator concentrating

from 60% to anhydrous. 1 70 In anhydrous melt.

21 Nil laboratory test in glass bottle. liz 8.1 In evaporator concentrating

from 60·75%. 2 28 In evaporator concentrating

from 60% to anhydrous. 1 90 In anhydrous melt.

67 1.8 Velocity 1.8 ft/sec. Salt saturated.

118 2.0

Table XII

Corrosion Rate of Copper-Nickel Alloy CA 715 in Alkaline Solutions Containing Sulfur Compounds

Corrosion Duration, Rate,

Conditions of Exposure days mils per year

1. In open tank used to boil 18·22 per cent NaGH to release mercaptans at 80 C (175 fl 30

2. In reboiler of caustic stripper, 1·2 per cent NaDH.3 per cent Na,S. 10 per cent sodium phenolate + 0.7 mg per liter as sodium

25· mercaptides at 124 C (255 Fl 131

3. In 10 per cent sodium sulfide in storage tank at atmospheric temperature 81

4. In 60 per cent sodium sulfide in flaker 14 feed tank at 171 C (340 f) 28

5. In regenerator reboiler for steam stripping of mercaptans from solutizer solution

25.2 per cent potassium hydroxide 37.8 per cent potassium isobutyrate

5.5 per cent potassium sulfide 1.9 per cent potassium mercaptides 2.1 per cent potassium carbonate

at 141 C (286 f) 140 15

6. In vapors from solution in item 5 140 12

~ Pitting up to 3 mils depth.

12

(.

E. Iron·Nickel·Chromium Alloys Based upon data obtained in several test exposures and shown in Tables Xln and XL, it appears that INCOLOY alloy 800 approaches INCONEL alloy 600 in resistance to caustic soda. However, Alloy 800 is more susceptible to stress-corrosion cracking than Alloy 600, as shown in Table XIV.

There has not been sufficient experimental work on the stre.ss-corrosion cracking of Alloy 800 to determine if stress-relieving in a temperature range which will cause sensitization (precipitation of chromium carbides in a continuous network) renders the alloy more susceptible to this form of attack. Therefore, it would appear prudent to anneal the alloy in the range of 1120 to 1150 C (2050 to 2100 F) or stress-relieve and stabilize at 870 C (1600 F) for one to two hours when it is to be used in higher strength caustic soda at elevated temperatures.

Table XIV

Laboratory Tests-Results of U-Bend Specimens in 90% Caustic Soda at 300 C (572 F)

Maximum Depth of Cracks, mils

Argon 15 psig 50 psig 150 psig atm. air air air

Material 1 week 1 week 8 weeks 1 week

INeOLOY alloy 800 10 7 120lal 1151bl INCONEL alloy 600 0 0 75 115 Type 304

Stainless Steel 100 110 11 10

(a) Removed at four weeks. (b) Two·week test. Note: Testing performed in autoclaves under static conditions without

replenishment of air or argon.

Table XIII

Plant Tests-Corrosion Rates in Caustic Production Equipment Using Electrolytic Diaphragm Cell Caustic

Exposure times vary from 24 to 29 days

Conditions

NaOH NaGI Temperature

Concen- Concen· Av Max Min tration, tration,

% % G F G F C

10 12 88 190 91 195 82 23 J.8 93 200 104 220 82

35-40 6·7 116 240 127 2~0 102 50 10·15 93 200 104 220 71 72 ? 121 250 124 255 119

F. Austenitic Chromium-Nickel Stainless Steels

F

180 180 215 160 245

Austenitic chromium-nickel stainless steels offer good corrosion resistance to boiling caustic soda solutions up to about 10 per cent concentration, but from 10 to 50 per cent, the temperature for satisfactory service probably would not exceed 93 to 100 C (200 to 212 F). Generally more severe but inconsistent corrosion rates occur in more


'" '" ~ ~ ~

'" '" '" ~ ~ = = ~ ~ M ~ '" ~ :s~ :'§a; :Sa:; .,.

~

" ~ " !9~ ~~ .e.e ~

'" '" " '" <n<n <n<n <n<n ~ ,. '" '" ~ .,. .,. .,. ] c 0 N ~ W '" '" '"

~

1': ~ 0; w Z M M M .;;; t;

0 ~ z 0 ~ ~ ~ ~

~

'" " " ~ 0 " ~ ~ ~ .~

~

~ Z ~ ~ ~ ~

"' ~

~ ~ ~ z ~

<0.1 <0.1 <0.1 <0.1 0.2 <0.1 0.2 0.2 5 4 0.2 0.1 0.3 0.1 0.4 2 0.4 1 9 2 0.6 0.4 1 0.5 1 2 0.9 3 46 49

<0.1 <0.1 0.2 <0.1 0.2 0.2 0.4 0.4 5 6

13

0.1 <0.1 OJ <0.1 4 3 0.3 5 4 16

concentrated solutions and at higher temperatures. An isocorrosion chart (Figure 8) summarizes the corrosion behavior of austenitic stainless steels in caustic soda.

Typical corrosion rates for several stainless steels are shown in Tables XV and XL. Type 316 stainless steel does not appear to offer any appreciable improvement in corrosion resistance over Type 304 stainless steel in caustic soda solutions.

700 .-----r-------,-----r----r------.-. 3 It

600

500

'-'-. 400

~ ~ " Q.

E ~lOO

200

100

. .o..pF=".Jre~· 5tre~~\·CO(f·Os~O':"I

\ .1\ ·:""";)-~oher;.: I C'OC';"1 Bou"cio,y

\ Bo;';n9 P;i", C~rve \ 0' 30moy

\. , f to ' .......

50 m py

.-: 1 ::npv

AU Grades

lib

2&0

Q)

19

17 S OL-____ ~--L-~----~L-----~----J o 20 &0 ,,-' 100

fig. 8 - Isocorrosion chart for austenitic chromium· nickel 'stainless steels in sodium hydroxide,

J. M. Stone observed that Type 304 stainless steel sensitized for one hour at 677 C (1250 F) was not susceptible to intergranular corrosion during 40-week exposures in: 1:!0

1. 10% NaOH at room temperature

2. 10% NaOH boiling at about 102 C (216 F) 3. 50% NaOH at room temperature. and 4. 50% NaOH at 60 C (140 F).

Therefore. post-weld heat treatment of regular (0.08 max) carbon grades or the selection of a low-carbon or stabilized grade of stainless steel does not appeal' to be required for these exposure conditions. However, intergranular corrosion of sensitized Type 304 stainless steel was observed by Agrawal and Staehle in boiling solutions of 20 to 80('; NaOH.I:t

Chromium-nickel stainless steels are subject to stress-corrosion cracking in caustic soda solutions at elevated temperatures. Nathorst H reported several cases of stress-corrosion cracking of austenitic stainless i3teels caused by alkalies. A comparison of the cracking behavior of Type 304 and Alloys 600 and 800 is given in Table XIV. A stress-corrosion cracking zone based upon these and other known failures reported in the literature is shown in Figure 8. A dashed line was used to indicate the temperature-concentration boundary because this zone is probably not completely defined. Agrawal and Staehle have shown that sensitized Type 304 stainless steel is more prone than annealed material to stress-corrosion cracking in boiling caustic sodaP A portion of their data is shown in Figure 9. The cracking obtained was predominantly intergranular in the sensitized material and predominantly transgranular in the annealed material.

Commercial standard grade 50r;. caustic soda from diaphragm cells can have up to 11.000 ppm chlorides. and commercial 50 c; caustic soda from mercury cells and reagent grade anhydrous caustic can have up to 50 ppm chlorides. It has been

Table XV

Corrosion of Stainless Steels by Caustic Soda Solutions

HaOH Concel!- Temperature Test Corrosion

AISI tration. Duration. Rate. Type % C F days mils per year Comments

302 20 50·60 122·140 134 <0.1 storage tank 309 20 50-60 122·140 134 <0.1 storage tank 310 20 50-60 122·140 134 <0.1 storage tank 304 22 50..60 122·140 133 <0.1 storage tank 309 34 65 149 37 <0.1 storage tank 310 34 65 149 37 <0.1 ;Iorage tank 309 50 21 70 134 <001 storage tank 3to 50 21 70 134 <0.1 storage tank 202 50 50·65 122·149 167 0.5 storage tank 304 50 50..65 122·149 167 <01 storage tank

14

10'

L

~ '" .} j

10

So;\;"9 So:"·<,~; lcood JOOe "_ ~J Y e':i >'tres,"

,O','="O----::l:?O=----=li:-O--.. -'=0---SOL----/,L.O---L70---180

Fig. 9 - Stress·corrosion cracking of annealed and sensi· tized Type 304 stainless steel in caustic soda solutions. l3

suggested that unreported chloride impurities are responsible for some of the stress-corrosion cracking. I;; Whether the reported cracking was caused by caustic solutions or the chlorides these solutions contain is an academic point. In any case. consideration should be given to the temperature and stress limitations of austenitic stainless steel in caustic soda solutions.

Autoclave tests have been run on some of the cast stainless steels at temperatures both above and below the boiling point. II; Data for ACI alloy CF-8 from these tests are shown in Table XVI.

Table XVI

Corrosion of Cast ACI AUoy CF·8 in Caustic Soda at High Temperature

HaOH Concentration,

%

20

40

60

80

Temperature

C f

119 245 138 280 185 365

119 245 138 280 185 365 219 425

119 245 138 280 185 365 219 425

119 245 138 280 185 365

Corrosion Rate Range, mils per year

0·5 20·50 20-50

0·5 20-50 50-200 >200

5-20 50-200 50-200 >200

0-5 5-20

20·50

15

Fig. 10 - Piping and certain internal parts of these two KAMYR ¢ digesters used in the pulp and paper industry are Type 316L stainless steel to resist caustic soda and sodium sulfide. Insulation sheathing is Type 304 stainless steel to resist alkaline spills. • See inside back cove<for registered trademarks_

The authors reported that the results for ACI CF -8M were similar. Alloys CF -8 (cast equivalent of wrought Type 304) and CF-8M (cast equivalent of wrought Type 316) both exhibited increasing corrosion rates with increasing temperature.

G. Iron-Base Nickel-ChromiumCopper-Molybdenum Alloys

and Nickel-Base Chromium-Copper-

Molybdenum Alloys The limited corrosion test data for wrought alloys such as CARPENTER 20Cb-3. INCOLOY alloy 825. HASTELLOY alloy G, and cast ACI CN-7M compositions in caustic soda solutions. shown in Tables III, XVII and XL, indicate appreciable corrosion resistance. These alloys fall between the austenitic chromium-nickel stainless steels su!:h as Type 304 or cast ACI CF-8 and the nickel-chromium alloys such as Alloy 600 in resistance to caustic soda solutions. They are markedly superior to Type 304 stainless steel and ACf CF-8 in concentrated solutions above 95 C (205 F)_

At least one plant has used WORTHITE stainless steel pumps for handling 73('; caustic soda at

140 C (284 F).I' However, the same reference also cites high corrosion rates for alloys of less than 70'; nickel, which would include WORTHITE

stainless steel, in a storage tank handling 73'"; caustic soda at temperatures ranging from 120 to 171 C (248 to 340 F). Thus, the 140 C (284 F) application may be at the upper limit of usefulness for this aHoy.

if these alloys are to be used in conjunction with nickel and high nickel alloy equipment in strong caustic soda solutions at elevated temperatures, consideration should be given to electrical insulation between the dissimilar alloys so as to prevent harmful galvanic effects.

H. Nickel-Base Molybdenum or Molybdenum-Chromium-Iron Alloys

Materials such as HASTELLOY alloys Band C-276. INCONEL alloy 625 and cast CHLORIMET alloys 2 and 3 have not been used to any great extent in caustic soda solutions. Ag a result. corrosion data for them are rather mea~er. Tables IV and XVIII show the results of some iabonltory corrOfiion tests. From these data. it is evident that HASTEL

LOY alloy B can be u:~ed in concentrations up to 50 per cent at the boiling point and that the temperature limit for HASTELLOY alloy C-276 would be somewhat less than with Alloy B. Temperature limitations in caustic soda concentrations ~reater

Table XVIII

Corrosion of HASTEllOY Alloys Band C in Caustic Soda Solutions 18

NaOH Temperature Corrosion Rate. mils per year Concen-tration. HASTEllOY HASTELlOY

% C F alloy B aUoyC

5 Room Room Nil Nil 5 66 150 Nil Nil 5 102 215 Nil Nil

to Room Room Nil Nil 10 103 217 <2 2·20 10 121 250 2·20 20 107 225 <2 2·20 25 Room Room Nil Nil 25 66 150 Nil Nil 30 166 240 <2 2·20 40 Room Room Nil Nil 40 128 261 <2 2-20 50 Room Room Nil Nil 50 66 150 Nil Nil 50 144 291 <2 2·20 50 400 750 152 60 165 328 2·20 2·20 70 191 375 2·20 2-20

Note: I) N.t means no measurable corrosion was observed in five 24·hour test periods.

2) 2-20 means corrosion (ate was within this range.

than 50 per cent C.lllnot be determined with the exi::;ting data. HASTELLOY alloy C and INCONEL

alloy 625 were both found to be subject to stre::;scorrosion cracking in seven-day tests in aerated 90'; NaOH at 300 C (572 F). but did not crack if ar~on was Sub5tituted for the air in tests at the Paul D. Merica Rese~lrch Laboratory of The International Xickel Company. Inc.

Table XVII

HaOH toncel}-tration.

%

10

10

13

74

Iron Base Nickel-Chromium-Copper-Molybdenum Alloys and

Nickel Base Chromium·Copper-Molybdenum Alloys in Caustic Soda Solutions

Temperature

C F

24 75

66 150

95·100 203·212

130 265

Test Period.

days

111

II trips of 7·9 days

16

Comments

laboratory test. INCOLOY alloy 825

laboratory test. INCOlOY alloy 825

Test tank simulating action of lank car. WORTHITE stainless steet

rest iOl tank car. CARP[NHR alloy 20

Corrosion Rate.

mils per year

<0.1

<0.1

0.2

0.3·0.9

I. Cast Irons and Ni-Resists The beneficial etred of nickel additions 011 the corro:,ioll t'esistance of cast irons in moderately concentrated caustic alkali is shown by data in Tables XIX, XX Hnd XX I. It is evident that nickel contents of 20 to 30 per cent pro\'ide vet'y marked improvement in resistance to corrosion as compared 'with unalloyed cast iron. It is also apparent that as lo'w as 3 to 5 c ; nickel may improve the corrosion resistance of cast iron in some concentration ranges,

Table XIX

Effect of Nickel Additions on Corrosion Rates of Cast Irons in 50 to 65% Caustic Soda

Temperature: Boiling under 26 in. (mercury) vacuum.

Duration: 81 days.

Nickel, %

o o o 3.5 5 15 20 20 (plus 2% Chromiuml 30


73 91 86 47 49 30

3.3 S.O 0.4

In practice. the nickel cast irons most widely used with caustic solutions, where minimum contamination of the caustic is desired. are the NiResist alloys and their spheroidHl graphite counterparts. the ductile Ni-Resist alloys, The corrosion rates of these alloys fora number of different exposures are shown in Table XXII.

Table XX

Corrosion of Nickel Cast Irons in the Evaporation of Caustic Soda from 37 to 50 Per Cent

Average Temperature: 120 C (248 F).

Duration: 51 days.

Corrosion Nickel, Chromium, Copper, Silicon. Carbon. Rate.

% % % % % mils per year

28.60 1.71 1.30 2.87 17 28.37 1.50 2.72 18 14.26 2.39 6.08 1.62 3.15 22 19.40 1.42 3.15 24 19.02 2.90 1.22 3.18 28 20.53 1.25 2.91 31

17

Fig. 11 - Moiten sodium hydroxide at an initial tempera· ture of 370 C (700 F) is converted to flake caustic by this flaker and breaker. All surfaces exposed to caustic are nickel except for l'li'Resist Type 3 cooling drum.

Table XXI

Plant Corrosion Test in 74% Caustic Soda in Storage Tank

Specimens exposed for total of 32 days (20 days in liquid and 12 days in vapor).

Corrosion rates based on 20 days exposure to liquid.

Temperature:. 125 C (260 n.

Material

MONEl. alloy 400 H.i-Resist Type 3 Hi·Resist Ductile rron Type 02 Hi-Resist TYlIe 2 Type 304 Stainless Steel Mild Steel Cast Iron


0.9 2.5 5 6

15 75 76

Copper-free Ni-Resist Type 2 may be used in preference to Xi-Resist Type 1 (6.50:-; copper) where it is desired to keep copper content of the solution at a minimum, The 30 r ;. nickel cast iron (Ni-Resist Type 3), in addition to having somewhat g-rcater resistance to corrosion by hot caustic solutions than Ni-Resist TypeS 1 and 2, has a low coefticient of expansion, an advantage for expOSUI"C conditions invo!\'ing sudden changes in temperature.

Of the fi\'e basic types of ~i-nesist. Type 3 appear:' to be the best suited to meet the requirements for caustic sen"ice. !'\i-l1e::;ist Type 3 or Type D3 can be con::;idered as alternate materials

to nickel and the high nickel alloys for caustic soda concentrations up to 73 per cent, but nickel is preferred for higher concentrations.

There have been occasional stress-corrosion cracking failures with the Ni-Resists in high-

chloride aqueous environments. Although these environments did not include caustic soda, it \\"ollld appear a reasonable precaution to stressreliew these alloys at 677 C (1250 F) for one hour before use in hot caustic soda solutions.

Table XXII

Corrosion Rates of the Ni-Resists in Caustic Soda

Corrosion Rate. mils per year NaOH

'" '" '" '" c

~.~~ C> Concen- Temperature Test "'~ ";:;fN '~("') "Vi v tration, Period, "'", "'", "'", "'", .- '" '70.. '70.. '70.. '70.. ~o:: ~ '" % C F Aeration Agitation days - >- .- >. .- >. .- >. =:Ii • ..!. >.. '" x>- X..- 201- XI- c2O>- c..>

8.5-9 82 180 None due to 32 plus 15·15.5% 2.5 0.8 1.5 15 filling tank NaCI in storage

tank

10 88 190 Moderate due to 279 plus 12');' NaCl 0.2 4 filling tank in storage tank

I

! 14 88 190 None due loevap. 90 lirst eHect of i 8

multiple effect !

evaporator I 23 93 200 Moderate Medium 48 plus 7-8% NaCI 1.2 21 in salt settler

30 85 185 Moderate Moderate 82 plus heavy con- 0.8 0.4 0.1 0.5 6 centratlOn of suspended NaG! in sail settler

35-45 116 240 Moderate Small 24 plus 6-7% NaGI 3.3 49 in salt settler. Intermittent ex-

I posure to vapor

49-51 55 149 None due to 30 storage tank I II filling tank

50 55 131 Moderate 1.8 Ips 173 plus heavy con- 0.5 0.2 I <0.1 0.2 1.2 cen,ration of

,

I I suspended NaC! I in transfer line I I

50 71-104 160-220 Moderate 1 Ips llS plus 10·15% NaG! OA 6 in cooling tank

50 95 203 Moderate 1.8 Ips 67 plus heavy con- 1.0 0.6 0.4 11 cent ration of suspended NaGl in transfer line

50 21-127 70-260 None None 10 days @250F

laboratory test 4.7 5.0

& 4 days @70r

50-65 Boiling None due to evap. 81 30 3.3 OA 86 50-70 121 250 None due to evap. 10 in evaporator 90 290

72 121 250 Moderate very small 1]9 star age lank 4.7 15 74 127 260 Above & Slight 20 specImens 6 2.5 5.5

Below exposed in storage liquid lank for 32 days level (20 days in liquid

and 12 days in vaporl. Corrosion rales based on 20 days

I None

exposure in liquid

!60 100 510 950 Moderate 14 concentratIOn in 70 87 534 60 141

open pot

18

PART III. CORROSION BY OTHER ALKALIES

A. Caustic Potash (KOH) Caustic potash is produced by the electrolysis of muriate (potassium chloride) brine. Several types and concentrations of KOH are available, but 45 and 50 per cent liquid and 85 and 90 per cent solid are most commonly marketed. Above about 50 per cent concentration, caustic potash has a higher boiling point than caustic soda of the same concentration. This differential is especially pronounced at high concentrations. For this reason. the commercial product is usually not concentrated above 90 per cent because of the high temperatures involved.

In general. those materials which are useful in caustic soda are also suitable for caustic potash.

Nickel 200 and INCONEL aHoy 600 are both suitable for service in hot caustic potash. as indicated by the data presented in Table XXIII. Negligible data exist for other nickel alloys in caustic potash. Gegner has suggested that because caustic potash is so similar to caustic soda. the corrosion data in

caustic soda of similar concentration and temperature can be used to approximate corrosion resistance in caustic potash. Iff

Under extreme conditions, some nickel alloys are subject to stress-corrosion cracking in caustic potash solutions. However, the information presented in Tables IX and X suggests that stress corrosion cracking of Alloy 600, Alloy 400 and Alloy K-500 is not quite as severe with caustic potash as with caustic soda.

The beneficial effect of nickel in cast iron exposed to caustic potash is shown in Table XXIV. The reductions in corrosion rates are similar to those obtained in caustic soda solutions.

Table XXV shows the results of laboratory corrosion tests of several Ni-Resist alloys in hot. concentrated caustic potash. Lower corrosion rates would be expected with a decrease in either temperature or caustic potash concentration. NiResist Type 3 appears to have equivalent, and sometimes superior. corrosion resistance in comparigon to Types 1 and 2.

Table XXIII

KO" COllcentration.

%

13

30

47

50

50

50

10

10

Temperature

C F

30 85

113 236

139 281

28 82

150 300

.

150 300

150 300

150 300

Aeration

None

None

None

None

None

None

None

None

Corrosion Tests in Caustic Potash Solutions

Agitation

due to filling tank

Boiling

Boiling

due to lilting lank

2\.61pm

3481pm' •

21.6fpm

3481pm"

Test Period. days

207

26

26

207

7

35

7

35

Comments

storage tank impurities-K Jeo J 3 gpl. KCf 170 gpl. KClO .. 0.1%

laboratory test-saturated with KCf. 0.05% KClO,

laboratory test-saturated with KG\. 0.18% KClO,

storage tank impurilies-K,CO. 0.3%. KGI 0.75%. KGto, 0.10%

laboratory test-U·Bend specimen showed no cracking

laboratory test

laboratory test

laboratory test

• nil-less than 0.05 mils pel'" year. l-liQuid V-Vapor

." Specimens m~ved at th!s veloc11y fof'" 8 hr each working day and at zero ft per mtn overn1ght and duong weekends. Th,s was equlvatent to ten 24 ·hour days at the high velocity rate.

19

Corrosion Rate. mils per year

Nickel 200

nil'

l. 0.2 V.0.3

l. 0.1 V.O.3

nil

nil

nil

0.4

1.6

INCONEl. alloy 600

nil

l. 0.1 V.O.1

l. 0.4 V.O.1

nil

0.5

0.5

0.7

5.7

MONEl. aUoy 400

nil

nil

Table XX'V

Effect of Nickel in Cast Iron on Corrosion by Caustic Potash

Concentration: 950 g KOH per liter.

Temperature: 400 C (750 F).

Nickel Content of Alloy Iron, %

o 3 6.5

12.4

Table XXV


21-30 3.0 2.0 0.4

Corrosion of Ni-Resists in Caustic Potash

Hi-Resist ClIrrosjon Rate, Type Exposure mils per year

1 68-hour test in 81 % KOlt at 225 C (437 f) 30

2 68-hour test in 81% KOH at 225 C (437 f) 20

3 68-hour test in 81 % KOH at 225 C (437 f) 10

2 36-hour test in 92% KOlt at 268 C 1516 f) 10

3 3S-hour test in 92% KOlt at 268 C (516 f) 10

B. Ammonia and Ammonium Hydroxide

Most of the nickel-base alloys, with the exception of the nickel-copper alloys and nickel itself, resist all concentrations of ammonium hydroxide up to the boiling point.

Among the nickel-containing alloys, the austenitic stainless steels are most frequently employed in ammonia and ammonium hydroxide solutions. Austenitic stainless steels exhibit good resistance to all concentrations of ammonia and ammonium hydroxide up to the boiling point. Tables XXVI through XXIX show the results of plant corrosion tests in ammonia- and ammonium hydroxide-containing process streams.

Stone .determined that Type 304 stainless steel, which had been sensitized at 677 C (1250 F) for one hour, was not subject to intergranular corrosion during a 40-week exposure in 28% NH40H at room temperature.12 However, this resistance does not extend to elevated temperatures in commercial solutions, as shown in Table XXVI.

20

Considerable amounts of Types 316 and 316L stainless steels are used in the ammonia-soda process for the production of soda ash (Na2C03 ).

The main reaction involves the carbonation of an ammoniated brine to form sodium bicarbonate and ammonium chloride. The ammonia is recov-

Table XXVI

Plant Corrosion Test in Ammonia Surge Vessel of Urea Manufacturing Plant

Solution: 22% NH, • 71 % H,O, 7% CO, and trace of NH.NO, .

Temperature: 66C (150 F).

Test Period: 300 days.

Aeration: None.

Agitation: Moderate.

Location: Uquid phase at bottom of aqua ammonia surge vessel.

Material

INCOlOY alloy 825 Type 347 Stainless Steel Type 316 Stainless Steel Type 304 Stainless Steet Type 316 Stainless Steel <Sensitized) CARPENTER alloy 20 INCONEL alloy 600 Type 304 Stainless SteellSensitized) Type 410 Stainless Steel

Mild Steel

Table XXVII

Corrosion Rate, mUs per year

<0.1 <0.1 <0.1

0.3 0.3 0.4 3.4 7.6

Missing-presumed corroded away

Missing-presumed corroded away

Plant Corrosion Test in Mixed Ammonia-Carbon Dioxide Gas Stream in a Chemical Plant

Gas: Mixture of NHJ and CO2 with probably some moisture present.

Temperature: 20 to SOC (68 to 176 F).


Aeration: None.

Agitation: Gas flow.

Location: Suspended in gas stream.

Material

Type 304 Stainless Steet Type 316 Stainless Steel MONEL alloy 400 tNCONEL alloy 600 Mild Steel Nickel 200 Silicon Bronze

• Nil == Less than 0.01 mill><" year.


Nil· Nil· 2.2 3.0 5.1 5.3

72

Table XXVIII

Plant Corrosion Test in Ammonia-Carbon Dioxide Gas Stream in a Metal Refining Plant

Gas: 26% NH J • 14% CO,. balance water vapor.

Temperatur-e: 66 ro 93 C (150 to 200 F); Average 82 C (l80 F).


Aeration: Moderate.

Agitation; 25 to 40 fps gas flow.

Location: NH1·CO~ stripping still overhead line.

Material

Type 202 Stainless Steel Type 304 Stainless Steel Type 316 Stainless Steel INCOlOY alloy 825 INCOLOY alloy 800 INCONEl alloy 600 Type 410 Stainless Steel Type 502 Stainless Steel Mild Steet

Table XXIX


<0.1 <0.1 <0.1 <0.1

1.5 4.1 0.1

20 22

Plant Corrosion Test in Contaminated Ammonia Vapors in a Coke By-Products Plant

Gas: Ammonia vapors plus H,S. CO,. HCN. phenols and steam.

Temperature: 100 to 110 C (212 to 230 F); Average 105 C (221 F).


Aeration: None.

Agitation: High velocity gas flow.

location: Ammonia liquor still vapor outlet.

Material

Type 304 Stainless Steel Type 316 Stainless Steel I NCONEl alloy 600 Mild Steel MONEL alloy 400

Nickel 200

• Nil "" 1.ess tban 0.04 mils per year.


KiINil' 0.1 S.O

>40 (corroded away)

>40 (corroded away)

ered in this process for reuse. Table XXX shows corrosion rates for metals and alloys in an ammonia recovery still in a soda ash plant. The possibility of pitting must be taken into account in the design of equipment where there are such high chloride levels, so as to avoid crevices where chlorides can concentrate to even higher levels and promote crevice corrosion.

21

Table XXX

Plant Corrosion Test in Ammonia Recovery Still, Ammonia-Soda Process for Production

of Sodium Carbonate

Middle Section

Temperature: 60 to 71 C (140 to 160 F).

Liquor Composition: 2% NH l , 9% NH.CI, 14% NaCl, 2% CO2 •


Top Section


Liquor Composition: 5% NHl • 9% NH.CI, 14% NaCl, 3.4% CO2 •



Middle lop Material Section Section

Type 316 Stainless Steel OJ 0.1 Zirconium 0.1 0.1 Titanium 0.1 0.2 HASTEllOY alloy C 0.1 14' Nickel 200 >33" >32" Mild Steet >73" >71"

• Specimen pitted in crevice beneath insulating wasber. •• Specimens cQrroded away ..

Nickel is not attacked by anhydrous ammonia, but is resistant to ammonium hydroxide solutions in concentrations only up to about one per cent. Aeration may induce passivity in concentrations under 10 per cent, but even in the presence of air, more concentrated solutions are highly corrosive to nickel. The corrosion data shown in Table XXXI were obtained in room-temperature laboratory tests ()f 48-hour duration in one normal ammonium hydroxide, following a previous 48-hour ex-

Table XXXI

Corrosionof Nickel 200 in One Normal Ammonium Hydroxide (1.7% NHa)

Test Condition

Total Immersion Quiet Air"'Agitated

Alternate Immersion Conti!luous lntermittent

Spray {4 to 30 Daysl

Corrosion Rate,· milspllryear

0.8 <0.1

2.7 0.4

<0.1

• Specimens exposed at room temperature tor 2 days after a previous 2-day exposure except for spray expo'Sure.

posure. The re~ults of 20-hour tests in highly agitated ammonium hydroxide solutions at room tempel·ature are shown in Table XXXII. Typical corrosion rates for Nickel 200 in several industrial exposures are also given in Table3 XXVII, XXIX and XXX.

Table XXXU

Corrosion of Nickel 200 in Ammonium Hydroxide Solutions

NH.OH Cancentration, %

1.1 12.9 20.2 27.1

CCIfI"lISion Rate, • mils per year

o 560 370 180

• Tests run to agitated solution at room temperature for 20 hours.

Nickel-copper alloys. such as Alloy 400, are resistant to anhydrous ammonia and are slightly more resistant than commercially pure nickel in ammonium hydroxide solutions. as shown in Table XXXIII. However, their usefulness is restricted to dilute solutions up to about 3<1· ammonium hydroxide. In solutions of higher concentration. corrosion rates are increased considerably by aeration and agitation.

TableXXXm

laboratory Corrosion Tests of MONEL alloy 400 in Ammonium Hydroxide

Temperature: Room. Test Period: 20 hours.

Agitation: 371 feet per minute.

HH3 Cancentration, %

2.7 3.6 5.5 8.2

11.1 18.3 25.8

Carrosian Rate, mils per year

o 70

298 317 327 231

36

C. Other Alkaline Solutions of Sodium and Potassium Salts

Salts su~h as sodium sulfide. sodium carbonate, sodium silicates, trisodium phosphate and others form alkaline water solutions. These alkaline

22

Fig. 12 - Sodium carbonate filters use Type 304 stainless steel or MONEL alloy 400 for the perforated backing sheet or winding wire. The same materials are also used for back· ing wire cloth and facing cloth .

salt solutions can be handled in the same materials suitable for caustic soda. As with caustic soda, dilute solutions at low temperature are not very corrosive to carbon steel and may even act as corrosion inhibitors, but concentrated solutions at high temperatures often require nickel or high nickel alloys.

The results of tests within an evaporator handling sodium metasilicate are shown in Table XXXIV.

Another plant test in a kettle during the dissolving of silicates in strong caustic soda gave the corrosion rates shown in Table XXXV.

The superiority of Alloy 400 and austenitic stainless steels for a phosphate hydration was demonstrated in a short-duration test shown in Table XXXVI.

Nickel and high nickel alloys offer good resistance to corrosion by sodium sulfide solutions. In

Table XXXIV

Corrosion Tests in Sodium Metasilicate

Composition: 50% sodium silicate. 50% sodium hydroxide.

Ave~age Temperature: HOC (230 F). Test Period: 6 weeks.

Material

Nickel 200 MONEL alloy 400 INCONEL alloy 600 Hi-Resist Type 1 Mild Steel Cast Iron


<0.1· <0.1 <0.1

0.4 13 18

Table XXXV

Corrosion T e.sts During Dissolving of Silicates in Caustic Soda

Location: Test specimens suspended near bottom of kettle.

Temperature: 77 C (170 f). Test Period; 32 days.

Material

Nickel 200 Ni:Resist Type 3 Ni:Resist Type 2 Nickel Cast Iron (3% Ni) Cast Iron Mild Steel

Table XXXVI

COffoswn Rate, mils per year

0.1 0.2 0.5 8

33 41

Conosion Tests in Phosphate Hydrator

Composition: 50% solution of sodit.t1l1 tripolyphosphate and sodit.tm tetrapotyphosphate.

Average Temperature: 74 C (165 F}. Test Period: 60 hours.

Aeration: Extensive. Agitation: Considerable.

Material

Type 3114 Stainless Steel Type 316 Stainless Steel MONEt. alloy 400 Mild Steel

Corroswn Rate, mils pet year

0.1 0:4 0.7

133

10:t sodium sulfide. the corrosion rates are quite Jow. as shown in Table XXXVII. The most severe service conditions are encountered in hot. concentrated solutions. The results of two plant corrosion tests in direct-fired evaporators which con-

Table XXXVII

Plant Corrosion Test in a Sodium Sulfide Storage Tank

Solution: 10% Na2S. Aeration: Open tank.

Temperature: Atmospheric. Agitation: Only due to filling tank.


Matl!rial

. Nickel lO!l MQNEL alloy 400 tNCONEL alloy 600 KASrELLOY all.oy B HAsrULOY atloy C Type 304 Stainless Steel Type 316 Stainless Steel ILLlUM G DURIMEr 20 Copper-Nickel alloy CA 715

Corrosiolt Rate. mils per year

<OJ <OJ <OJ <0.1 <OJ <0.1 <0.1 <fU <0.1

1.3

centrate sodium sulfide from 25 to 60 per cent are given in Tables XXXVIU and XXXIX. Operating experience over a number of years with evaporator tubes in sodium sulfide evaporation has shown that Nickel 200 and Alloy 400 are satisfactory for this application.

23

Experience has also demonstrated that Alloy 600 is useful .for direct-tired pans in which sodium sulfide is eoncentrated from 25 per eent to 60 per cent. Operating temperatures on the order of 150 to 177 C (300 to 350 F) prevaiL Under sueh conditions. Alloy 600 has given a service life of up to eight years.

Table XXXVIlI

Plant Corrosion Test in Direct·Fired Open Pot Used for Concentrating Sodium Sulfide to 60 Per Cent

Temperature: 100 to 180C (212 to 356 f).


Material

MONEL altoy 400 fMCOJltEl.. alloy 600 Nickel 200 Copper-Nickel alloy CA 715 HASTELLOY alloy B ILLiUM G Type 316 Stainless Steel Type 3114 Stainless Steel KASrELLOY alloy C DURIMEr alloy 20

~ Specimens corroded away.

Table XXXIX


8 10 16 20 22 48

>72* >73* >85*

110

Plant Corrosion Test in Gas-fired Open Tray Used for Concentrating

Sodium Sulfide from 25 to 60 Per Cent

Temperature: 125to 175C (257 to 347 f).


Material

MONEL alloy 400 Type 310 Stainless Steel Type 309 Stainless Steel Nickel 200 INcoNELa1loy 600 Type 304 Stainless Steel Type 3D2 Stainless Steel Type 330 Stainless Steel Type 316 Stainless Steel Mild Steel

• Spedmens corroded away.

Corroswn Rate. mils per year

3 7 8

11 22 84

230 >290* >300' >600*

PART IV. INDUSTRIAL APPLICATIONS

A. Caustic Soda Manufacture Service records, often dating back for 20 to 30 years, have demonstrated the satisfactory service of nickel and nickel alloys in caustic soda manufacture. In one plant, nickel centrifugal pumps handling 50% caustic soda from mercury cells are 27 years old and are still in operation. In another plant, nickel evaporators continue to give good service after 30 years' use. Nickel 200, low-carbon Nickel 201, Alloy 600, Alloy 400 and their cast counterparts are "standard" materials of construction, either solid or as a cladding for

equipment such as evaporators, heat exchanger tubing, pumps, crystallizers, valves, fittings, etc., used in the concentration and handling of caustic soda. Corrosion test data cited earlier in this bulletin were obtained largely in caustic soda manufacturing processes.

A comparison of the corrosiveness of caustic soda produced from mercury cells with that produced by diaphragm cells was made by Committee T5A-3D of the National Association of Corrosion Engineers. Data obtained in this survey are shown in Table XL. It appears that there is not a

Fig. 13 - Triple-effect evaporators for the concentration of diaphragm cell liquor to 50% caustic soda_ All threeevaporators are constructed entirely of Nickel 200 and Nickel 200-clad steel.

Photograph courtesy of Blaw-Knox Company_

24

Table Xl

"Round Robin" Test Program by Four Caustic Soda Producers-Comparison of Corrosiveness of Diaphragm Cell vs. Mercury Cell Caustic

Conducted by NACE Committee TSA-3D

Average Temperature

Company

1 2 3

Material Corredent C f C f C F

Nickel 200 50% NaOH-Oiaphragm Cell 35 95 29 85 88 190 Hickel 200 50% NaaH·t>iaphragm Cell 40 104 - - - -Hickel 200 50% NaOH·Mercury Ceil 38 100 105 221 82 180 Nickel 200 50% NaOH·Mercury Cell 37 98 45 113 - -Nickel 20:0 50% NaOH·Mercury Cell - - Ambient - -Nickel 20:0 73% NaOMHapluagm Cell 119 246 - - 99 210 Nickel 200 73% NaOH-Oiaphragm Cell 125 257 - - - -Nickel 200 73~4 NaOH·Mercury Cell 114236 - - - -INCONEL alloy tioo: 50% NaOK-Diaphragm Cell 35 95 29 85 88 190 INCONEL alloy 601} 50% HaOM·Diaphragm Cel! 40 104 - - - -INCONEL alloy 600 50% NaOH·Mercury Cell 38 100 105 221 82 180 INCONEL alloy 600 50~{' NaOH·Mercury Cell 37 98 45 113 - -INCONEl alley 600 50~{' NaOH-Mercury Cell - - Ambient - -INCONEL alley 600 73% NaOH-Diaphragm Cell 119 246 - - 99210 INCONEl alloy 6{}0 73% NaOH·Diaphragm Cell 125 257 - - - -INCONEL alley 600: 73% NaOH·Mercury Cell 114 236 - - - -MONEL alloy 400: 50% NaOH·Oiaphraglll Cen 35 95 29 85 88190 MONR alloy 400 50% N~OH'Diaphragm Cell 40 104 - - - -MONEL aliDY 41111 50% NaOH;Mercury Cell 38 100 105 221 82180 MONEL aUoy 400 50% NaOH·Mercury Cell 37 98 45 113 - -MONEL alloy 400 50% NaOH-Mercury Cell - - Ambient - -MONEL alloy 400 73°kNaoH·Olaphraglll Cell 119246 - - 99 210 MONEL alley 400 73% NaOH-Diaphraglll Cell 125.257. - ,...,... - ,.... MONEL aUoy 400 13% HIlOa·Mercury Cell 114236 - - - -INCOLOY alloy 800 50% NIlOH·Oiaphragm Cell 35 95 29 85 88 190. INCOLOY alloy 800 50%: HIlOH·tliaphragm Cell 40 104 - - - -INCOLOY alloy 800 50% NaOH·Mercury Celt 38100 105 221 82180 INCOLOY alloy 800 5{)%NaOH.Mercury Cell 37 98 45 113 -INCOLOY alloy 800 50~i. NaOH.Mercury Cell - - Ambient - -IHCOLOY alloy 800 73% NaOIH)iaphragm Cell 119246 - - 99.210 IHCOLOY alloy 800 73% NaOH-Diaphragm Cell 125251 - - - -INCOLOY alloy 800 73% NaoN·Mercury Cell U4.236 - - - ~

CARPENfER alloy 20 Cb·3 50'!';' NaOH·Diaphragm Cell 35 95 29 85 88 190 CARPEtffER alloy 20 Cb-3 50'!" NaOH·Oiaphragm Cell 40 104 - - - -CARPENfER alloy 20 Cb·3 50% NaOK-Mercury Cell 38100 105 221 82 1811 CAftPEtffER alloy 20 Cb~ 50% NaOH,Mercury Cell 37 98. 45.113 - ~

CARPENTER alloy 20 Ch·3 50% NaON-Mercury Cell - - Ambient - -CARPENTER allDY 20 (:b·3 73% NaOH·Oiapllragm Cell U9246 - - 210 CARPENTER alley 20 tb·3 73% NaOH·Oiilplnagm Cell 125 257 - - - -CARPENTER alllly20 (:b·3 73% NaOH·Mercury Cell H4236 - - - -ACI tN-7M 50% NaOH·Oiaphragm Cell 35 95 29" 85 8'8 190 ACt CN·7M 50% NaOH·Diaphragm Cell 40 104 - - - -ACI CtOM 50?{. NaOH·Mer(;ury Cell 38 100 105 221 82 180 ACl eN·7M - 50°4 NaOH·Men:ury Cell 37 98 45 il3 - -ACt CN-1M 50% NaOH·MeJ.eury Cell - - Ambient - -ACI CN-1M 73'l~NaOH-Dlaplmlgm Cell 119 246 - - 99210 ACI CN-7M 73'}~ NlIOH,OillPhraCm Cell 125 257 - - - -ACI CN"7M 13% NaOH.MetClifY Cell 114 236 - - - -Ni·Re.sist Type 3 50% NaOH·Diapbragm Cell 35 95 29 85 88 190 Ni-Resist Type 3 50% NaOH·Diaphragm Cell 40 104 - - - -Hi-Resist Type 3 50% NaOH·Mercury Cell 38 100 105 221 82 180

4

C F

54 130 Ambient 60 140

Ambient Ambient - -- -- -

54 130 Ambient 60 140

Ambient Ambient

- -- -

- -54 130

Ambient 60140

Ambient Ambient - -- -- -54130

Ambient 60 140 Ambient Ambienl - -- -

-54 130

Ambient 60 140

Ambient Ambient - -- -- -54130

Ambient 60 140 Ambient Ambient - -- -- -' 54 130

Ambient 60 140

1

<0_1 <0.1 <0'\ <0.1

<0.1 0.2 0.3

<0.1 (l) <0.1 <0J <O.!

<0.1 0.3 0.2

<0.1 <OJ <OJ <O.}

<0.1 0.4 0.5

<OJ <IU <0.1 <0.1

OJ 0.5 O.3IU

<0.1 <0.1 <(j.l <0.1

0.4 0.5 0.4

<0.1 <0.\ <0.1 <0.1

lU


C.Dmpany

2 3

<0.1 <0.1

<0'\ 1.0 <0.1 <0J

0.2 --

<0.1 <0.1

<0.1 <0.1 <0.\ <0.1

0.2

<0.1 <0.1

0.1 0.2 <0J <0.1

0.8

<0.1 <0.1

«l.l <OJ <0.1 <0.1

4..l (2)

<0.1 <0.1

<OJ <0.1 <OJ <OJ

1.5(3)

<0.1 <0.1

<0.1 <0.1 <OJ <0.1

9.3 1.2 (J)

G.4

0.2 <0.1 0.4 (4) 0.6

<0.1 <0.1 <0.1

4

<OJ <0.1 <0.1 <0.1 <0.1

<0.1 <0.1 <OJ <0.1 <0.1

<0.1 <0.1 <0.1 <0.1 <0.1

<0.1 <0.1 <OJ <0.1 <OJ

<0.1 <0.1 <0.1 <0.1 <0.1

<0.1 <OJ <0.1 <0.1 <0.1

<0.1 0.2

<01

Table XL (Cont'd.)

"Round Robin" Test Program by Four Caustic Soda Producers-Comparison of Corrosiveness of Diaphragm Cell vs. Mercury Cell Caustic

Conducted by NACE Committee T5A-3D

Average Tempe,ature

Company

2 3 4

Material Corrodent C f C f C f C f

Hi·Resist Type 3 50% NaOM.Mercury Cell 37 98 45 113 - - Ambient Ni:Resist Type 3 50% NaOJ1,Mercury Cell - - Ambient - - Ambient Ni:Resist Type 3 73% NaOH-Oiaphragm Cell 119 241> - - 99 2W - -Hi:Resist Type 3 73% NaOM-Diaphragm Cell 125 257 - - - - - -Hi:Resist Type 3 73% NaOH·Mercury Cell 114 236 - - - - - -Type 3.16 Stainless Steel 50% NaOH~iapllragm Cell 35 95 29 85 88 190 54 130 Type 3Ui Stainless Steel 50% NaOH~iaphragm Cell 40 104 - - - - Ambient Type 316 Stainless Steel 50% NaOH·Mercury Cell 38 100 105 221 82 180 60 140 Type 316 Stainless Steel 50% NaOH·MercuI}' Celt 31 98 45 113 - - Ambient Type 316 Stainless .Steet 5~% NaOH·Mercury Cell - - Ambient - - Ambient Type 316 Stainless Steel 73% NaOH~.iapllragm Cell 119 246 - - 99 210 - -Type 3l6.stainless Steet 13% NaOH-Oiapbragm Cell 125 257 - - - - - -Type :116 Stainless Steel 13% NaOH'Mercury Cell 114236 - - - - -. -Type 304 Stainless Steel 50% NaOH·Diapllragm Cell 35 95 29 85 88190 54 130 Type 304Stainle$$.Sieei 50% NaOlH)iallhragm Cell 40 104 - - - - Ambient Type ~04 Stainle$~ Steel 50% NaOn-Mercury Cell 38 100 105 221 82 180 .60 140

Type 304 Stainless Steel 50% NaOH·Mercury Cell 37 98 45 113 - - Ambient 1ype304 StaiRtessSteel 50% NaGH.Mercury Cell - - Ambient - - Ambient Type 304 Stainles$.steel 73~ NaOH-Oiapbragm Cell 119246 - - 99 210 - -Type 304 Stainless Steel 73% NaOH-Oiaphragm Cell 125 257 - - - - - -Type 304 Stainless Steel 73% NaOH·Mercury Cell 114 236 - - - - - -Ductile Cast Iren 50% NaOll·Diaphragm Celt 35 95 29 85 88 190 54130 Ductile Cast Iron 50% NaOli-Diaphragm Cell 40 104 - - - - Ambient Ductile Cast Iron 50% NaOH·Mercury Cell 38 100 105 221 82 180 60 140 Dllctil~cast Iron 50% NaOli·Mercury Ce:t 37 98 45113 - - Ambient Ductile Cast IllIn 50% NaOJt..Men:l.lry tet! - - Ambient - - Ambient D1Ictile Cast Iren 73% NaOH~iapllfagm Cell 119246 - - 99 210 - -Ductile Cast illln 73% NaOH~iaph[agm Celt 125 251 - - - - - -Ductile cast Iron 13% NaOH.MercuI)' Celt 114236 - - - - - -Gray Cast Iron 50% NaOH-Oiaphragm Cell 35 95 29 85 88 190 54 130 Gray Cast Iroll 50% Na(lli-Diaphragm Cell 40 104 - - - - Ambi'lnt Gray Cast Iron 50% NaQli-Mercury Cell 38 100 105 221 82 100 60140 Gray Cast Iron 50% NaOH·Mercury Cell 31 98 45 113 - - Ambient Gray Cast Iron 50% NaOli-Mercury Cell - - Ambient - - Ambient Gray Cast Iron 73% NaOli-Dia.phragm CeU H9246 - - 99 2It} - -Gray Cast Iron 73% NaOn-Diaphragm Cell 125 257 - - - - - -Gray Cast Iron 73% NaOK-MereuI}' Cell 114 236 - - - - - -MildSteeJ 50% HaOH-Oiapbragm Cell 35 95 29 85 88 190 54130 Mild Steel 50% NaOH~iallhragmCeIi 4D 104 - - - - Iynbient Mild Steel 50% NaOH·Mercury Cell 38 100 105 221 82 180 60 140

Mild Steel 50% NaOH·Mercury Cell 37 98 45 113 - - Ambient Mild Steel - 50% NaOM-Mercury Cell - - Ambient - Ambient -Mild Steel 13% NaOH·l)iapilragm Cell 119 246 - - 99210 - -Mild Steel 73% NaOfl..Diapbragm Cell 125 251 - - - - - -Mild Steel 73% NaOM·Mercury Cell 114 236 - - - - - -

(1) Pitted to a maximum dept" Qt 1 mHo (2) Pitted to a maximum depth of 4 mils. (3) Pitted t<> a maximum depth of 5 mils. (4) Stress-corrosion crack through

(5) Pitted to a ma.ximlJm depth Qt 3 mils. (6) Mercury droplets in tao". 2 rates shown

are for the duplicate speCimens

one of the identifying punch marks.

(not averaged): specimen with high rate showed stress-acce1erated local attack.

26

1

0.2

0.3 2.3 1.2

<0.1 0.2

<0.1 <0.1

6 (4) 13.1(5) 10 {4}

<0.1 <0.1 <0.1

<0.1

15 (4)

19.4 t5} 15 (4)

1.4 l.5!S) 0.4 1.8

73 106 103

2.1 1.9 0.4 2.1

54 44 82

UJ5} 1.5 (5) O.SISI

1.4

59 dissolved >38

71


Company

2 3

0.1 <0.1

8.4

<0.1 3.3

<0.1 0.2 0.1

<0.1 8.7

<0.1 1.1

0.1 n. 61 0.3 11.0

<0.1 <OJ

13 m

3.1 12

0.3 0.6 2 (81 3.7

18

1.9 13

2 t9) 2.6 3 4.5

21

1.4 nO} 20

3 {6.1ID 1.8m 21 2 5.1

5.7

4

0.1 0.2

<0.1 <0.1 <0.1

0.3 <0.1

<0.1 <0.1 <0.1

0.4 <0.1

2.6 52 1.0 6.2(2 2.6

2.9 7.9 1.7 5.1 3.5

2.1 3.2 1.2

2.9(5 1.9

(1) Pitted to a maximum depth of 8 mils. (8) Pitted to a maximum depth of 2 milS. (9) Pitted to a maximum depth of 12 mils.

(10) Pitted to " maximum depth of 11 mils.

great deal of difference in the corrosiveness of the caustic produced by these two types of cells, al,'ld that other variables such as temperature and concentration are more important in influencing corrosion rates.

Prior to about 1946, the concentration of 50% or 73 % caustic soda to anhydrous was carried out entirely in direct-tired caustic pots in a batch operation. These pots Were usually constructed of gray cast iron. Nicl<:el could not be used because of the practice of "sulfur shading" (sulfur addition for the removal of iron and other contaminants to achieve higher purity and better product color), which caused severe sulfur embritllement of the nickel at the high temperatures involved.

Subsequent to 1946, these pots have been repla<:ed to a very great extent by nicl<:el and nickel anoy equipment for continuous vacuum evaporation, which has proven to yield a higher quality product more economically.3.20 The production of anhydrous caustic soda in corrosionresistant Nickel 201 and AHoy 600 equipment has eliminated the necessity of sulfur shading.

fig. 14 --Evaporator bodies and vapor piping at a large chlorine-caustic soda plant for concentraticn t.o 730/0 NaOH. All evaporatcrs are constructed entirely .of Nickel 200 and Nickel 200·clad steel.

Chlorates are removed from diaphragm cell caustic soda when concentrating to anhydrous in nickel or high nickel alloys to minimize corrosion

rates. Some corrosion test data showing the effects of chlor;;ltes upon the corrosion of Nickel 200 and INCONEL alloy 600 in high temperature caustic soda are shown in Tables XLI and XLII. There are several means by which chlorates can be re-

27

Table XU

Plant Corrosion Test During Concentration of Diaphragm Cell Caustic Soda from

50 Per Cent to Anhydrous

Feed liquor c.ontained 0.24% scdium chl.orate and 1.0% scdium chloride .on S.olid caustic basis. Rapid circulation of liqu.or.

Temperature: 400 C (750 fl. Test Period: 243 hcurs .operaticn.

Material

Hickel 2DD tHCONEt. anoy 6IJD


liquid Vapor

51.0 87.0

TableXUl

0.5 5.0

laboratory Corrosion Test in Evaporation of Caustic Soda from 73 tQ 96 Per Cent

with and without Chlorate

Temperature: 180 C (360 F) t.o 450 C (840 F).

lest Pericd: 24 h.ours.

Material

Nickel 2IltJ IHCOHEt. alloy 6flO

Corrosion Rate, mils pef year

Without Witll fl.3% Clllorate Chlorate (Solid C;tIIStic Basis)

1.5 2.2

260 380

moved; the addition of sucrose (U. S. Patent 2,610,105) or dextrin (British Patent 778.226) appear to be the most common. While these additions minimize corrosion and attendant metal contamination of the product,- they do increase the carbonate concentration. Bradbury and Cooper have shown that the addition of sorbitol and subsequent heating will also remove chlorates but with the formation of less carbonate.21

Liquid-liquid extraction with ammonia is also widely used to remove chlorates and chlorides.22

In the continuous vacuum concentration and production of anhydrous caustic soda, low-carbon Nickel 201 and Nickel 201-clad steel equipment have given excellent service as evaporator tubes. tube sheets and shells, and as receiving tanks and piping.

Alloy 600 has also been used extensively for producing anhydrous caustic soda and is the preferred material of construction where the heating is accomplished with any media in which there is a po:.;sibility of sulfur compounds being present. Alloy 600 for this service should be stress-relieved or annealed as indicated in the discussion of nickel-chromium alloys in Part II of this bulletin.

Fig. 15 - Tubes fabricated from Nickel 200 afe used in the inclined heat exchanger in front of the Nickel 200-dad

evaporator which prodl.l<.:es 50% caustic soda. separator atop the system. which separates

sodium chloride and other salts from the caustic solution. is also Nickel 200.

B.Caustic Potash Manufacture The production of caustic potash is carried out in nickel and nickel alloy equipment in a similar manner t9 the production of caustic soda. One important difference. however, is the higher boiling point encountered in caustic potash above 50 per cent concentration. Because of the higher temperahx ~~; involved. cathodic protection is often used for low-carbon Nickel 201 or high nickel alloy equipment u:=;ed for the production of

caustic potash at concentratiolls of 90 per cent and above.

28

C. Caustic Soda Storage and Transportation

After extreme care has been taken to assure high purity in the production of caustic. it is important that storage and transportation facilities provide for continuing product purity. NickeIclad steel tank cars have been used for transporting iron-free caustic since 1930. Nickel and nickelclad steel barrels are also in use for the transportation of smaller quantities. More recently. nickelplated steel tank cars and piping have been employed.

The first tank-trailer constructed of INCOLOY

alloy 825 was put in service in 1963 to haul 50% caustic soda. This alloy was selected because of its versatility in its ability to transport other corrosive materials induding sulfuric. nitric and phosphoric acids. INCONEL aHoy 600 was selected for barge tanks to carry 73 (-; caustic one way and return with ammonia-base fertilizers or jet fueL

Fig. 16 - Marine terminal where caustic soda is unloaded from barge. Nickel 200 lined caustic transfer pipe is at right foreground.

Tt'an;;fer of materia! to and from storage tanks usually requires pumps. Table XLIII shovv's the results of a plant test in which Xickel 200, MONEL

alloy 400 and INCONEL alloy 600 corrosion coupons wet'e subject to turbulent flow just downstream of a pump handling 50('~ caustic soda. Similar corrosion resistance would be expected fl'O!1l the cast counterparts of these wrought materials (ACI CZ-IOO, M-35 and CY-40). Pumps cast from ACI CZ-IOOhave given over 25 years service as previollsly noted.

Table XLIII

field Test in 50% Caustic Soda Just Downstream of a Pump

Temperature: 60 to 70 C (140 to 158 f); AI/erage 65 C (149 f).


Aeration: Moderate.

flow; 100 gpm in 3·inch pipe.

Material

Nidle1200 lNCON~L alloy 600 MONEL aUoy 400

_ ..


a.OJ 0.03 0.10

fig. 17 ~ This barge carries 50%. caustic soda from a mercury cell plant to storage facilities'along the Tennessee. Ohio and Mi Rivers. Four cylindrical tanks have a total capacit liquid tons. To insure a long sef\lice life for the ge and to protect product purity. all cargo piping is either solid Nickel 200 or Nickel 2oo-clad steel with Nickel 200 fittings: The cargo fromal! four tanks empties into a Ni,ckeI2oo-clad steel well from which ,it ispl.lmped to on-shore facilities_' .' . '

The use of nickel;..cla<i steel .tanks offers particularj\dvantage.<; in the storage and shipment of 13~ caustic soda. To a\'Qid solidification;caustic of. this st~ngth must be loaded hot and main-

fig. J,8 -:- INCOLOY aUoy 8,25 ,is used for the inner tank and all internal parts that come into contact with corrosil/e cargoes in these two trailers-inner shell and heads. manhole ring and cover. dip tubes. spill dam and discharge pipe. Although presently used for hauling 50% caustic soda. the corrosion resistance of this material will allow the hauling of a variety of corrosives.

tained above the freezing point. Tables XLIV and XLV give the results of tests in transportation and storage facilities.

It is common practice to load and unload cars of 73% caustic through Nickel 200 heat exhangers, pumps and piping.

Table XLIV

Field Test in Tank Car Transporting 74% Caustic Soda


Test Period: 11 trips of 7·9 days.

Aeration: None. Agitation: By movement of tank cars.

Material

Nit;lle1200 INCOlOY alloy 825 MONEL alloy 400 CARPENTER alloy 20 Type 316 Stainless Steel


0.3 0.3 0.4 0.9 8.4

Fig. 19 - 1300 feet of transfer pipe with a rolled and welded internal lining of Nicke1 200 carries 50% caustic soda from a marine terminal to a Nickel 2oo·c1ad storage tank. Nickel 200-clad tank cars are in the background.

Table XLV

Field Test in Storage Tank for 73% Caustic Soda



Aeration: None. Agitation: None except for filling of tank.

-Material

Nickel 200 I NCONEl alloy 600 MONEL alloy 400 Zirconium Titanium Mild Steel


0.3 0.4 0.7 L4 4.7

Destroyed during test

30

D. Soap Manufacture Soaps are made by the reaction, called "saponification,"between alkali and fatty oils (gly_ cerides) and fatty acids of animal or vegetable origin, or a mixture of both. The largest production, and the most familiar, is "hard" soap made with caustic soda as the saponifier. Caustic potash produces a "soft" or liquid soap.

In certain high grade soaps, it is necessary to avoid contamination by such metals as iron and copper in order to obtain a high quality product. Therefore, pure caustic must be used in combination with corrosion-resistant equipment. The matter of iron contamination is particularly significant in soap-boiling kettles because the soap spends so much time there. particularly in the fuHboiled process. This is especially significant in the upper parts of the kettles where corrosion rates are highest. Table XLVI shows the results of one plant corrosion test in a soap-boiling kettle. Some of the earliest applications of ~orrosion-resistant materials were in the construction of soap kettles.

Table XLVI

Plant Corrosion Test in Soap·Boiling Kettle

Specimens immersed near the top of the settling cone duro ing saponification and graining.



Material

Hit;ke1200 MONEL alloy 400 .NCOffEl. alley 600 Hi·Resist Type 1 Mild Steel Cast Iron


<0.1 <0.1 <0.1

0.1 3.2

11.0

The first step. in most cases, was to line the upper portions of existing steel kettles with Nickel 200. AHoy 400 or either Type 304 or Type 316 stainless steel. Because of occasional difficulties win these lined vessels (usually weld cracks in thE liner because of differential thermal expansior between the steel kettle and the liner) , new vessel: were sometimes constructed completely from cla( plate. The same materials are also used for heat ing coils. swing pipe, kettle covers and othe accessory equipment.

Much of the corrosion test work in soap plants has been concerned with the treatment of spent soap lye and recovery of glycerine. since these processes represent particularly corrosive conditions. The pH of the solution during acid treatment is usually 4 to 4.5 and sometimes as low as 3, due to the presence of hydrochloric or sulfuric acids. Agitation of the mixture with air, a common practice, tends to increase the corrosion rate of steel. The results of six tests during acid treat-

ment in four different soap plants are shown in Table XLVII.

Alloy 400 and Nickel 200 or steel clad with these materials are used for both acid-treating and caustic-treating tanks because of their resistance in both environments. Austenitic chromiumnickel stainless steel and Alloy 600 are also used but pitting is possible with these alloys under certain conditions as shown in Table XLVII. NiResist Type 3 is used instead of Types 1 or 2 for

Table XLVII

Plant Corrosion Tests in Acid Treatment of Spent Soap lye

Test 1: Immersed in acid treating tank in mixture of 13% NaCI and 4.5% glycerine to which is added 150 Ib of 28% HCI and 75 Ib of 17% aluminum sulfate per 30,000 Ib soap lye. Temperature: 1 to 82 C (30 to 180 F). Test Period: 167 days. Plant L

Test 2: Immersed half·way down in acid treating tank in mixture of 18% NaCI plus glyce.rine to which is added 0.5% solution of aluminum chloride. Aerated. Average tempera· ture: 74 C (160 F). Test Period: 65 days. Plant 2.

Test 3: Immersed in trough of filter in acid treated filtrate from Test 2. Aerated. Average temperature: 71 C (160 F}. Test Period: 65 days. Plant 2.

Test 4: Immersed half·way down in acid·treating tank in mixture of 8 to 10% NaCI and 4.5% glycerine made acid to pH 4.5 with HCI and ferric chloride. Air agitation. Temperature: 21 to 71 C (70 to 160 F). Test period: 28 days. Plant 3.

Test 5: In acid treating tank in spent soap lye made acid to pH 4.5 with HCI and ferric chlo· ride, and aluminum sulfate. Agitated wiUl1iIir. Temperature: 54 to 79 C (130 to 114 F). Test Period: 45 days. Plant 3. a. Immersed in bottom of tank near air inlet. b. In vapor in top of tank. .

Test 6: Immersed half·way down in solution containing 13 to .16% NaCI plus Na2S0. and 10 to 12% glycerine made acid to pH 4.5 with sulfuric acid and ferric chloride. Aerated. Temperature: 32 to 100 C (90 to 212 fl. Average 85 C (180 Fl. Test Period: 105 days. P\ant4.

C.onosion llate.lIllJs per year

Test 1 Test 2 Test 3 . TesH Test Sa Test Sh TestS Material Plant 1 Plant 2 Plant! Plant 3 Plal1t3 Plant 3 fitaltt4

MOHEL alloy 400 .9 0.3 4.8 2.9 5.6 4.4 16.0 Hickel 200 1.1 0.9 3.7 L8 5.1 4.7 10.0

IHCOHEL aUoy SOO .6 <.1 .7 0.8 .7 Type 302 stainless steel <.1 .7 (a) O.5!d

Type 304 stainless steel .5!d LOlh) Type 316 stainless steel <.1 A (b) LO(i)

Copper·Nickel alloy CA 715 27.0 Aluminum. Type 1100 4.4 (d)

Hi·Resist Type 3 3.0 Hi· Resist Type 2 2.7 1.0 4.0

Hi·Resist Type 1 2.5 0.9 3.4 5.0 Mild Steel 5.3 3.0 17.0 16.0 29.0 (e) 34.0(gl 14.0

Wrought Iron 18.0 24.0 (f) 44.01gl 14.0 Cast Iron 11.0 6.0 16.0 14.0

(a) Perlorated by pitting. original thickness 31 mils. (f) Pitted to maximum depth of 9 milS. (bl Pitted to maximum depth of 11 mils. (gl Pitted to maximum depth of 8 mils. (e) Pitted to maximum depth of 5 mils. (h) Pitted to maximum depth of 6 mils. (d) Pitted to maximum depth of 3 milS. (i) Pitted to maximum depth of 14 mils. (e) Pitted to maximum depth of 20 mils.

31

the construction of soap lye filters and filter plates to withstand possible therma·' shock when the hot solutions enter a cold filter process. Pumps of iron-base nickel-chromium-copper-molybdenum alloys such as WORTHITE or DURIMET 20 have given good performance handling both acid- and alkali-treated soap lye. Austenitic chromiumnickel stainless steels, usually Type 304, have been used to advantage for "finishing and packaging" equipment.

Although a considerable amount of the world's soap is still produced batch-wise, efforts to reduce the 4 to 11 days required with the fun-boiled kettle method have resulted in a number of continuous processes for soap manufacture. In one such process, blended fats with zinc oxide catalyst are reacted countercurrently with water in a 65-foothigh. Type 316 stainless steel hydrolyzing tower maintained at 282 to 260 C (450 to 5(,)0 F) and 600-700 psi. Fatty acids are continuously drawn off the top and crude glycerol off the bottom of the column. The fatty acids arevacuum-distiUed and then neutraiized in a high-speed mixer· with a caustic soda solution containing salt. thus producing soap in about four hours.24

In other continuous processes which usually utilize centrifuges. the corrosives encountered are much the same and considerable quantities of austenitic chromium-nickel stainless steel. Alloy 400 and Nickel 200 are utilized. Tables XLVIII and XLIX indicate the corrosion rates eneoun-

Table XLVIII

Plant Corrosion Testinfourth·Stage lye Tank Immersed in tank containing soap lye with 2% MaOH and 11% MaC!..

Temperature: 88 to 96C (190 to 205 F).


Corrosion Rate, Material mils per year

Nickel zno Nil o

MONEL alloy 400 Nil INcolfEl aUoy 600 Nit Type 316 Stainless Steel Nil Type 341 Stainless Steel Nil Type 304 Stainless Steel 0.1 Hi·Resist Type 1 0.1 Mild Steel 1.0 Cast Iron 3.0

• Less than 0.05 mils pel' year.

Maximum Depth of Pitting, mils

None None None None None None None

7 22

tered in a lye tank and a centrifuge in one of these processes.

Table XLIX

Plant Corrosion Test in Third-Stage Centrifuge

Specimens located at soap discharge. Mixture contained 15% NaOH and 11 % NaC!. Low aeration, flow 350 gallons per hour through 4·inch pipe. .



32

C orrusion Rate, Maximum Depth Material mils pef year of Pitting, mils

Nickel ZOO Nil o None MONEL alloy 400 Nil None I/'ICONE.L alloy 600 Nil None Type 304 Stainless Steel 0.1 None Type 316 Stainless Steel 0.1 None Type 341 Stainless Steel 0.1 None Hi-Resist Type 1 0.4 None Mild Steel 10 Perforated Cast Iron 12 55

• Less than 0.05 mils per year.

E. Pulp and Paper Industry Over a million tons of caustic soda are used annually in the pulp and paper industry, principally for the extraction of alkali-soluble impurities in multistage bleaching and for pH control. Small amounts are used for preimpregnation of wood chips and for the production of soda pulp.

More than two-thirds of aU paper is pX'Qduced by the Kraft process. Digestion of certain soluble portions of wood chips is accomplished by a hot alkaline liquor consisting of a mixture of dilute caustic soda and sodium sulfide with a total alkalinity of about 3 per cent. The following are principal areas where carbon steel may corrode at an excessive rate and nickel-containing alloys (usually austenitic chromium-nickel stainless steels) can be used to advantage.

1. Digesters Batch-type Kraft digesters are commonly built of carbon steel with a corrosion allowance in excess of one inch. Until recent years this resulted in a service life of about 15 years, but with the increasingly severe conditions imposed. by modern pulping methods, service life was reduced to about 7 to 9 years. Weld overlays employing A WS E310,

or A WS E310-Mo. have been employed to extend the service life of corroded steel batch digesters. Table L indicates the excellent corrosion resistance of stainless steel and several other nickel alloys in one Kraft digester.

Table l

Plant Corrosion Test in a Sulfate Process, Alkaline, Wood Pulp Digester

Temperature: 177 C (350 f).


Aeration: None.

Agitation: Violent boiling during cook. Top--Vapors in the top of the digester. Occa· sional splashing of chips, pulp and cooking liquors. Bottom-Liquid and slurry on bottom screen of digester.

Specime(ls: Combination of stress and general COrrosion. Strips were stressed beyond the yield point by bolting down over a fulcrum. Some specimens were welded or contained weld overlays as noted. No stress· corrosion cracking occurred.

Material Condition

INCONEL alloy 600 Plate. as·received CARPENT£R alloy 20 Welded INCoNEL alloy 600 Welded INCOlOY alloy 825 Plat.e. as·received INCOlOY alloy 825 Welded Type 316 Stainless Steel Plate, as·received Type 316t. StainJess steel Plate. as·received Type 31Sl Stainless Steef Welded Type 316 Stainless Steel Welded AViS Eltflllveriay on Steel Weld Overlay

Note: A dash indicates no coupon was exposed.


Top Bottom

0.02 0.21 0.03 0.09 0.03 0.23 0.03 0.09 0.03 0.04 0.15 0.05 0.06 0.17 0.06 0.15 0.05 0.17

There are a few Kraft digesters that utilize a duplex process in which the charge is initially acid (pH 4) and later alkaline. Table LI shows corrosion rates in this process.

There are several hundred continuous digesters operating on wood chips in the United States. These are constructed primarily from carbon steel with high corrosion rate areas lined or clad with Type 316L stainless steeL These high corrosion rate areas include the upper section. where fresh, hot alkaline liquor is injected, and the bottom section in the area of the blow valve. Internal accessories such as scrapers and chip screens are usually fabricated from Type 316L stainless steeL Construction of digesters with clad Type 316L stainless steel would allow for con-

Table LI

Plant Corrosion Test in a Digester Utilizing a Duplex Sulfate Process

Temperature: 100 to 171 C (212 to 340 f).

Cycle: Chips steamed for one hour, temperature rises from 100 C to 118 C (212 to 244 f). Acid liquor removed. Alkaline liquor containing 82 gpl NaOH and 25 gpl Na,S added. Charge brought to 171 C (340 f). cooked for total of 5 hours.

location: In vapor. Test Period: 731 days.

33

Material

Type. 316 Stainless Steel Type 341 Stainless Steel CARPENTER alloy 20 INCj)NELalloy Spo MONEL alloy 400 Titanium Mild Steel


0.1 0.1 0.1 OJ

23 55

107

Fig. 20 ~ This top separator on a KAMYR continuous di· gester separates the chips from the flushing liquor. With the exception of the drive mechanism. this separator is can· structed of Type 304 stainless steel.

siderably reduced wall thickness and much lower maintenance costs.

2. liquor Heaters Shell and tube heat exchangers are used to heat the digester liquor prior to its introduction into

both batch and continuous digesters., Results of a corrosion test in such a heater are shown in Table LII. These data may indicate a lower than actual corrosion rate for carbon steel, since the heat exchanger tube walls are at a temperature higher than the liquor in which the test specimens were exposed. The liquor contains a large proportion of fresh caustic and sulfide in addition to some black liquor recovered from a previous digester cook. Experience over many years has proven the adequacy of annealed Type 304 stainless steel for this service. The use of "as-welded" tubes has sometimes resulted in failure by intergranular corrosion immediately adjacent to the weld. This type of corrosion has not been observed when tubes are used that have been made in compliance with ASTM A 249. This specification caUs for welded, drawn, quench-annealed tubing. Tubes of this type have been known to last in excess of.10 years, but service life is dependent on specific operating conditions. In a few instances, the Type 304 stainless steel tubes have been subject to failure by chloride stress-corrosion crackng. Alloy 600 and Alloy 20 have been successfully employed to resist this type of attack.

3. Black liquor Evaporators To permit recovery of chemical values in the digester liquor when chip cooking is complete, it is necessary to concentrate the liquor, together with the chip wash water. This is required to raise the solids content to more than 50 per cent. which will permit burning in the recovery furnace.

Kraft liquor vacuum evaporators are multiple units usually consisting of one or more sets of six long tube vertical effects connected in series. Corrosive conditions on the tubes are somewhat less severe than in digester liquor heaters since the vacuum operation results in lower boiling temperatures. The first effect operates at the highest temperature of about 135 C (275 F). Temperatures decrease in each succeeding effect. It has been customary to use Type 304 stainless steel for tubes in the first effect and often in the second effect. A number of installations have used Type 304 stainless steel tubes in all effects, resulting in less frequent downtime fer cleaning, long service Hfe, and maintenance of high heat transfer rates.

34

Table lit

Corrosion Test in Kraft Pulping Exposed 68 Days in Head of liquor Heater

Flow rate of 2400 gpm at temperature of 173 C (344 F).

Material

Type 304 Stainless Steel Type 316 Stainless Steel INCONEL alloy 600 MONEL alloy 400 Nickel 200 Mild Steel Cast Iron


0.5 0.8 0.9

38 57 95

342

Vapor domes in the hottest effects are often clad with Type 804Lor Type 316L stainless steel, since carbon steel in this area may corrode at a rate exceeding 100 mils per year. There are also installations where sta.inless-elad steel has been used for the entire evaporator body. Advantages of such construction.inelude less carry..over of corrosion products and less fouling of the evaporator tubes by these products.

Defiectorpla.tes and auxiliary piping are usuany made of solid Type 804L stainless steeL For valves and pumps, Ni-Resist Type 2, CF-8 and CF -8M castings are used.

4. Recausticizing As part of the operation to regenerate chemicals reclaimed from the recovery furnace, sodium carbonate is treated calcium hydroxide (milk of lime) to produce sodium hydroxide.

Table lin Corrosion Test in Kraft Pulping

Exposed 68 days in green Uquor. 175 to ~5 gpl as Na2 CO, in flow bQx.from recovery furnace to claSSIfiers.

Temperature: 66 to 99 C.(150to 210 F).

Some aeration and agitation.

Material

tN(:ONEL alloy 600 Type 302 Stain1ess Steel Type 309 Stainless Steel Type 310 Stainless Steel Type 316 Stainless Steet Nickel 200 MONEL alloy 400 Mild Steel Cast Iron


<0.1 0.1 0.1 0.1 0.2 0.3 0.5

115 176

Carbon steel, with a corrosion allowance. has been used for most of the equipment. As shown in Table LIII, fairly high rates can occur on carbon steeL Light gauge Type 304 stainless steel is an economic selection for troublesome areas.

F. Aluminum Indu~try Despite extensive use of caustic soda by the aluminum illdustry for the extractioRofhydrated alumina from bauxite in the Bayer process, rela-

Fig. 21 - This ll·foot long. 16·inch diameter pipe has been electmplated with nickel to yield a 30-mil thick deposit on the inner diameter and about 2 mils on the outer diameter. Sections like this are welded toget:he( to form piping used in bauxite refining in the aluminum industry. Lengths of greater than 11 feet can also be plated.

Photo by courtesy of Plating Engineering Company. Milwaukee. Wise.

35

tively smallam{llwts of nickel and nickel altoys have been utiJized in these plants.

Alloy 400 tubes have been successfully employed for digester preheaters. and Alloy 400 insert ferrules have been used to overcome the inlet end corrosion in other steel preheater tubes. Relatively thick (30 mils minimum) electroplated nickel {lll steel has been used to advantage for piping and digester preheater channels. Nickel weld-overlays haVe proven \Iseful on pump casings, and cast nickel (ACI CZ-IOO) has given good service as pump impellers. valve bodies and for other instrumentation.

However. the present practice with bauxite digesters is to use thick-walled carbon steel at low stress levels. Some cases of stress-corrosion cracking (If steel have occurred in plants handling caustic soda solutions in the Bayer extraction process.z::>

The recent trend awaYI;r;om ores high in gibbsite C{lntent toward the use of Ores relatively high in boehmite c()ntent has necessitated digester operation at higher pressures and temperatures. This increases the pos!;;ibHity of caustic embrittlement of steeL Thus. nickel or nickel-clad steel should be given consideration for the processing of these higher boehmite bauxites.

G. Caustic FuSions Nickel 200 and Nickel 201 are useful as materials of construction for vessels for the caustic fusi{ln of·· organic comp{lunds. Where temperatures exceed 316 C (600 F). the low.-carb{ln Nickel 201 is preferred to preclude grain boundary precipitation of carbon which greatly reduces duetility. For those reactions where sulfur compounds are present at temperatures over 250 to 300 C (482 to 572 F). either in the process or the heating medium, nickel may be attacked intergranularly and Alloy 600 is preferred.

One process for the production of resorcinol has involved the caustic fusion of benzene meta disulfonic acid at 325 C (617 F). Equipment for this production has been made of wrought Alloy 600 and ACI CY -40 castings. Both alloys should be stress-relieved as indicated in the section on nickel-chromium alloys (Part II B).

H. Petroleum Refining Cau~tic soda or, occasionally, caustic potash or sodium carbonate is used in petroleum refining for acid neutralization and the removal of undesirables such as mercaptans and hydrogen sulfide. Aqueous solutions may range from 2 to 50 per cent.

For many of the applications where temperature and concentration are low, the corrosive conditions are mild enough that steel can be used. Where the corrosive conditions are more aggressive, Nickel 200, AHoy 400 or Alloy 600 are used. Very often Alloy 400 is used because it appears to have a greater tolerance for the impurities present in the process.

Fig. 22 - Nickel·copper alloy 400 was used for the walls of the caustic stripper towers. reboiler tube bundles and hot caustic lines in this refinery. After 10 years of service. the Alloy 400 components continue to withstand the corrosive mineral acids. sulfur compounds and hot caustic soda in the fluid hydroformer and caustic regenerating equipment.

In the t'egeneration of caustic solutions, it is common practice to use Alloy 400 in critical portions of the system where steel is unsuitable. These locations include the regenerator reboiler, preheaters and piping for handling hot caustic solutions and sometimes for the bottom sections of the regenerator towers. These components may be either solid or clad. ACI CZ-IOO, ACI M-35 ductile Ni-Resists and WORTHITE stainless steei have been used for valves and pumps. The results of plant corrosion tests in the reboilers of caustic regenerator units are shown in Table LIV.

36

Table ltV

Plant Corrosion Tests in Caustic Regeneration Units

Test A-In open tank used to boil 18 to 22% caustic soda plus merca pta ns and cresolates for regeneration of caustic solution. Test specimens were immersed in solution above heating coils.


Temperature 38 to 104 C (100 to 220 F).

Average 80 C (175 F).

Test a-Just ab~ve reboiler inlet below bottom tray of reo generating tower. Solution 18% caustic soda for· tified with naphthenic acid. cresols and phenols to 22 to 28 ·Be. Solution also contained 0.040/0 mer· captan sulfur.


Temperature: 21 to 116 C (70 to 240 f).

Average 107 C (225 F).

Test C-At bottom of stripping tower 18 inches above reo boiler tubes. Solution 7 % caustic soda with trace of mercaptans.



Average 135 C (275 Fl.

Test o-In vapor sectj~n of caustic soda regeneration unit. Solution entenng contained' 13.2% caustic soda. 0.37% sulfide sulfur and 0.80% mercaptide sulfur.




Material TestA TestS Test C Test D

INCONEl alloy 600· <0.1 <0.1 <0.1 0.3 Type 304 Stainless Steel" • 0.1 0.1 0.1 Nickel 200 0.1 0.2 1.1 2.0 MONEL alloy 400 0.3 0.1 0.9 2.0 Type 316 Stainless Steel"· 0.4 0.2 Copper·Nickel alloy CA 715 1.1 4.5 Hi·Resist Type 1 3.8 4.0 13.0 Cast Iron 13.0 10.0 Carbon Steel·· 29 12.0 33.0

.., Subject to pitting. "') May be 'Subject to stress-corrosion cracking_

In view of its good resistance to caustic alkalies containing hydrogen sulfide and mercaptans, Alloy 600 is also a useful material for evaporator tubes or other parts of regenerator systems. Alloy 600, rather than Nickel 200 or AHoy 400, should be used in this service where metal temperatures in excess of about 250 to 300 C (482 to 572 F) are encountered, since Nickel 200 and Alloy 400 are subject to sulfidation at higher temperatures.

A caustic stripper, at a major Louisiana refinery, constructed of MONEL alloy 400, exhibited no detectable metal loss after more than 4 l'2 years' service handling up to 45<; caustic soda at temperatures up to 143 to 149 C (290 to 300 F). It is still giving repair-free service after 15 years.

In one Texas refinery, mercaptains are removed by the Dualayer Process * which utilizes two layers of immiscible solvents. The first solvent layer, a water solution of caustic· potash and potassium cresylate, removes the mercaptans. The second and lower layer is a water solution of caustic potash that maintains the composition of the upper layer. Water and potassium hydroxide migrate between lower and upper levels, sustaining the equilibrium. MONEL alloy 400 was used for the stripper preheater, reboiler. and stripping tower trays and. in a cast form (ACI M-35), for the bottom pump. The tower itself was lined with MONEL aHoy 400 and stress-relieved. This equipment continues in operation after 20 years.

Nickel-copper alloy 505 has excellent nongalling properties and can be combined with cast nickel-copper alloy ACI M-35 in pump assemblies to avoid seizing. particularly in mixtures containing gasoline or similar solvent materials where lubrication is practically impossible.

L Caustic Oescaling Several processes involving molten caustic soda are in commercial use for the descaling of various metals and-alloys, particularly the stainless steels. Some of these processes involve addition to the caustic of reducing agents to reduce the metallic oxides to metal or lower metal oxides, most of which flake off in the subsequent water quench.

• Patented Process. Mohil Oil Corp.

37

I n one process, the parts to be descaled are immersed in a 370 C (700 F) bath of molten caustic soda containing 1.5-2% sodium hydride. Other processes operate with molten caustic at 480 C (900 F) or higher.

Carbon steels are often used for the equipment handling these fused caustic baths up to about 480 C (900 F). In cases where carbon steel has not proven satisfactory. Nickel 201 and Alloy 600 have been demonstrated to give good performance. Nickel 201 is used for sodium hydride generators in one process. Both Nickel 201 and Alloy 600 are used for sheathing on electric heating elements in caustic baths. AHoy 600 is used for gas-fired heater tubes in some cases. In cases where the caustic baths are operated at higher temperatures than usual, such as 566 to 621 C (1050 to 1150 F), Nickel 201 is used instead of carbon steel for pickling tanks and associated equipment. A plant corrosion test in a commercial molten caustic pickling bath operating at 482 C (900 F) showed a corrosion rate of one mil per year for nickel in a 60-day test.

J. Reclaiming Caustic for Economy and Pollution Control

In the diverse industries which make use of caustic solutions, numerous companies have found that it is economically attractive to reclaim and concentrate the caustic values of their effiu-ents. Even the return on investment for caustic recovery units is not high enough for justification on this basis alone. pollution control is desirable and may become mandatory as local anti-pollution laws become more stringent.

Recovery and concentration plants are commercially available for some industries. Of the total caustic soda purchased yeariy by a textile mill for mercerizing cotton. often as much as 65 per cent can be recovered from the mercerizingframes and up to 95 per cent at the mercerizer. The concentration of caustic soda is from about 5 per cent in contaminated liquor to the de~;jred concentration for the particular mercerizing operation. These plants utilize nickel or high nickel alloys for evaporators and associated equipment .

PART V. WELDING

A. Fabrication of Nickel-Clad Equipment

In the welding of nickel-clad equipment, a certain amount of iron dilution of the nickel weld deposit occurs. Special precautions are usually taken in order to minimize this dilution. With special precautions, the nick~l welds in a nickelclad tank for a chemical tanker were limited to an iron content of 0.35-3.29%.26 Gegner has suggested that considerably more iron than this can be tolerated.27

Although iron-contaminated nickel weld metal and nickel-iron alloys are not severely attacked in 73% caustic soda at 121 C (250 F). as shown by data in Tables LV and LVI, nickel-iron alloys can be the anode in an electrolytic cell with nickel, as shown in Table LVII. Note that the 20% iron alloy corroded at three to five times the rate it did when it was not coupled to nickel. Even greater . increases in rate would be expected in large pieces of equipment where the relative ratio of cathodic area (cladding) to anodic areas (weld) is greater than the 10:1 ratio of the test.

Table LVII

Table LV

Corrosion of Iron·Contaminated Nickel Welds in 73% Caustic Soda at 121 C (250 F) 27

Iron in Weld,

%

0.51 5.56

11.43 13.15 17.62 22.85


First Exposure, 30 days

8 8 7 7 7 6

Second Exposure, 60 days

5 5 5 4 5 4

Table lVI

Total Exposure, 90 days

6 6 5 5 5 5

Corrosion of Nickel and Nickel·lronAlloys in 73% Caustic Soda at 121 C {250 f)27

Corrosion Rate, Iron. % mils per year

0 7 0 7 5 8 5 8

10 6 to 8 20 8 20 8

Galvanic Corrosion Tests in Caustic Soda of Nickel to Nickel-Iron Alloy Couples

Area: Nickel Nickel·lron Alloy

Motion: None.

0.5 sq dm O.05sq dm

Aeration: None.



Couple No.

2

3

Notes:

Couple Materials

5% Fe·Ni Nickel

10% fe·Ni Nickel

20% Fe·Ni Nickel

23% NaOH at 105 C (221 Fl Coupled Uncoupled

1.6 1.4 0.6 0.4

2.6 L2 0.4 0.4

3.6 0] 0.6 0.4

1. The iron·nicket alloys were in the form of castings. 2. No tests were made, uncoupled. in the 500/0 NaOH solution.

38

50% HaOH at 15C (161 Fl 15% NaOH at 126 C (259 Fl

Coupled Coupled Uncoupled

2.4 1.0 1.0 0.8 1.6 1.5

2.0 1.5 1.4 0.6 LO 1.5

1.6 1.8 0.6 0.4 1.1 1.5

B. Repair of Equipment in Caustic Service

Before doing any repair or maintenance welding of nickel or nickel-containing alloys or dadsteel plate that has been in caustic service, it is necessary to remove products of corrosion, and any other foreign material, from the vicinity of the area to be welded. (The caustic soda and other impurities present can cause loss of ductility and cracking if present during welding.) Therefore, great care should be taken to obtain a clean, bright metal surface over an area extending 2 to 3 inches from the site of welding on both sides of the piece. Cleaning mechanicalIy, by grinding with either a

fine wheel or a disc grinder, or chemically, by pickling, is recommended. After cleaning, the welding procedures outlined for new metal should be followed in every detaiL

Flash pickling solutions are effective for cleaning nickel and high nickel alloy surfaces. These may be applied with long-handled swabs or brushes where equipment is large, or may be held in glass or ceramic crocks for pieces that are easily handled. such as the ends of nickel caustic evaporator tubes that have been removed from evaporator service and are to be used for pipelines. The tubes can be dipped vertically and cleaned for a minimum distance of 3 inches from the end.

AVAILABLE LITERATURE The following Corrosion Engineering Bulletins are available for your use:

"Resistance of Nickel and High Nickel Alloys to Corrosion by Sulfuric Acid"

"Corrosion Resistance of Nickel and Nickel-Containing Alloys in Caustic Soda and Other Alkalies"

"Resistance of Nickel and High Nickel Alloys to Corrosion by Hydrochloric Acid, Hydrogen Chloride and Chlorine"

"Corrosion Resistance of Nickel-Containing Alloys In Phosphoric Acid"

"Corrosion Resistance of Nickel-Containing Alloys in Hydrofluoric Acid, Hydrogen Fluoride and Fluorine"

39

REFERENCES

1. Swandby, R. K., "Corrosion Charts: Guides to Materials Selection", Chen!. Eng., Vol. 69, No. 11, Nov. 12, 1962, p. 197.

2. Fontana, M. G., "Corrosion at Elevated Temperatures and Pressures", The Ohio State University Research Foundation, Report No. 10, Project 350, May 1, 1951, p.F2.

3. Badger, W. L. and Standiford, F. C., "Anhydrous NaOH: Today's Technology", Chern. Eng., 61, Feb. 1954, pp. 183-187.

4. Gregory, J. N., Hodge, N. and Iredale, J. V. G., "The Static Corrosion of Nickel and Other Materials in Molten Caustic Soda", AERE CIM 272, March, 1958.

5. Gregory, J. N., Hodge, Nand Iredale, J. V. G., "The Corrosion and Erosion of Nickel by Molten Caustic Soda and Sodium Uranate Suspensions Under Dynamic Conditions", AERE CIM 273, March, 1956.

6. Lad, R. A. and Simon, S. L., "A Study of Corrosion and Mass Transfer of Nickel by Molten Sodium Hydroxide", Corrosion, 10, December. 1954, pp. 435-439.

7. Smith; G. P., Sieidlitz, M' E. and Hoffman, E. E., "Corrosion and Metal Transport in Fused Sodium Hydroxide", Co)Tosion. 13, September, October, 1957. pp. 561t-564tand 627t-630t.

8. Forestieri, A. F. and Lad, R. A., "The Use of Metallic Inhibitors for Eliminating Mass Transfer and Corrosion in Nickel and Nickel Alloys by Molten Sodium Hydroxide", Lewis Flight Propulsion Laboratory, Cleveland, Ohio, February, 1955, NACA RM E54L13.

9. May, C. E_, "Correlation Between Hydro~en Pressure and Protective Action of Additives in the Molten Sodium Hydroxide-Nickel System", Lewis Flight Propulsion Laboratory, Cleveland, Ohio, February, 1966. NACA RM E55LOl.

10. Wallace, T. and Fleck, A., "Some Properties of Fused Sodium Hydroxide", Jourl/al Chelll. Suc., 119, 1921, p. 1839.

11. Uhlig, H. H., Ed., Corrosion Handbook. N. Y., John Wiley, and Sons, Inc., 1948, pp. 576-577.

12. Stone, J. M., "Solutions Causin~ Intergranular Corrosion of Stainless Steels", Information from Internal Document by courtesy of E. L du Pont de Nemours and Co., October, 1955.

13. Agrawal, A. K. and Staehle, R. W., "Stress-Corrosion Cracking of Fe-Cr-Ni Alloys in Caustic Environ-

40

ments", Report No. COO-2018-21 (Q6) for period April 15, 1970-July 14, 1970, Ohio State University, Columbus, Ohio.

14. Nathorst, H., "Stress Corrosion Cracking of Stainless Steels-Part I Practical Experiences", Welding Research Council Bulletin, No.6, October, 1950, pp. 6-7 and 10.

15. ASM Committee on Stainless Steel in Chemical Corrosion Service,lHetals Handbook, Am. Soc. Metals, 1961, p.566.

16. Beck, F. H. and Fontana, M. G., "Corrosion by Aqueous Solutions at Elevated Temperatures and Pressures", Corrosiun, Vol. 9, No.8, August, 1953. pp. 287-293.

17. Pratt. W. E., "Corrosion Resistance of Worthite in Caustic Soda", Chemical EnginC(!ring, Vol. 56, No. 12, 1949, pp. 213-214 and VoL 57, No.1, 1950, pp. 213-214.

18. "Hastelloy Corrosion-Resistant Alloys", Union Carbide Corporation, 10th Edition, May, 1957 and private communication from Haynes Stellite Co.

19. Ge~ner, P. J., "Corrosion in Alkaline Environments", ProcecdiJlgs of Shm·t Course 0)/ Pl"(lCeSS Industry Corrosio)t, National Association of Corrosion Engil!eers, September 12-16. 1960, p. 1 L

20. McCallion, J., et at, "Switch to Continuous Evaporation Boosts Capacity But Not Manpower", Chemical P"ocessing, August, 1968. pp. 20-21.

21. U. S. Patent 3,380,806, April 30, 1968.

22. Twiehaus, H. C. and Ehlers, N. J., "Caustic Purification by Liquid-Liquid Extraction", Chemical Industries, August, 1948, pp. 230-233.

23. Friend, '.V. Z. and Mason, J. F., "Corrosion Tests in the Processing of Soap and Fatty Acids", Corrosion, Vol. 5, No. 11, 1949, p. 358.

24. Kirk-Othmer, Ellcyclopl'tiia <If Chcmical Technology, Second Edition, 1969, Vol. 18, p. -123.

2;). Champion, F. A., "Some Aspects of the Stress-Corrosion of Steel in Caustic Soda Solutions", Chemistr1l {fllel Il/dustl'1f, July 13, 1957, pp. 967-975.

26. Phelps, H. C .. "Nickel·Lined Ship for Liquid Chemicals", The Weldil/g Engil/eer, Vol. 39, No.4, 1954, pp. 41-44.

27. Gegner, P. J., "Corrosion in Caustic of Nickel-Iron Welds Obtained in Fabrication of Nickel-Clad Vessels", CUI'rosiol/, Vol. 12, No.6, 1!:5C, pp. 26lt-262t.

TRADEMARKS

Following is a list of the registered trademarks referred to in this publication together with the names of the trademark owners .

• • • • • • •

ALOYCO Registered trademark of Aloyco Inc.

CARPENTER Registered trademark of Carpenter Technology Corporation.

CHLORIMET Registered trademark of The Duriron Company. Inc.

DURANICKEL Registered trademark of The International Nickel Company. Inc.

DURIMET Registered trademark of The Duriron Company. Inc.

HASTELLOY Registered trademark of Cabot Corporation.

ILLIUM Registered trademark of Stainless Foundry & Engineering, Inc.

I NCOLOY Registered trademark of The International Nickel Company. Inc.

INCONEL Registered trademark of The International Nickel Company. Inc.

KAMYR Registered trademark of Kamyr Inc.

MONEL Registered trademark of The International Nickel Company. Inc.

NIMONIC Registered trademark of The International Nickel Company. Inc.

WORTHITE Registered trademark of Worthington Corp.

corrosion resistance of nickel and nickel- containing

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