H2S RESISTANT MATERIALS FOR OIL&GAS PRODUCTION

J. K16wer, Krupp VDM GmbH

P.O. Box 18 20 58778 Werdohl / Germany

H. Schlerkmann, R. P6pperling Mannesmann Forschungsinstitut

Ehinger Str. 200 47259 Duisburg / Germany

ABSTRACT

The corrosion behaviour of alloy 31 (UNS N08031 - 31 Ni - 27Cr - 6.5Mo - 1.2Cu - 0.2 N - bal. Fe) and other corrosion resistant materials (CRAs) was tested in a wide variety of corrosion tests comprising laboratory and field tests in seawater with and without additions of CO2 and/or H2S, slow strain rate tests, and SSC (sulphide stress corrosion)-tests according to NACE MR0175.

Because of its high chromium and molybdenum concentration the critical pitting temperature and crevice corrosion temperature of alloy 31 are higher than that of UNS N08028 (alloy 28) and UNS N08825 (alloy 825). CCT and CPT temperatures are in the same range as those of nickel base alloys like UNS N06625 (alloy 625). It is not sensitive to chloride-induced stress corrosion cracking, sulphide- induced stress corrosion cracking or sulphide stress cracking even at high strength levels. UNS N08031 was tested in the cold-worked condition (HRC < 35) for sour gas applications according to NACE TM 0177-96 A and has been approved in NACE MR0175 up to LEVEL VI.

The combination of properties makes UNS N08031 a suitable alloy for sour service especially under conditions, where high strengths, resistance to seawater and resistance to H2S is required. Typical examples would be casings, wirelines, downhole tubing and topside facilities.

Keywords: Pitting corrosion, crevice corrosion, seawater, sour service, stress corrosion cracking, sulphide stress cracking, alloy 31, UNS N08031.

f l 1 n n ~ / 1

INTRODUCTION

Depletion of shallow water reservoirs in combination with predicted growth of hydrocarbon demand suggest an global increase in deepwater activity, resulting in more corrosive conditions and - as a consequence - in a higher demand for Corrosion Resistant Materials (CRAs) in oil & gas production 1.

Depending on the process conditions, corrosion by CO2 and H2S, chloride-induced localised pitting or crevice corrosion, stress corrosion cracking and/or H2S-induced sulphide stress cracking may be encountered by metallic materials used for wires, casings, tubing and piping in oil and gas field service.

The chemical composition of some Corrosion Resistant Alloys (CRAs) typically used in oil & gas production are listed in Table 1. The corresponding mechanical properties of these alloys are listed in Table 2. Alloy 316L, like the 13Cr steel, shows reasonable resistance to chloride-induced corrosion and COz-corrosion but it is not suitable for application in severe sour gas (H2S) conditions 2. The same applies to the Duplex (2205) and Superduplex (2507) stainless steels, which show limited resistance to sulphide stress cracking (SSC) in the presence of H2S 2. Alloy 825 (UNS N08825) is widely used in oil & gas production systems for a variety of applications because of its excellent resistance to CO2- and HaS- corrosion and its high resistance to stress corrosion cracking. However, it has only limited resistance to chloride-induced crevice and pitting corrosion. Hence if both chlorides and H2S are encountered in oil & gas production, oRen nickel base alloy like alloy 625 must be used.

Alloy 31 (UNS N08031) is a high alloyed stainless steel containing 3 1 % Ni, 27 % Cr and 6.5 % Mo, which has found wide application in chemical and petrochemical process industry due to its high resistance in a variety of corrosive environments 3'4. However, its high resistance to localised corrosion in both acid solutions of chlorides and in seawater suggested that it may be a candidate material for service in the oil and gas industry.

To check the potential of UNS N08031 for oil and gas applications, a wide variety of corrosion tests comprising tests in natural seawater and in various chloride-containing solutions, slow strain rate tests, and SSC-tests according to NACE MR0175 both in the cold-worked and in the solution-annealed condition have been carried out. This paper summarises the results.

EXPERIMENTAL PROCEDURE

A summary of all test parameters is presented in Table 3. Laboratory tests were carried out to determine the critical temperatures for crevice and pitting corrosion according to ASTM G-48, the pitting corrosion potential and the repassivation potential (Tests 1-3). The determination of the repassivation behaviour was performed on sheet specimens connected to PTFE crevice formers 5'6. Alloy 926 (UNS8926) and alloy 24 were included in these tests as reference materials. To simulate the conditions in chlorinated seawater, aerated seawater was used with the specimens being potentiostatically polarised at 600 mV SCE. The potential was kept constant during the entire test time and the temperature was stepwise increased by 5 °C every two hours, until a distinct increase in current was observed.

However, such basic laboratory tests can only indicate a ranking of materials; they do not allow us to predict material behaviour in field conditions. For this reason laboratory tests were paralleled by field tests in the Baltic Sea, the North Sea and the Arabian Gulf with and without additions of chlorides, CO2 and H2S (Tests 4-10). Test details are listed in Table 3, too.

Crevice corrosion tests on samples connected with PTFE-formers were carried out in German North sea water. The test arrangement is shown in Figure 1 and described in detail elsewhere 6. Amongst various test conditions and arrangements those where chlorinated seawater is pumped through pipe spools is considered as most practice-related. Such tests were performed in North sea water (Tests 5 and 10) and in Baltic sea water (Test 6). The test arrangement of the Loop Tests (Test No. 5 and 6 ) is shown in Fig. 2. The seawater was pumped into the test stations, heated up to temperature and passed into a tank where the chlorination was performed by adding hypochlorite solution. Temperature and chlorine concentration were continuously monitored. Corrosion attack was detected by visual inspection. Additional tests were carried out at the R&D center in A1-Jubail in Saudi-Arabia (Test No. 7) at 50°C to determine the Critical Crevice Solution (CCSpH) 7 and at CAPCIS in UK 8.

In many applications in oil & gas production the resistance to Stress Corrosion Cracking (SCC) and Sulphide Stress Cracking (SSC) is the biggest concern 9. To check its resistance to SCC and SSC, several tests were performed. The resistance to chloride-induced stress corrosion cracking of UNS N08031 was tested on U-bend specimens according to ASTM G 30 for a testing time of 2000 hours.

The stress corrosion cracking behaviour of UNS N08031 in H2S-containing environment was tested in slow strain rate tests at the Marmesmann Research Institute in Germany 1°, The test conditions were chosen according to an EXXON specification. The bar samples were tested in a cold-worked condition (34 % cold word, Rp0.2 = 982 Nmm 2 corresponding to 142 ksi, fracture elongation: 17.5 %). The test temperature was 177 °C (350 °F). The strain rate was adjusted to 4 * 10 -6 s "1 which corresponds to a crosshead displacement rate of 1.02 * 104 mm s "1. The gas atmosphere was 7 bar N2 at room temperature (inert gas) and 7 bar H2S (corrosive gas) respectively. The corrosive test solution was 25 % NaC1 + 0.5 % acetic acid. at room temperature.

The resistance of UNS N08031 to sulphide stress cracking (SCC) was tested according to NACE TM 0177-96A. SSC - tests were performed at Level II up to Level VI at Corrosion Laboratory, Houston. The applied stresses and test conditions are listed in Table 10.

RESULTS AND DISCUSSION

Results of Laboratory tests

Crevice corrosion is a typical failure mechanism in seawater piping systems 6. The critical temperatures for crevice corrosion of various CRAs are listed in Table 4. As expected, the resistance to crevice corrosion increases with increasing concentrations of chromium and molybdenum and, consequently, with increasing Pitting Resistant Equivalent (PRE-Number). It should be noted that some materials (especially the Superduplex Stainless Steel) show a large gap between the resistance to pitting corrosion and the resistance to crevice corrosion. UNS N08031, on the other hand, has a pitting corrosion temperature 85 °C and a crevice corrosion temperature of 65 °C, both values being in the range of nickel base alloys. The high resistance of UNS N08031 to crevice corrosion is especially advantageous for piping systems with crevices in sealing areas of threads, flanges and other connections.

The results obtained in the field test in German North Sea water on crevice corrosion formers (Test 4) are summarised in Table 4. It can be seen that none of the alloys tested showed any evidence of

localised corrosion in the as-supplied condition. However some welded samples of alloy 317, 904L and 926 showed slight (alloy 926) to severe (alloy 317) crevice corrosion. Stainless steels 317, 904L, 926, 28 and alloy G-3 were attacked by general corrosion, while UNS N08031 and all nickel base alloys investigated did not suffer corrosion under the present conditions.

The results from the Pipe Spool Loop Tests in chlorinated North sea water (Test No. 5) are listed in Table 6 and summarised in Fig. 4. It can be seen that UNS N08031 is resistant up to about 40°C at 1.5 % chloride. A similar ranking was obtained in Baltic seawater.

These results obtained on UNS N08031 to localised chloride-induced corrosion are in agreement with those found at AI-Jubail and at CAPCIS, which also showed high resistance of UNS N08031 in Arabian Gulf water TM and North Sea water g respectively.

The high resistance of UNS N08031 to chloride-induced localised corrosion was expected. It can easily be explained by its high concentrations of chromium and its molybdenum content of 6 %, both resulting in high resistance to pitting and crevice corrosion which is superior to that of stainless steels, UNS N08028 and nickel base UNS N08825 and which is equal to that of the nickel base alloys 625.

High concentrations of chromium, on the other hand, do not necessarily guarantee high resistance to stress corrosion cracking. However, the results of the SCC and SSC tests show that UNS N08031 is insensitive to stress-induced cracking, too.

In the U-Bend tests in 62% CaC12 the time until cracking was found to be more than 2000 hours, while alloy 926 and UNS N08028 did not even achieve 1000 hours (Table 7).

The results of the slow strain rate tests in H2S-containing environments are listed in Table 8. UNS N08031 in the work-hardened condition fulfils completely the Exxon specification, which requires that both the ratio of time-to-failure in corrosive atmosphere to time-to-failure in inert atmosphere and the ratio of reduction-of-area in corrosive atmosphere to reduction-of-area in inert atmosphere are higher than 90 %. In all tests the ratio of "time-to-failure" was about 100 %, the ratio of "reduction-of-area" was between 95 and 100 %, both values indicating that the material was not affected by either chlorides or H2S. No secondary cracks were found in the gauge section in any of the materials tested.

A comparison of Slow Strain Rate Tests of UNS N08031 compared to earlier test results obtained on UNS N08825 and UNS N08028 is listed in Table 9. It can be seen that alloy 31 is more resistant to stress corrosion cracking than UNS N08028, which passed this tests under the same conditions at a temperature of 149 °C (300 °F) but failed at 177 °C. UNS N08825 passed the corrosion test but only in the grade 110 (110-125 ksi) 1°.

The results of the SSC tests according to NACE TM-0177 for approval in NACE MR-0175 are listed in Table 10. The results show that UNS N08031 can be used in sour service up to Level VI representing a HzS partial pressure of 3.5 MPa, a CO2 partial pressure of 2.5 MPa and 20 % sodium chloride up to a temperature of 175 °C. UNS N08031 has been approved by NACE for sour service up to Level V112.

Generally a high nickel concentration is considered to be beneficial in conditions where stress corrosion cracking may o c c u r 9, However, all test results obtained in this study show that the resistance of UNS N08031 to chloride and sulphide induced stress corrosion cracking is superior to that of UNS N08028 and at least comparable to that ofUNS N08825 despite the lower nickel concentration of UNS N08031 compared to UNS N08825. It is likely that the high molybdenum concentration (6.5 % molybdenum of UNS N08031 compared to 3 % molybdenum in UNS N08825) compensates the lower nickel concentration. However, further investigations were necessary to understand the effect of alloying elements on stress corrosion cracking and sulphide stress cracking behaviour of CRAs.

CONCLUSIONS

The results of the crevice corrosion tests and pitting corrosion tests show that UNS N08031 (alloy 31) has a high resistance to chloride-induced localised corrosion. Due to its high chromium and molybdenum concentration its critical pitting temperature and crevice corrosion temperature in chloride- contaminated seawater are significantly higher than that of alloy UNS N08028 (alloy 28) and UNS N08825 (alloy 825). They equal that of typical nickel base alloys like UNS N06625 (alloy 625).

Field tests in natural seawater show that the corrosion resistance of UNS N08031 in chloride- contaminated water is superior to that of UNS N08028, UNS N08926 and alloy 24.

Alloy 31 is not sensitive to chloride-induced stress corrosion cracking, sulphide-induced stress corrosion cracking or sulphide stress cracking even at high strength levels. It was tested in the cold- worked condition (HRC < 35) for sour gas applications according to NACE TM 0177-96 A and has been approved up to LEVEL VI.

The combination of properties makes UNS N08031 a suitable alloy for sour service especially under conditions, where high strengths, resistance to seawater and resistance to H2S is required. Typical examples would be casings, wirelines, downhole tubing and topside facilities.

REFERENCES

1) L. Smith, Stainless Steel World July/August 2000, 34.

2) Bruce D. Craig, NiDI Technical Series No. 10073: "Selection Guidelines for Corrosion Resistant Alloys in the Oil and Gas Industry".

3) U. Heubner, R. Kirchheiner, M. Rockel, Paper No. 321, Corrosion/91, NACE International, Houston, TX, 1995.

4) U. Heubner, M. Rockel, E. Watlis, Werkstoffe und Korrosion, 40 (1989) 418.

5) M.R. Jasner, E. Altpeter, Paper No. 499, Corrosion/93, NACE International, Houston, TX, 1995.

6) M. Jasner, U. Heubner, Paper No. 279, Corrosion/95, NACE International, Houston, TX, 1995.

7) A. Malik, N. Siddiqui and I. Andijani, WRPC World Conference on Desalination and Water Treatment, Vol. 1, Yokohama, Japan.

8) S.A. Ashton, "Rawwater" Materials Testing, CAPCIS Report July 1999.

9) EFC Working Party on Corrosion in the Oil and Gas Production (Ed.): "Corrosion Resistant Alloys for Oil and Gas Production", EFC Publication No. 17, Institute of Materials, London, 1996.

10) H. Schlerkmann, "Slow Strain Rate Tests of Material Nicrofer 3127hMo in H2S-containing Environment", Internal Report No. 52/2000, Mannesmann Forschungsinstitut, Duisburg, 2000.

11) P.O. Gartland and S. Valen, Paper No. 511, Corrosion/91, NACE International, Houston, TX, 1991

12) NACE Standard MR 0175-2000, NACE, Houston, 2000

TABLE 1

N O M I N A L CHEMICAL CO MPO SITION OF SOME HIGH ALLOYED STAINLESS STEELS AND NICKEL-BASE ALLOYS FOR APPLICATION IN OIL & GAS PRODUCTION.

I !?~ii~,~ :~ iiiii~::~:ili:il: ii:::~:::~:~:~:~.i:: ~ i~S~:~ i:~ii ..... : ~ ~:~:~ . . . . . . . . . . . . . !il !~ .... i:: iii~: i !i~: ::iii ~::::: ~ii:~i:'~i~,i.~, ~, i~i~i::iiiiii:~ii~iiiiiii!iii~i:!~i:iiiiii':i:i',ii~iii!ii~i::! i li!i::iiiii~,iiii

li~:::::::::::: ....... : : ~ ............................................................................... ::;ii? ;ii:iiiiiiiii::i:i~i~i!i:iiiili~i~:::-:!:i:ii:i:i:i::iii ;~i!i~i!i:i~!:!:i!!!i!i:~:ii::iiii!~:iii~:i~:iiiii~i: :, ~i,ii:::: ::::iiiiiiiiii :~iii ii:'::i~i::! :::!i!::!:

316L $31600 12 bal. 17 2.5 --- 1 Mn

926 N08926 25 bal. 21 6 0.18 0.9 Cu

24 18 b~. 24 4.3 0.45

2205 $31803 5.5 b~. 22 3 0.16

2507 $32550 7 bal. 25 4 0.3

825

28

31

N08825

N08028

N08031

40 31 23

31 35

31 32

27

27

3.2

3.5

6.5

0.05

0.2

6.2 Mn, 1.6 Cu

0.7 Cu, 0.7 W

2.2 Cu, 0.8 Ti

1.3 Cu

1.3 Cu

625 N06625 62 4 22 9 --- 3 . 4 N b

59 N06059 bal. < 1 23 16

TABLE 2

MIN. M E C H A N I C A L PROPERTIES OF SOME HIGH ALLOYED STAINLESS STEELS AND NICKEL-BASE ALLOYS FOR APPLICATION IN OIL & GAS P R O D U C T I O N

316L

ii:ii i̧liii ¸iiiii!!!iiii i!i i i̧iii!i!iiiiiii!iiiiiill i i i ii!ili!iii!i i,ili iiiiiiiiiiiiiiiii $31600

iiii iii iiiiiiiiiiiiii! i iiiii iiiiiii!i !ii i i! i!!ii!iiiiiii iii iiiiiiiiiii ii!!iiiiiiiiiiiii iii ,

!ii ii!ii!ii!i ii!iii i iiiiiiiii !!iiii!iiiii iiiiii

170 485

2205 $31803 450 640 25

2507 $32550 550 800 20

825 N08825 220 550 30

28 N08028 215 500 35

31 N08031 280 650 40

625 N06625 380 760 35

59 N06059 340 690 40

iiiiiiiiiiiiiii!iii!i!iii~i!i:~iiiiiiiii iii!iii~i!!!!iiiiiiiiii iiii i iiiii i!iiiiiiii iiiiiiiiiiii i i iiH i iiiiJJiii iW iiii ii iiii i iiii

4O

TABLE 3

TESTING OF ALLOY 31 (N08031) FOR APPLICATION IN OIL&GAS PRODUCTION

. . . . . . . . " ' : ' : " : ' "" : : . . . . : : ' " ' : : : ~ : : : : : ~ ' : : : : : ' : ~ ' ~ " : : ; : : : : : : : : : ' : : : : : : : : : : : : : : : : : : : : : : : : : : : ' " " " ::: : : ~ i F I : : ~ : i : : : i i i i ~ i i i ? : - ' " " : ' : i i ~ i i i : : . . . . i ' : ' " : " " ' " " ~ : i i i : : : ~ : : : : ~ i i i i i i i "

1 CCP ASTM-G 48 10 % FeCI3 ~2 i CCT ASTM-G 48 10 % FeCls 3 Repassivation Artificial seawater, Stepwise Until increase

Polarisation: increase: 5°C in current 600 mV

4

9

10

11

Corrosion in Field test, partly welded seawater , and/or with PTFE Corrosion in Field test / pipe spools seawater Corrosion in seawater Corrosion in seawater, CCSpH*

Corrosion in seawater

Corrosion in seawater + COz Corrosion in seawater + CO2 + H2S

SCC

12 SCC

13 SSC

Field test / pipe spools

Lab test: Polarisation - 600 mV - + 100 mV samples with PTFE crevice forms Polarisation: -600 mV - +100 mV

Field test at Capcis, UK (pipe spool)

Field test at CapcisUK

U-Bend ASTM-G 30 Slow strain rate testing (at MFI / D) strain rate: 4 * 10"6g "1 samples work hardened 34 % (Rv = 142 ksi) NACE TM 0177-96 (MR 0175 /Level I-VI) ys = 100 %

German North Sea 50°C and 70°C 1 year water

I I

North Sea water 30-50°C < 85 days + C12

I I

Baltic Sea water 30-50°C 30 days + C12 Natural Arabian 50°C 150 days Gulf water

Natural Arabian Gulf water + NaC1, deaerated Seawater + CO2 pH 4.9 - 5.2 3rn/s, 2.6 ppm O: Seawater + CO2 to give pH 5 + 50 ppm H2S 3 m/s

50°C I until 10 ~tA/cm 2

70-78°C 134 days

69-75°C 134 (CO2) + 120 (+HES)

62 % CaCI2 125°C 2000 h

25 NaC1 + 0.5 % CHsCOOH, 7 bar HzS

20 % NaCI p(H2S): 3.5 MPa p(CO2): 3.5 MPa

177°C

Level VI: 30 d 175°C

* Determination of Critical Crevice Solution pH

TABLE 4

CRITICAL PITTING AND CRITICAL CREVICE TEMPERATURE OF SOME HIGH STAINLESS STEELS AND NICKEL-BASE ALLOYS (TESTED ACCORDING TO ASTM-G 48 FOLLOWING THE MTI PROCEDURE); PRE: PITTING RESISTANT EQUIVALENT, CALCULATED BY THE

EQUATION: Cr + 3.3 Mo + 30N

• :~!~! i i ? ~ i ! ! i ' " : i : : : i i : : ! i i i i : i I : i ~ i ! ! i i ' : :~ i i i " : : : ! ! i l i l i I~ I ! ! ! I ! I [E : I ! i : "?~i ! ! i ! ' i i i i ! : ' ? : : : i i i : !~ [ ! ! ! : : : : • i : : : : ~ : IXXX I I - rr :

2205

i : 1 . . . . . . .

3 0

r

2O 37

2507 60 35 47

825 30 < 5 33

28 50 25 38

31 85 65 54

625 77

> 85 59

57

> 85

51

76

TABLE 5

EVALUATION OF SHEET SPECIMENS MADE FROM UNS N08031 (ALLOY 31) AND REFERENCE MATERIALS; GERMAN NORTH SEA, 70°C, 0.1 M/S, 1 YEAR

as MMA MMA TIG TIG + Met/ I Met/ matedal supplied weld PTFE weld PTFE Her I PTFE

1 2 5 4 5 6 3z7 013 Oil OD Om O~n 0[3 o~n 904L © E 3 0 C 3 0 ~ O C 3 0 E 3 0 D O l 928 b E 3 0 ~ 0[3 Oil O~ 0 [ 3 0 ~ n 28 O E 3 0 D o n OD OD o n O I G-3 0[3 or~ On On O~ Orq O~n 625 On 0[3 0 [30E3 0[3 o n OE3 C-276 O 1 - 1 0 D O D OI -3 O n O D O i-1

59 O E 3 0 E 3 0 D OD o n OE3 On 3z On o n 0 [ 3 0 D o n 0 [ 3 0 E 3

Symbols:

Overall attack

severe severe moderate severe moderate none none none none

> O.l,..< 0.5 > 0.5

TABLE 6

RESULTS OF LOOP TESTS IN CHLORINATED NORTH SEA WATER ON UNS N08031 (ALLOY 31 ) AND REFERENCE MATERIALS

30 TO 85 DAYS, NORWAY

I~l~ ::::~i~ii ...... m~ ' ~ ~ : :? ~ [ ~ ! ! l l ~ l I

926

24

31

30 30 30 35 30

35 40 45 50

0.5 1.5

1.5 1.5 2.0

2.0 2.0 1.0 0.5

0 0 7 7 2

11 12

6 8

85 85 30 35 32

30 30

30

30

0/12 3/12 0/10 1/10 8/10

0/10 1/10 0/10 3/10

TABLE 7

RESULTS OF SCC TESTS ON U-BEND SPECIMENS * 62 % CaC12, 125°C

: ' ! : : i ! ' R E E E I W i : i : : : " 'EE~ : : : i : ? : i . E E E ' i : : : : ' r

.:!:!i!ii: i::iii!!:.:iiiiiiiMatenal,iii::ii:: ~!i:ii. ::::::::::: ii

926 607 + 113

28 591 ± 236

31 > 2000

825 > 2000

* 3 samples, according to ASTM-G 30-72

........ 10

TABLE 8

RESULTS OF THE SLOW STRAIN RATE TESTS IN H:S-CONTAINING ENVIRONMENT

I I ! ::==::i:: ! ::.i:i{iiiiii=i~!ii::iiii::ii!::i::ii:::iiii::=:::i:i::i::!::!::i~!:: == :=ii::i:i:i::::i::iii :::::::::::::::::::::::::::::::::: ii=:!:=ii~==i~=?=i==::iii::i::i::i::i~::i~ii::i'i:~iiii::.i:: ::i::==::=:::==::!~iiiii::i::ii=:iii!~i~iii::i:::::::::.:::/:::i ::::::::::::::::::::::::

i:i:i:,i:i:iiiii i ::i:i ii i: iiiiiii i: i : i i:.i iiii iiiiii i i i i i i=i i = i i:i:iii:i i i ii: iiiii i iiii i i i: i::i i.i i!i: i!iiii =i:i:iiiiiiiii::!iiiii !ili i::i::i:i:/,!i!i #i ,i ',:,iiii!iiiii ,iii:ii{ii',ilili',ili',} Nii N i} i',iiiiiiiii:iiiii'i' i',Ni: i' iliJ,',ii: il ii ,iiiiii!ii 6 iii ';i',iii

Nicrofer 3127 hMo

Heat 55973

(cold worked round bar)

1 N 1 A 28.0 8.13 64

1 S 1 B 28.5 8.19 100.7 62 96.9

1 S 2 B 28.3 8.30 102,1 64 100

1 S 3 B 28.0 8.28 101.8 61 95.3

Mechanical properties: 34 % cold work, Rm: 1096 MPa (159 ksi), Rp0,2:982 MPa (142 ksi), As: 17.5, %, Z: 63 %

Environment: A) deionised water, 7 bar N2 at room temperature B) 25 % NaCI, 0.5 % acetic acid, 7 bar H2S at room temperature

Test temperature: 177°C (350°F)

Ratio = Ratio of test time (and reduction of area) in inert environment "A" compared to test time (and reduction of area, respectively) in corrosive environment "B"

TABLE 9

RESULTS OF SLOW STRAIN RATE TESTS ACCORDING TO EXXON SPECIFICATION

825 110 - 125 177 passed

28 130 177 failed

31 142 177 passed

Strain rate: 4 . 10 "6 s "1

25 % NaCI, 0.5 % acetic acid, 7 bar H2S at room temperature

Passed: ratio of failure time in corrosive environment and failure time in inert environment > 90 %

T A B L E 10 R E S U L T S OF SSC T E S T S O N U N S N08031 ( A L L O Y 31) A C C O R D I N G TO N A C E T M 0177-96 A

( M A T E R I A L : ~ 30 % C O L D - W O R K E D , H A R D N E S S : H R C 35, Y I E L D S T R E N G T H : ~ 960 MPa, T E S T I N G TIME: 720 H O U R S ) . A V E R A G E OF 6 S A M P L E S

i .= :=~i [ . :: i m ~ s t ii :~i!ii:=! =iiii=:==== = iiiii::: i=~!:~== ~:=i=~=~===:=~,~==:~==: =:~====~==~ ,=:====~==~== ~.=== :======~==pH==~== ::v: :::::::::::::: :: ::: : :::::=..=C /: =:= i:i -~=~ ii. i~ill !/: ~ii~,:=i:~ !:=!!ii=,,ii~,~il; :: ,,.i~i ,,,,==~===)=i:= =:=!!=/ :~i i ~==:=: ~=:=,==, ..... ~:.i: ,,,,i,:,, , !~i :::::::::::::::::::::::::: iii=.===i~ i::iii.lii:iiiiiii~=~i=i~ii:ii:i,i,,~,i=,!!,i,~/~j~ili,~/:iiii~ii

H2S-saturated solut ion with II 138 5 wt. % NaCI + 0.5 "art. % 2.78 2.8 25 o.k.

glacial acetic HzS-satura ted solut ion wi th

III 138 5 wt. % NaC1 + 0.5 wt. % 2.78 3.7 25 o.k.

glacial acetic 15 % NaCI

V 137 p(H2S) = 0.7 M P a 6.87 3.9 150 o.k.

p(CO2) = 1.4 M P a

20 % NaC1 VI 122 p(H2S) = 3.5 M P a 5.85 4.0 175 o.k.

p(CO2) = 2.5 MPa

r -

1 as

Supplied !

2 N.M.A, weld

I

TIG PTFE weld

I

metal/metal PTFE metal/PTFE

7 U bend

8 U bend + weld

FIGURE 1 Arrangement of sheet series, long-time tests German North Sea, 70°C, 0.1 m/s

pipe no: 1 2 :5 4

t 5 Ag/Ag CI ref. elec.

inlet

pipe & flange grade: N08926 and N08031 I filler metal: alloy 625 / 59

gaskets: 2"x 150 + CNA ONNIA 1.5 mm (Aramide fibre)

FIGURE 2:Loop test arrangement for testing piping and components made from UNS N08031, UNS N08926 and alloy 24 in chlorinated seawater

1400

E t ~

c -

O e ~

t -

O 0 m t ~ o L_ O t O

t -

e-,

1200

1000

800

600

~nwiuwm~mmm@@@@e ~

~%~-o

\ I II i

\ " %

• " ~ " " " 9 2 6 ~:~ - '-¢--,, 31 " . o - - e - - - 28

400 20 5'0 ,10 i0 60 70 8'0 9'0

temperature, *C 100

FIGURE 3:Pitting corrosion potential of alloys 31,926, 28 and 24 as a function of temperature, ASTM seawater, areated, stirred

~... Based on 30 - 85 days 50 "- . , . . , loop tests

/ ~ "" - -~ .~ in the North Sea, | ~ ' ~ ~,~,,~Norway

' ° --,

i Alloy 926 ~ / / / J u

~o_ A,,0~24 ~A 1

0 I I I ~

0 0.5 1 1.5 2 ppm CI2

FIGURE 4: Guidelines for selection of flange material made from UNS N08031, UNS N 08926 and alloy 24 in chlorinated seawater.

. . . . . . . . . . . . . . . . . . . . . . 14

1200

~lO00- z 800 ._

~:n 600

400

~" 200

~205 6~

D316 ~8 |

)31

C-276/ 159

0 o :~o ~o do 8o loo

CCT in *C

FIGURE 5: Yield strength of nickel-base alloys and stainless steels as a function of crevice corrosion temperature. For UNS N08825 and UNS N08031 maximum yield strength with respect to SCC is given.