effect of nitrogen on crevice corrosion and repassivation

Effect of Nitrogen on Crevice Corrosion and Repassivation

Behavior of Austenitic Stainless Steel*

Haruo Baba and Yasuyuki Katada

National Institute for Materials Science, Tsukuba 305-0047, Japan

Austenitic stainless steels were produced based on a Fe-23mass%Cr-4mass%Ni alloy with varying nitrogen (0.7–1mass%) andmolybdenum contents (0–1mass%), through electro-slag remelting (ESR) under high nitrogen gas pressure. The effects of nitrogen on crevicecorrosion behavior in an acidic chloride solution were investigated, and the passive film of the crevice corrosion area after corrosion tests wasanalyzed using X-ray photoelectron spectroscopy (XPS). At the same time, the effects of nitrogen on the passivation behaviors after scratchingwere also investigated. During crevice corrosion at a noble potential of 0.7V (SCE), the nitrogen in solid solution in the steel dissolves into thesolution as NO3

�, and its concentration increases with the nitrogen content in the steel. It was also established that the number of corrosion spots,the corrosion loss, and the maximum depth of corrosion all decrease with the increase in the nitrogen content present in the steel and the appliedpotential. Such results can be attributed to the presence of NO3

� dissolved into the aqueous solution. On the other hand, results from scratch testsshow that the increase in the amount of added nitrogen decreases the peak value of passivation current as well as the amount of electricity duringrepassivation, suggesting that nitrogen stimulates the passivation process and suppresses the occurrence of crevice corrosion. XPS analysisshows the presence of nitrogen as nitrides and NH3 in the surface layer of crevice corrosion and the internal layer of passivation films.[doi:10.2320/matertrans.MRA2007273]

(Received November 7, 2007; Accepted December 17, 2007; Published February 25, 2008)

Keywords: stainless steel, nitrogen, crevice corrosion, X-ray photoelectron spectroscopy, polarization, scratch test, repassivation

1. Introduction

It is well known from many studies that nitrogen has theeffect of enhancing the resistance to crevice corrosion andpitting corrosion of austenitic stainless steel.1–6) Compared toother additives such as chromium or molybdenum, a minutenitrogen content is effective in improving resistance tolocalized corrosion. Moreover, nitrogen addition helps torefine the microstructure and increase the strength of thematerial, and it can be used instead of nickel as an austenite-forming element.

Currently, the addition of nitrogen during fusion ofaustenitic stainless steel at ordinary pressure is limited byits solubility, and obtaining a stainless steel with a nitrogencontent as high as 1% is extremely difficult. Because of this,the behaviors and localized corrosion control mechanisms ofsolid solution nitrogen are not yet understood.7) Austeniticstainless steel obtained through the nitrogen gas pressurizedelectroslag remelting (ESR) method increases the nitrogensolubility, and makes the use of manganese, which reducesthe corrosion resistance of the material, unnecessary. Thebehavior of nitrogen on the surface of nitrogen-bearingaustenitic stainless steel has been investigated using X-rayphotoelectron spectroscopy (XPS) and Auger electron spec-troscopy (AES), and many research works have reportedenhancement of the resistance to localized corrosion.8–13)

On the other hand, formation of a chromium oxide filmpreserves the passivation of stainless steel, and alternatedissolution and regeneration of the passivation film inaqueous solutions keeps a constant thickness of this film.However, in an aqueous solution with a high concentration ofchloride ions, the passivation film is locally destroyed and thecorrosion advances at an accelerating rate. As the passivation

film on stainless steels is difficult to remove by cathodicreduction, mechanical means such as scratching or polishingare usually employed to destroy the film and expose a newsurface so that the corrosion resistance can be evaluated fromthe repassivation mechanism.14–17) In general, steels withhigh corrosion resistance are easily re-passivated, but it ismore difficult to rebuild the passivation film, and easier topromote local corrosion on the steel loosing its anticorrosioncharacteristics. One available method to investigate the effectof nitrogen on local corrosion is to observe the repassivationbehavior after mechanically destroying the passivation film.

In this study, high nitrogen-bearing austenitic stainlesssteels manufactured through the nitrogen gas pressurizedESR method were used to investigate the effects of nitrogenon crevice corrosion characteristics in an acidic chloridesolution, and the passive film on the crevice corrosion aftercorrosion tests was analyzed. The controlling mechanismsfor localized corrosion resistance were elucidated using XPS.Also, the effect of nitrogen on corrosion resistance wasinvestigated by observing the repassivation behavior afterscratching the passivation film instantaneously.

2. Experimental

2.1 Sample preparationHigh nitrogen-bearing austenitic stainless steels with the

compositions shown in Table 1 were used for the samples.Austenitic stainless steels were produced based on a Fe-23mass%Cr-4mass%Ni alloy with varying nitrogen (0.7–1mass%) and molybdenum contents (0–1mass%), throughelectro-slag remelting under high nitrogen gas pressure. Afterhot forging, hot rolling and cold rolling, the steels weresolution treated at 1250�C for 30 minutes. It was confirmedthat the steels consisted of a single-phase austenite structureand no grain boundary precipitation of Cr nitrides wasdetected. Samples with dimensions of 50mm� 50mm�

*This Paper was Originally Published in Japanese in J. Japan Inst. Metals

71 (2007) 570–577.

Materials Transactions, Vol. 49, No. 3 (2008) pp. 579 to 586#2008 The Japan Institute of Metals

http://dx.doi.org/10.2320/matertrans.MRA2007273

3mm were cut from the steel bars. After wet polishing withemery paper No. 600, the samples were washed in water,degreased with acetone, washed in alcohol, and dried.

2.2 Measurement of polarizationSamples were immersed in a solution of 1 kmol/m3

NaCl + 0.1 kmol/m3 HCl, degassed in an argon atmosphere,at 35�C. After cathodic reduction by applying a voltage of�0:7V (SCE) for 10 minutes, potentiodynamic polarizationin the anodic direction was carried out at 20mVmin�1. Asaturated calomel electrode (SCE) was used as the referenceelectrode and a Pt plate as the counter electrode, and thepotential value was expressed in the SCE standard.

2.3 Crevice corrosion test at constant potential andquantitative analysis of nitrogen products dissolvedin aqueous media

The constant potential multiple crevice corrosion test wascarried out at a constant potential to assess crevice corrosion.The sample had a central hole of 10mm in diameter. Amultiple crevice device with 20 crevices was prepared from apolysulfone resin disc with a diameter of 25.4mm accordingto the ASTM G78 standard. Samples were held from bothsides by the crevice forming material, and a torque of 8.5 Nmwas applied through a torque wrench.

The multiple crevice device shown in Fig. 1 was immersedin the anodic solution (260 cm3) in a glass electrolytic cellseparated into anodic and cathodic compartments by a glassfilter. A calomel electrode (SCE) was used as referenceelectrode, using a Pt plate as counter electrode of cathodicside. Crevice corrosion tests under potentiostatic conditionswere carried out to apply potentials of 0.2 V (SCE) and 0.7V(SCE) for 72 hours to the solution of 1 kmol/m3 NaCl +0.1 kmol/m3 HCl, pH 1 at 35�C, and the solution of 1kmol/m3 NaCl + 0.1 kmol/m3 HCl + 0.02 kmol/m3

NaNO3, pH 1.2 at 35�C. Amounts of anodic current, andthe weight loss caused by corrosion, maximum depth of

corrosion and quantity of corrosion of the crevice corrosionproduced were measured.

The amounts of NH4þ, NO2

� and NO3� eluted into the

anodic solution as a result of crevice corrosion were alsocalculated using absorption spectroscopy (ASTM D1426-93and ATM D3867-90). Microscopic corrosion test equipmentwith a CCD laser displacement sensor (Nittetsu-ELEX) andan optical microscope were used to measure the maximumdepth.

2.4 Surface analysis by X-ray photoelectron spectrosco-py (XPS)

Chemical bonding conditions for each element in thepassive film and in the surface film of the crevice corrosionarea after the corrosion tests of the high nitrogen-bearingaustenitic stainless steels were analyzed using XPS. Theequipment employed is a Quantum 2000 made by PhysicalElectronics. Mono-Al K�-rays were used as an X-rayexcitation source at a take-off angle of 90� to the surface offilm. Wave separation was carried out after smoothing andbackground adjustment of the obtained spectra.

2.5 Measurement of repassivationThe passivation film of the sample immersed in a solution

of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl was scratched witha diamond needle to observe if repassivation occurs. Aschematic diagram of the equipment used for scratch tests isshown in Fig. 2. Inside an electrochemical cell composed of aSCE and Pt opposite electrode, a load of 100 g or 200 g wasset on the tip of the diamond needle. The diamond needle wasset on the surface of the sample, the stage was moved at aspeed of 20mm/s at a designated horizontal displacement,and the instantaneous scratch left the surface newly exposed.At the same time, the peak of current density was measuredfrom the current decay curve at a constant potential, and thequantity of electricity measured was used to estimate therepassivation behavior of the sample.

Table 1 Chemical compositions of steels (mass%).

Sample No. C Si Mn P S Ni Cr Mo N Al(Total) O

(1) 0.7N-1Mo 0.020 0.11 0.06 0.005 0.0002 4.15 22.55 1.02 0.73 0.14 0.0014

(2) 0.8N-0Mo 0.024 0.13 0.08 0.006 <0:0001 4.16 22.96 <0:01 0.81 — 0.0029

(3) 0.9N-1Mo 0.024 0.12 0.09 0.006 0.0004 4.23 22.44 1.04 0.93 0.13 0.0019

(4) 0.9N-0Mo 0.034 0.11 0.10 0.005 0.0020 4.53 23.30 0.02 0.96 0.018 0.0022

PotentiostatThermostat

Glass filter

Water bath

Saturated calomelelectrode

Pt counter electrode

Electrolyte

Multiple crevice device

Titanium (bolt,nut,washers)

Specimen

Glass cell

Cathodic side Anodic side

Fig. 1 Schematic illustration of electrochemical cell used for dissolved

nitrogen compound analysis and crevice corrosion measurements.

Weight

Diamond bit

Pt counter electrode

Luggin probe

Reference electrode

Electrolyte

Specimen

Stage

Electrochemicalcell

Fig. 2 Schematic of electrochemical equipment used for scratch test.

580 H. Baba and Y. Katada

3. Results and Discussion

3.1 Polarization curveFigure 3 shows the potentiodynamic polarization curves

of 0.7N-1Mo, 0.8N-0Mo, 0.9N-1Mo, and 0.9N-0Mo steels ina solution of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl at 35�C.Regardless of the nitrogen content, the critical passivecurrent density (icrit) shows a tendency to decrease with theincrease of molybdenum content. In the case of no addition ofMo, the critical passive current density shows a peak in thevicinity of �0:4V (SCE). For all the steels, a steady passivecurrent density was observed in the range from �0:2 to+0.8V (SCE).

3.2 Potentiostatic crevice corrosion characteristicsFigures 4(a) and 4(b) show the electric current vs. time

curves corresponding to a potentiostatic crevice corrosiontest carried out at 0.2 V (SCE) and 0.7V (SCE). A tendencyfor the current to decrease as the nitrogen and molybdenumcontents increased was confirmed. Especially, the electriccurrents for the 0.9N-1Mo and 0.9N-0Mo steels wereconsiderably lower at 0.7V (SCE) than at 0.2V (SCE).Figures 5(a) and 5(b) show the current vs. time curves forpotentiostatic corrosion tests in a solution after adding0.02 kmol/m3 NaNO3. It was confirmed that the presence ofNO3

� in the solution causes a sharp decrease in the current atthe high potential value of 0.7 V (SCE) value, inhibitingcrevice corrosion.

Figures 6 and 7 represent relationships between weightloss and number of crevice corrosion spots against thenitrogen and molybdenum contents in samples immersed ina solution of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl for anapplied potential of 0.2V (SCE) and 0.7V (SCE). Thenumber of crevice corrosion spots for a potential of 0.2V(SCE) remained almost constant at 40/40 regardless of thenitrogen or molybdenum content. In contrast, at a highpotential of 0.7V (SCE), the number of spots showed atendency to decrease as the nitrogen content increased. Thephenomenon that the number of crevice corrosion spots showa sharp decrease at high potential was observed. Thecorrosion weight loss at potentials of both 0.2 V (SCE) and0.7V (SCE) showed a tendency to decrease as the nitrogencontent increased, but this tendency was especially evident at

the high potential of 0.7V (SCE). This phenomenon has beenreported for high nitrogen-bearing austenitic stainless steelsimmersed in solutions containing chloride ions, establishingthe dependence of the number of crevice corrosion spots andcorrosion weight loss on the potential value.18)

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

1 kmol/m3 NaCl + 0.1kmol/m3 HCl 35oC

0.7N-1Mo 0.8N-0Mo 0.9N-1Mo 0.9N-0Mo

Cur

rent

den

sity

, I /

A m

-2

Electrode potential, E / V vs. SCE

102

10

1

10-1

10-2

10-3

Fig. 3 Potentiodynamic polarization curves of 0.7N-1Mo, 0.8N-0Mo,

0.9N-1Mo and 0.9N-0Mo steels in a 1 kmolm�3 NaCl + 0.1 kmolm�3

HCl solution.

(1)

(2)

(4)

(3)

(1)

(2)

(4)

(3)

0 10 20 30 40 50 60 700.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1 kmol/m3 NaCl+0.1 kmol/m3 HClpH 1 35oC

0.2V(SCE) 72h

(1) 0.7N-1Mo(2) 0.8N-0Mo(3) 0.9N-1Mo(4) 0.9N-0Mo

Cur

rent

, I /

mA

(a)

0 10 20 30 40 50 60 700.0

0.2

0.4

0.6

0.8

1.0

1.2

1.41 kmol/m3 NaCl+0.1 kmol/m3 HCl

pH 1 35oC0.7V(SCE) 72h

(1) 0.7N-1Mo(2) 0.8N-0Mo(3) 0.9N-1Mo(4) 0.9N-0Mo

Cur

rent

, I /

mA

Time, t / h

(b)

Fig. 4 Current-time curves for potentiostatic crevice corrosion of 0.7N-

1Mo, 0.8N-0Mo, 0.9N-1Mo and 0.9N-0Mo steels in a 1 kmolm�3

NaCl + 0.1 kmolm�3 HCl solution. (a): at 0.2V (SCE), 72 h and (b): at

0.7V (SCE), 72 h.

(1)

(2)

(4)

(3)

(1)

(2)(4)(3)

(a)

(b)

0 10 20 30 40 50 60 700.0

0.2

0.4

0.6

0.8

1.0

1.2

1.41 kmol/m3 NaCl+0.1 kmol/m3 HCl

+0.02 kmol/m3 NaNO3 pH 1.2 35oC0.2V(SCE) 72h

(1) 0.7N-1Mo(2) 0.8N-0Mo(3) 0.9N-1Mo(4) 0.9N-0Mo

Cur

rent

, I /

mA

0 10 20 30 40 50 60 700.0

0.1

0.2

0.3

0.4

0.5

1 kmol/m3 NaCl+0.1 kmol/m3 HCl+0.02 kmol/m3 NaNO3pH 1.2 35oC0.7V(SCE) 72h

(1) 0.7N-1Mo(2) 0.8N-0Mo(3) 0.9N-1Mo(4) 0.9N-0Mo

Cur

rent

, I /

mA

Time, t / h

Fig. 5 Current-time curves for potentiostatic crevice corrosion of 0.7N-


NaCl + 0.1 kmolm�3 HCl + 0.02 kmolm�3 NaNO3 solution. (a): at 0.2

V (SCE), 72 h and (b): at 0.7V (SCE), 72 h.

Effect of Nitrogen on Crevice Corrosion and Repassivation Behavior of Austenitic Stainless Steel 581

Figure 8 shows the relationship between maximum depthof crevice corrosion and the nitrogen content in crevicecorrosion tests carried out in a solution of 1 kmol/m3

NaCl + 0.1 kmol/m3 HCl at potentials of 0.2V (SCE) and0.7V (SCE). Maximum depth at both potentials decreasedas the nitrogen content increased, especially at the highpotential of 0.7V (SCE), the depth tended to becomeshallow.

When 0.02 kmol/m3 NaNO3 was added, the number ofcrevice corrosion spots, the corrosion weight loss and themaximum corrosion depth decreased further as the nitrogencontent increased. In the same way, the depth becameshallow at high potential of 0.7V (SCE).

3.3 XPS surface analysis of passivation film and crevicecorrosion spots

Figures 9 and 10 show the XPS spectra of N 1s and Mo3p3/2 corresponding to the surface film of the crevicecorrosion area and to the passivation film after corrosion testscarried out on a 0.7N-1Mo steel in a solution of 1 kmol/m3

NaCl + 0.1 kmol/m3 HCl at 35�C under potentials of 0.2V(SCE) and 0.7V (SCE). As the bonding energy of the N 1sand Mo 3p3/2 spectra overlap, waveform separation wascarried out. At both potentials, N 1s spectra for the surface ofthe crevice corrosion area and for the passivation film showpeaks in the vicinity of 399.9 eV and 397 eV. The first peakcorresponds to NH3, whereas the later one indicates thepresence of nitride; Mo 3p3/2 spectrum indicated thepresence of Mo0 and Mo6þ, whereas the surface film of thecrevice corrosion area showed a NH4

þ peak in the vicinity of401 eV.9,19) Peaks corresponding to oxides, hydroxides andmetals were detected in the Cr and Fe spectra, it wasconfirmed that the passivation film is composed of chromiumand iron oxides.

Table 2 shows the quantitative value of nitrogen contentcalculated from the area of the N 1s spectrum after waveformseparation. At 0.2 V (SCE) and 0.7V (SCE), nitride and NH3

contents in the surface film of the crevice corrosion area werelower than those in the passivation film. This can beattributed to elution of nitrogen into the solution by effectof the crevice corrosion. Nitrogen content in the crevice

Num

ber

of c

revi

ce c

orro

sion

(n

/40)

Num

ber

of c

revi

ce c

orro

sion

(n

/40)

0.7N-1Mo 0.8N-0Mo 0.9N-0Mo 0.9N-1Mo

0

10

20

30

40

1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35oC

0.2V(SCE) 72h


0.7V(SCE) 72h

0

10

20

30

40

Fig. 6 Crevice corrosion number of spots caused at 0.2V and 0.7V for a

0.7N-1Mo, 0.8N-0Mo, 0.9N-1Mo and 0.9N-0Mo steels in a 1 kmolm�3

NaCl + 0.1 kmolm�3 HCl solution.

Cor

rosi

on l

oss,

/ m

gC

orro

sion

los

s, /

mg

0.7N-1Mo 0.8N-0Mo 0.9N-0Mo 0.9N-1Mo

0

20

40

60


0.2V(SCE) 72h


0.7V(SCE) 72h

0

20

40

60

80

Fig. 7 Crevice corrosion weight loss at 0.2V and 0.7V for 0.7N-1Mo,

0.8N-0Mo, 0.9N-1Mo and 0.9N-0Mo steels in a 1 kmolm�3 NaCl +

0.1 kmolm�3 HCl solution.

Max

imum

dep

th, d

/ mm

Max

imum

dep

th, d

/ mm

0.7N-1Mo 0.8N-0Mo 0.9N-0Mo 0.9N-1Mo

0.00

0.05

0.10

0.15

0.20

0.251 kmol/m3 NaCl+0.1 kmol/m3 HCl 35oC

0.2V(SCE) 72h


0.7V(SCE) 72h

0.00

0.05

0.10

0.15

0.20

0.25

Fig. 8 Maximum depth of crevice corrosion at 0.2V and 0.7V for 0.7N-


NaCl + 0.1 kmolm�3 HCl solution.


corrosion surface film was a little lower for 0.2V (SCE) thanfor 0.7V (SCE).

3.4 Electrochemistry in production of ammonium, ni-trite and nitrate salts

Potentiostatic electrolyses of the samples (shown inFigure 1) were carried out after being immersed into theanodic solution in the electrolytic cell, followed by quanti-tative analyses of the nitrogen component eluted into theanodic solution. The calculated values for nitrogen contentwere used to produce the results shown in Fig. 11, which isthe potential-pH equilibrium diagram20) in the NH3-H2Osystem at potentials of A (0.2V, SCE) and B (0.7 V, SCE) for

potentiostatic crevice corrosion tests. This figure shows thatNH4

þ is relatively stable at low potential, whereas NO3� is

stable at high potential.Figure 12 shows the ratio of two values; the nitrogen

content eluted into the solution obtained from calculating the

800

850

900

950

1000

1050

1100

1150

Inte

nsity

(A

rb.

unit)

MoO 3

NH3 NitrideMet.Mo

NH4+

1100

1200

1300

1400

1600

1700

1500

404 402 400 398 396 394 392

Binding energy, E / eV

Inte

nsity

(A

rb.

unit)

NH3

MoO3

NitrideMet.Mo

N 1s, Mo 3p3/2 0.7N-1Mo 0.2V(SCE)

(a)

(b)

Fig. 9 N 1s and Mo 3p3/2 XPS spectra with wave identification for 0.7N-

1Mo steel after crevice corrosion at 0.2V (SCE) in a 1 kmolm�3

NaCl + 0.1 kmolm�3 HCl solution of pH 0.93 at 35�C. (a): surface film of

the crevice corrosion area and (b): passivation film.

900

950

1000

1050

1100

1150

1200

Inte

nsity

(A

rb.

unit)

NH3MoO3

NitrideNH4+

1100

1200

1400

1500

1600

404 402 400 398 396 394 392

Binding energy, E / eV

Inte

nsity

(A

rb.

unit)

Met.Mo

MoO3

Nitride

NH3

1300

N 1s, Mo 3p3/2 0.7N-1Mo 0.7V(SCE)

(a)

(b)

Fig. 10 N 1s and Mo 3p3/2 XPS spectra with wave identification for 0.7N-

1Mo steel after crevice corrosion at 0.7V (SCE) in a 1 kmolm�3



Table 2 Quantitative values for nitrogen content in a 0.7N-1Mo steel after

crevice corrosion at 0.2V (SCE) and 0.7V (SCE) in a 1 kmolm�3



(at%)

0.2V (SCE) 0.7V (SCE)

(a) (b) (a) (b)

Nitride 0.2 0.5 0.4 0.7

NH3 0.3 1.8 0.7 1.3

NH4þ 0.2 — 0.2 —

0 2 4 6 8 10 12 14-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

pH

NH3

NH4+

NO3

_HNO2

25oC

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

NO2

A

B

_

O2 / H2O

H+ / H2

Ele

ctro

de p

oten

tial,

E/ V

vs.

SH

E

Ele

ctro

de p

oten

tial,

E/ V

vs.

SC

E

Fig. 11 Potential-pH diagram for an ammonium salt, nitrite and nitrate

system. A,B indicate potential of the crevice corrosion tests.


weight loss values after crevice corrosion tests of a 0.7N-1Mosteel and a 0.8N-0Mo steel in a solution of 1 kmol/m3

NaCl + 0.1 kmol/m3 HCl at 35�C under potentials of 0.2Vand 0.7V (SCE) during 72 hours; and the nitrogen content inNH4

þ, NO2�, and NO3

� measured by absorption spec-troscopy. Figure 13 shows similar results for 0.9N-0Mo steeland 0.9N-1Mo steel. For 0.7N-1Mo steel and 0.8N-0Mosteel, after potentiostatic crevice corrosion tests of 0.2V(SCE), the ratio of the nitrogen content eluted into thesolution calculated from weight loss measurements and thenitrogen content calculated from stoichiometric NH4

þ valueis almost 1, indicating that almost all the nitrogen in thesolution is present as NH4

þ.There are several reports regarding nitrogen compounds

formed by nitrogen dissolved into the bulk solution after

being solidified in the steel by the effect of localizedcorrosion. The results in the present work coincide with thereport by Osozawa et al.2) regarding the presence of NH4

þ inthe solution in the vicinity of natural potential. According tothese results, the amount of NH4

þ formed from Hþ in thepitting area and N counterbalances the amount of NH4

þ asnitrogen eluted from the steel. Results also coincide in thatthe formation of NH4

þ promotes a repassivation effect as itraises the pH in the pitting area. After a potentiostatic crevicecorrosion test in the high potential region of 0.7 V (SCE),small nitrogen content remained in existence as, apart fromNH4

þ, NO3� and NO2

�.For the 0.9N-0Mo steel with a high nitrogen content, after

the potentiostatic crevice corrosion test at 0.2V (SCE), theratio of the nitrogen value into the solution measured from

(a) (c)

(d)

0

0.5

1

1.50

0.5

1

1.5

0 0 0 0 0

0.7N-1Mo0.2V(SCE) 72h

0.7N-1Mo0.7V(SCE) 72h

Det

ecte

d n

itrog

en /

diss

olve

d n

itrog

en

NH4+ NO2

- NO3-


1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35oC(b)

0

0.5

1

1.5

0

0.5

1

1.5

0 0 0 0

0.8N-0Mo0.2V(SCE) 72h

0.8N-0Mo0.7V(SCE) 72h

Det

ecte

d n

itrog

en /

diss

olve

d n

itrog

en

NH4+ NO2

- NO3-

(c) 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35oC


Fig. 12 Ratio between the total amount of N from the steel and NH4þ, NO2

� and NO3� dissolved after the crevice corrosion tests, (a):

0.7N-1Mo steel, 0.2V (SCE); (b): 0.7N-1Mo steel, 0.7V (SCE); (c): 0.8N-0Mo steel, 0.2V (SCE); (d): 0.8N-0Mo steel, 0.7V (SCE).

(a)

(b)

(c)

(d)

0

0.5

1

1.5

0

0.5

1

1.5

0 0 00 0

0.9N-0Mo0.2V(SCE) 72h

0.9N-0Mo0.7V(SCE) 72h

Det

ecte

d n

itrog

en /

diss

olve

d n

itrog

en

NH4+ NO2

- NO3-



0

0.5

1

1.5

0

0.5

1

1.5

2

00

0.9N-1Mo0.2V(SCE) 72h

0.9N-1Mo0.7V(SCE) 72h

Det

ecte

d n

itrog

en /

diss

olve

d n

itrog

en

NH4+ NO2

- NO3-



Fig. 13 Ratio between the total amount of N and NH4þ, NO2

� and NO3� dissolved after the crevice corrosion tests, (a): 0.9N-0Mo steel,

0.2V (SCE); (b): 0.9N-0Mo steel, 0.7V (SCE); (c): 0.9N-1Mo steel, 0.2V (SCE); (d): 0.9N-1Mo steel, 0.7V (SCE).


weight loss to the stoichiometric nitrogen value calculatedfrom NH4

þ in the solution was almost 1, confirming thatalmost all the nitrogen in the steel solid solution was presentin the solution as NH4

þ after elution. In the case of the highpotential of 0.7V (SCE), for 0.9N-0Mo steel and 0.9N-1Mosteel, the nitrogen eluted forms besides NH4

þ, NO3� and

NO2�. The amount of NO3

� shows a tendency to increase,as does the nitrogen content in the steel. As shown in Fig. 6and 7, as the nitrogen content in solid solution in the steelincreases, the number of crevice corrosion spots and thecorrosion weight loss decrease, indicating that when NO3

�

was present in the solution, the number of crevice corrosionspots in the high potential region was suppressed.

3.5 Repassivation characteristics after exposing newsurface by scratch test

Figure 14 shows the current density decay caused by theappearance of a new surface after scratching of 0.2 cm withthe load of 200 g, which was made on a 0.7N-1Mo steelimmersed in a solution of 1 kmol/m3 NaCl + 0.1 kmol/m3

HCl at a potential of 0.2V (SCE). There was an instantaneoussurge in current density when a new surface appeared afterthe passivation film was destroyed by scratching, and thevalue of current went back to 0mA/cm2 when repassivationinstantaneously occurred. This maximum value of currentdensity was the peak. After 3.6 seconds, the total quantity ofelectricity corresponds to the sum of the current due torepassivation and the current caused by dissolution of the

0.7N-1Mo steel. It can be observed that the lower the peak ofcurrent density and total quantity of electricity, the easierrepassivation occurs.

Figure 15 (a) and (b) illustrate the relationship betweenpotentials and peaks of current density for 0.7N-1Mo steeland 0.9N-1Mo steel immersed in a solution of 1 kmol/m3

NaCl + 0.1 kmol/m3 HCl, after using loads of 100 g and200 g to make a 0.2 cm scratch respectively. For both steels,the peak of current density increases as the potentialincreases, and it also increase with the scratching load. Onthe other hand, the peak is lower for the steel with the highernitrogen content, suggesting that the nitrogen in solidsolution in the steel promotes the repassivation.

Figure 16 illustrates the relationship between potential andtotal quantity of electricity for repassivation for 0.7N-1Mosteel and 0.9N-1Mo steel immersed in a solution of 1kmol/m3 NaCl + 0.1 kmol/m3 HCl, after using a load of200 g to make a 0.2 cm scratch. In the case of 0.7N-1Mo steel,there is a tendency for the total quantity of electricity toincrease as the potential increases. In the case of 0.9N-1Mosteel, the electricity remains almost constant up to thepotential region of 0.7 V (SCE), but in the transpassivationregion there is a sudden increase in the value of total quantityof electricity the same as in the case of 0.7N-1Mo. The highnitrogen content in solid solution in the steel reduced thevalue of the total quantity of electricity repassivation.Scratching caused a sudden drop in the natural potential,but as the scratch was stopped, potential returned rapidly to

Cur

rent

den

sity

, I/

A m

-2

0

0.01

0.02

0.03

0.04

0.05

1 kmol/m3 NaCl+0.1 kmol/m3 HCl

0.7N-1Mo 0.2V(SCE)200g, scratch 0.2cm

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Time, t / s

Fig. 14 Current density decay after exposing a new surface on a 0.7N-1Mo

steel at 0.2V (SCE) in a 1 kmolm�3 NaCl + 0.1 kmolm�3 HCl solution.

Scratch length: 0.2 cm; scratch load: 200 g.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

1 kmol/m3 NaCl+0.1kmol/m 3 HCl 200g scratch 2cm

0.7N-1Mo 0.9N-1Mo

Qua

ntity

of e

lect

ricity

, Q

/ C

m-2

Potential, E / V vs. SCE

0

2

4

6

8

10

12

14

16

Fig. 16 Total amount of electricity for repassivation against applied

potential for a 0.7N-1Mo steel and for a 0.9N-1Mo steel, in a 1 kmolm�3

NaCl + 0.1 kmolm�3 HCl solution. Scratch length: 2 cm; scratch load:

200 g.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

(a) 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 100g scratch 0.2cm

0.7N-1Mo 0.9N-1Mo

Pea

k cu

rren

t den

sity

, I /

A m

-2


-0.2 0.0 0.2 0.4 0.6 0.8 1.0

(b) 1 kmol/m3 NaCl+0.1 kmol/m3 HCl200g scratch 0.2cm

0.7N-1Mo 0.9N-1Mo

Pea

k cu

rren

t den

sity

, I /

A m

-2


0

0.02

0.04

0.06

0.08

0.1

0

0.04

0.08

0.12

0.16

0.20

0.24

Fig. 15 Current density peak against potential for a 0.7N-1Mo steel and for a 0.9N-1Mo steel in a 1 kmolm�3 NaCl + 0.1 kmolm�3 HCl

solution. Scratch length: 0.2 cm. (a) scratch load: 100 g, and (b) scratch load: 200 g.


its former value. From analyses of the XPS spectra withmodified take-off angles for a stainless steel with a nitrogencontent of approximately 1%, Sagara et al.13) have suggestedthe possibility that there is a high nitrogen concentration inthe inner layer of the passivation film, and that this nitrogenconcentration in the inner layer increases with the polar-ization potential. From these observations, it is establishedthat the nitrogen concentration in the inner layer of thepassivation film has the effect of promoting repassivation.

On the other hand, it has been reported that in a NO3�

solution at stable high potentials, NO3� tends to increase

with the potential, and the number of crevice corrosion spotsas well as weight loss due to corrosion are markedlysuppressed.18) It was also reported that the presence of NO3

�

in the solution increases the resistance to pitting corrosion,21)

and that NO3� has a controlling effect on pitting corrosion at

the high potential region.22,23) In the present research, whenNO3

� is present in an acidic chloride aqueous solution, aconsiderable decrease in electric current is observed at highelectric potential, corroborating that the crevice corrosion iscontrolled. At relatively low potentials, solidified nitrogendissolves with crevice corrosion to produce NH4

þ in thesolution and control the acidification inside the pit.2)

In the present research, increase of the nitrogen content innitrogen-bearing austenitic steel results in (1) decreased ofnumber of crevice corrosion spots and the corrosion weightloss, with a tendency to further decrease as the polarizationpotential reaches high values; (2) the amount of NO3

� elutedinto the solution showed a tendency to increase, and at thesame time, the eluted NO3

� was adsorbed onto the surface ofthe passivation film, which had a inhibitor effect suppressingthe dissolution of the base metal; (3) the peak current densityand the total quantity of electricity for the repassivationprocess decreased and indicated a high corrosion resistancewhich promotes repassivation.

4. Conclusions

Austenitic stainless steels with a nitrogen content rangingfrom 0.7 to 1mass% were produced by the electroslagremelting (ESR) method and the effect of nitrogen on thecrevice corrosion in an acidic chloride aqueous solution aswell as XPS analyses of the surface film after crevicecorrosion were carried out. At the same time, the repassiva-tion process after scratching of the passivation film wasobserved, leading to the following conclusions.(1) After a crevice corrosion test in the high potential, the

nitrogen in solid solution in the steel eluted into thesolution and was present as NO3

�, The concentration of

nitrogen in the solution showed a tendency to increasewith the nitrogen content in the steel. The number ofcrevice corrosion spots, the corrosion weight loss, andthe maximum depth of corrosion decreased with theincrease in nitrogen content, and further decreased withthe values of polarization potential.

(2) The NO3� eluted into the aqueous solution was

adsorbed onto the surface of the passivation film, andacted as an inhibitor preventing the dissolution of thebase material.

(3) As the nitrogen content added to the steel and thepolarization potential increased, the current densitypeak and the total quantity of electricity for repassiva-tion decreased, promoting the repassivation and sup-pressing the occurrence of crevice corrosion.

(4) The XPS analyses confirmed the presence of nitrogen inthe form of nitrides or as NH3 at the crevice corrosionsurface film and the inner layer of passivation film.

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