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-23 mass%Cr-4 mass%Ni alloy with varying nitrogen (0.7–1 mass%) and
molybdenum contents (0–1 mass%), through electro-slag remelting (ESR) under high nitrogen gas pressure. The effects of nitrogen on crevice
corrosion behavior in an acidic chloride solution were investigated, and the passive film of the crevice corrosion area after corrosion tests was
analyzed using X-ray photoelectron spectroscopy (XPS). At the same time, the effects of nitrogen on the passivation behaviors after scratching
were also investigated. During crevice corrosion at a noble potential of 0.7 V (SCE), the nitrogen in solid solution in the steel dissolves into the
solution as NO3
À
, 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 applied
potential. Such results can be attributed to the presence of NO3
À
dissolved into the aqueous solution. On the other hand, results from scratch tests
show that the increase in the amount of added nitrogen decreases the peak value of passivation current as well as the amount of electricity during
repassivation, suggesting that nitrogen stimulates the passivation process and suppresses the occurrence of crevice corrosion. XPS analysis
shows 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 the
effect of enhancing the resistance to crevice corrosion and
pitting corrosion of austenitic stainless steel.1–6)
Compared to
other additives such as chromium or molybdenum, a minute
nitrogen content is effective in improving resistance to
localized corrosion. Moreover, nitrogen addition helps to
refine the microstructure and increase the strength of the
material, and it can be used instead of nickel as an austenite-
forming element.
Currently, the addition of nitrogen during fusion of
austenitic stainless steel at ordinary pressure is limited by
its solubility, and obtaining a stainless steel with a nitrogen
content as high as 1% is extremely difficult. Because of this,
the behaviors and localized corrosion control mechanisms of
solid solution nitrogen are not yet understood.7)
Austenitic
stainless steel obtained through the nitrogen gas pressurized
electroslag remelting (ESR) method increases the nitrogen
solubility, and makes the use of manganese, which reduces
the corrosion resistance of the material, unnecessary. The
behavior of nitrogen on the surface of nitrogen-bearing
austenitic stainless steel has been investigated using X-ray
photoelectron spectroscopy (XPS) and Auger electron spec-
troscopy (AES), and many research works have reported
enhancement of the resistance to localized corrosion.8–13)
On the other hand, formation of a chromium oxide film
preserves the passivation of stainless steel, and alternate
dissolution and regeneration of the passivation film in
aqueous solutions keeps a constant thickness of this film.
However, in an aqueous solution with a high concentration of
chloride ions, the passivation film is locally destroyed and the
corrosion advances at an accelerating rate. As the passivation
film on stainless steels is difficult to remove by cathodic
reduction, mechanical means such as scratching or polishing
are usually employed to destroy the film and expose a new
surface so that the corrosion resistance can be evaluated from
the repassivation mechanism.14–17)
In general, steels with
high corrosion resistance are easily re-passivated, but it is
more difficult to rebuild the passivation film, and easier to
promote local corrosion on the steel loosing its anticorrosion
characteristics. One available method to investigate the effect
of nitrogen on local corrosion is to observe the repassivation
behavior after mechanically destroying the passivation film.
In this study, high nitrogen-bearing austenitic stainless
steels manufactured through the nitrogen gas pressurized
ESR method were used to investigate the effects of nitrogen
on crevice corrosion characteristics in an acidic chloride
solution, and the passive film on the crevice corrosion after
corrosion tests was analyzed. The controlling mechanisms
for localized corrosion resistance were elucidated using XPS.
Also, the effect of nitrogen on corrosion resistance was
investigated by observing the repassivation behavior after
scratching the passivation film instantaneously.
2. Experimental
2.1 Sample preparation
High 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-23
mass%Cr-4 mass%Ni alloy with varying nitrogen (0.7–1
mass%) and molybdenum contents (0–1 mass%), through
electro-slag remelting under high nitrogen gas pressure. After
hot forging, hot rolling and cold rolling, the steels were
solution treated at 1250
C for 30 minutes. It was confirmed
that the steels consisted of a single-phase austenite structure
and no grain boundary precipitation of Cr nitrides was
detected. Samples with dimensions of 50 mm Â 50 mm Â
*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

3 mm were cut from the steel bars. After wet polishing with
emery paper No. 600, the samples were washed in water,
degreased with acetone, washed in alcohol, and dried.
2.2 Measurement of polarization
Samples 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
À0:7 V (SCE) for 10 minutes, potentiodynamic polarization
in the anodic direction was carried out at 20 mV minÀ1
. A
saturated calomel electrode (SCE) was used as the reference
electrode and a Pt plate as the counter electrode, and the
potential value was expressed in the SCE standard.
2.3 Crevice corrosion test at constant potential and
quantitative analysis of nitrogen products dissolved
in aqueous media
The constant potential multiple crevice corrosion test was
carried out at a constant potential to assess crevice corrosion.
The sample had a central hole of 10 mm in diameter. A
multiple crevice device with 20 crevices was prepared from a
polysulfone resin disc with a diameter of 25.4 mm according
to the ASTM G78 standard. Samples were held from both
sides by the crevice forming material, and a torque of 8.5 Nm
was applied through a torque wrench.
The multiple crevice device shown in Fig. 1 was immersed
in the anodic solution (260 cm3
) in a glass electrolytic cell
separated into anodic and cathodic compartments by a glass
filter. A calomel electrode (SCE) was used as reference
electrode, using a Pt plate as counter electrode of cathodic
side. Crevice corrosion tests under potentiostatic conditions
were carried out to apply potentials of 0.2 V (SCE) and 0.7 V
(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 1
kmol/m3
NaCl + 0.1 kmol/m3
HCl + 0.02 kmol/m3
NaNO3, pH 1.2 at 35
C. Amounts of anodic current, and
the weight loss caused by corrosion, maximum depth of
corrosion and quantity of corrosion of the crevice corrosion
produced were measured.
The amounts of NH4
þ
, NO2
À
and NO3
À
eluted into the
anodic solution as a result of crevice corrosion were also
calculated using absorption spectroscopy (ASTM D1426-93
and ATM D3867-90). Microscopic corrosion test equipment
with a CCD laser displacement sensor (Nittetsu-ELEX) and
an optical microscope were used to measure the maximum
depth.
2.4 Surface analysis by X-ray photoelectron spectrosco-
py (XPS)
Chemical bonding conditions for each element in the
passive film and in the surface film of the crevice corrosion
area after the corrosion tests of the high nitrogen-bearing
austenitic stainless steels were analyzed using XPS. The
equipment employed is a Quantum 2000 made by Physical
Electronics. Mono-Al K-rays were used as an X-ray
excitation source at a take-off angle of 90
to the surface of
film. Wave separation was carried out after smoothing and
background adjustment of the obtained spectra.
2.5 Measurement of repassivation
The passivation film of the sample immersed in a solution
of 1 kmol/m3
NaCl + 0.1 kmol/m3
HCl was scratched with
a diamond needle to observe if repassivation occurs. A
schematic diagram of the equipment used for scratch tests is
shown in Fig. 2. Inside an electrochemical cell composed of a
SCE and Pt opposite electrode, a load of 100 g or 200 g was
set on the tip of the diamond needle. The diamond needle was
set on the surface of the sample, the stage was moved at a
speed of 20 mm/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 measured
from the current decay curve at a constant potential, and the
quantity of electricity measured was used to estimate the
repassivation 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 calomel
electrode
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
Electrochemical
cell
Fig. 2 Schematic of electrochemical equipment used for scratch test.
580 H. Baba and Y. Katada

3. Results and Discussion
3.1 Polarization curve
Figure 3 shows the potentiodynamic polarization curves
of 0.7N-1Mo, 0.8N-0Mo, 0.9N-1Mo, and 0.9N-0Mo steels in
a solution of 1 kmol/m3
NaCl + 0.1 kmol/m3
HCl at 35
C.
Regardless of the nitrogen content, the critical passive
current density (icrit) shows a tendency to decrease with the
increase of molybdenum content. In the case of no addition of
Mo, the critical passive current density shows a peak in the
vicinity of À0:4 V (SCE). For all the steels, a steady passive
current density was observed in the range from À0:2 to
+0.8 V (SCE).
3.2 Potentiostatic crevice corrosion characteristics
Figures 4(a) and 4(b) show the electric current vs. time
curves corresponding to a potentiostatic crevice corrosion
test carried out at 0.2 V (SCE) and 0.7 V (SCE). A tendency
for the current to decrease as the nitrogen and molybdenum
contents increased was confirmed. Especially, the electric
currents for the 0.9N-1Mo and 0.9N-0Mo steels were
considerably lower at 0.7 V (SCE) than at 0.2 V (SCE).
Figures 5(a) and 5(b) show the current vs. time curves for
potentiostatic corrosion tests in a solution after adding
0.02 kmol/m3
NaNO3. It was confirmed that the presence of
NO3
À
in the solution causes a sharp decrease in the current at
the high potential value of 0.7 V (SCE) value, inhibiting
crevice corrosion.
Figures 6 and 7 represent relationships between weight
loss and number of crevice corrosion spots against the
nitrogen and molybdenum contents in samples immersed in
a solution of 1 kmol/m3
NaCl + 0.1 kmol/m3
HCl for an
applied potential of 0.2 V (SCE) and 0.7 V (SCE). The
number of crevice corrosion spots for a potential of 0.2 V
(SCE) remained almost constant at 40/40 regardless of the
nitrogen or molybdenum content. In contrast, at a high
potential of 0.7 V (SCE), the number of spots showed a
tendency to decrease as the nitrogen content increased. The
phenomenon that the number of crevice corrosion spots show
a sharp decrease at high potential was observed. The
corrosion weight loss at potentials of both 0.2 V (SCE) and
0.7 V (SCE) showed a tendency to decrease as the nitrogen
content increased, but this tendency was especially evident at
the high potential of 0.7 V (SCE). This phenomenon has been
reported for high nitrogen-bearing austenitic stainless steels
immersed in solutions containing chloride ions, establishing
the dependence of the number of crevice corrosion spots and
corrosion 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/m
3
NaCl + 0.1kmol/m
3
HCl 35
o
C
0.7N-1Mo
0.8N-0Mo
0.9N-1Mo
0.9N-0Mo
Currentdensity,I/Am
-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 kmol mÀ3
NaCl + 0.1 kmol mÀ3
HCl solution.
(1)
(2)
(4)
(3)
(1)
(2)
(4)
(3)
0 10 20 30 40 50 60 70
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1 kmol/m3 NaCl+0.1 kmol/m3 HCl
pH 1 35o
C
0.2V(SCE) 72h
(1) 0.7N-1Mo
(2) 0.8N-0Mo
(3) 0.9N-1Mo
(4) 0.9N-0Mo
Current,I/mA
(a)
0 10 20 30 40 50 60 70
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
pH 1 35o
C
0.7V(SCE) 72h
(1) 0.7N-1Mo
(2) 0.8N-0Mo
(3) 0.9N-1Mo
(4) 0.9N-0Mo
Current,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 kmol mÀ3
HCl solution. (a): at 0.2 V (SCE), 72 h and (b): at
0.7 V (SCE), 72 h.
(1)
(2)
(4)
(3)
(1)
(2)
(4)(3)
(a)
(b)
0 10 20 30 40 50 60 70
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
+0.02 kmol/m3 NaNO3 pH 1.2 35o
C
0.2V(SCE) 72h
(1) 0.7N-1Mo
(2) 0.8N-0Mo
(3) 0.9N-1Mo
(4) 0.9N-0Mo
Current,I/mA
0 10 20 30 40 50 60 70
0.0
0.1
0.2
0.3
0.4
0.5
1 kmol/m3 NaCl+0.1 kmol/m3 HCl+0.02 kmol/m3 NaNO3
pH 1.2 35o
C0.7V(SCE) 72h
(1) 0.7N-1Mo
(2) 0.8N-0Mo
(3) 0.9N-1Mo
(4) 0.9N-0Mo
Current,I/mA
Time, t / h
Fig. 5 Current-time curves for potentiostatic crevice corrosion of 0.7N-
HCl + 0.02 kmol mÀ3
NaNO3 solution. (a): at 0.2
V (SCE), 72 h and (b): at 0.7 V (SCE), 72 h.
Effect of Nitrogen on Crevice Corrosion and Repassivation Behavior of Austenitic Stainless Steel 581

Figure 8 shows the relationship between maximum depth
of crevice corrosion and the nitrogen content in crevice
corrosion tests carried out in a solution of 1 kmol/m3
NaCl + 0.1 kmol/m3
HCl at potentials of 0.2 V (SCE) and
0.7 V (SCE). Maximum depth at both potentials decreased
as the nitrogen content increased, especially at the high
potential of 0.7 V (SCE), the depth tended to become
shallow.
When 0.02 kmol/m3
NaNO3 was added, the number of
crevice corrosion spots, the corrosion weight loss and the
maximum corrosion depth decreased further as the nitrogen
content increased. In the same way, the depth became
shallow at high potential of 0.7 V (SCE).
3.3 XPS surface analysis of passivation film and crevice
corrosion spots
Figures 9 and 10 show the XPS spectra of N 1s and Mo
3p3/2 corresponding to the surface film of the crevice
corrosion area and to the passivation film after corrosion tests
carried 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 0.2 V
(SCE) and 0.7 V (SCE). As the bonding energy of the N 1s
and Mo 3p3/2 spectra overlap, waveform separation was
carried out. At both potentials, N 1s spectra for the surface of
the crevice corrosion area and for the passivation film show
peaks in the vicinity of 399.9 eV and 397 eV. The first peak
corresponds to NH3, whereas the later one indicates the
presence of nitride; Mo 3p3/2 spectrum indicated the
presence of Mo0
and Mo6þ
, whereas the surface film of the
crevice corrosion area showed a NH4
þ
peak in the vicinity of
401 eV.9,19)
Peaks corresponding to oxides, hydroxides and
metals were detected in the Cr and Fe spectra, it was
confirmed that the passivation film is composed of chromium
and iron oxides.
Table 2 shows the quantitative value of nitrogen content
calculated from the area of the N 1s spectrum after waveform
separation. At 0.2 V (SCE) and 0.7 V (SCE), nitride and NH3
contents in the surface film of the crevice corrosion area were
lower than those in the passivation film. This can be
attributed to elution of nitrogen into the solution by effect
of the crevice corrosion. Nitrogen content in the crevice
Numberofcrevicecorrosion(n/40)Numberofcrevicecorrosion(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 35
o
C
0.2V(SCE) 72h
o
C
0.7V(SCE) 72h
0
10
20
30
40
Fig. 6 Crevice corrosion number of spots caused at 0.2 V and 0.7 V for a
0.7N-1Mo, 0.8N-0Mo, 0.9N-1Mo and 0.9N-0Mo steels in a 1 kmol mÀ3
HCl solution.
Corrosionloss,/mgCorrosionloss,/mg
0.7N-1Mo 0.8N-0Mo 0.9N-0Mo 0.9N-1Mo
0
20
40
60
80
o
C
0.2V(SCE) 72h
1 kmol/m3
NaCl+0.1 kmol/m3
HCl 35
o
C
0.7V(SCE) 72h
0
20
40
60
80
Fig. 7 Crevice corrosion weight loss at 0.2 V and 0.7 V for 0.7N-1Mo,
0.8N-0Mo, 0.9N-1Mo and 0.9N-0Mo steels in a 1 kmol mÀ3
NaCl +
0.1 kmol mÀ3
HCl solution.
Maximumdepth,d/mmMaximumdepth,d/mm
0.7N-1Mo 0.8N-0Mo 0.9N-0Mo 0.9N-1Mo
0.00
0.05
0.10
0.15
0.20
0.25
1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35o
C
0.2V(SCE) 72h
C
0.7V(SCE) 72h
0.00
0.05
0.10
0.15
0.20
0.25
Fig. 8 Maximum depth of crevice corrosion at 0.2 V and 0.7 V for 0.7N-
HCl solution.

corrosion surface film was a little lower for 0.2 V (SCE) than
for 0.7 V (SCE).
3.4 Electrochemistry in production of ammonium, ni-
trite and nitrate salts
Potentiostatic electrolyses of the samples (shown in
Figure 1) were carried out after being immersed into the
anodic solution in the electrolytic cell, followed by quanti-
tative analyses of the nitrogen component eluted into the
anodic solution. The calculated values for nitrogen content
were used to produce the results shown in Fig. 11, which is
the potential-pH equilibrium diagram20)
in the NH3-H2O
system at potentials of A (0.2 V, SCE) and B (0.7 V, SCE) for
potentiostatic crevice corrosion tests. This figure shows that
NH4
þ
is relatively stable at low potential, whereas NO3
À
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
Intensity(Arb.unit)
MoO 3
NH3 Nitride
Met.Mo
NH4
+
1100
1200
1300
1400
1600
1700
1500
404 402 400 398 396 394 392
Binding energy, E / eV
Intensity(Arb.unit)
NH3
MoO3
Nitride
Met.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 0.2 V (SCE) in a 1 kmol mÀ3
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
Intensity(Arb.unit)
NH3
MoO3
NitrideNH4
+
1100
1200
1400
1500
1600
404 402 400 398 396 394 392
Binding energy, E / eV
Intensity(Arb.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 0.7 V (SCE) in a 1 kmol mÀ3
Table 2 Quantitative values for nitrogen content in a 0.7N-1Mo steel after
crevice corrosion at 0.2 V (SCE) and 0.7 V (SCE) in a 1 kmol mÀ3
(at%)
0.2 V (SCE) 0.7 V (SCE)
(a) (b) (a) (b)
Nitride 0.2 0.5 0.4 0.7
NH3 0.3 1.8 0.7 1.3
NH4
þ
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
25o
C
-1.2
-0.8
-0.4
0.0
0.4
0.8
1.2
NO2
A
B
_
O2 / H2O
H+
/ H2
Electrodepotential,E/Vvs.SHE
Electrodepotential,E/Vvs.SCE
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-1Mo
steel 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 0.2 V
and 0.7 V (SCE) during 72 hours; and the nitrogen content in
NH4
þ
, NO2
À
, and NO3
À
measured by absorption spec-
troscopy. Figure 13 shows similar results for 0.9N-0Mo steel
and 0.9N-1Mo steel. For 0.7N-1Mo steel and 0.8N-0Mo
steel, after potentiostatic crevice corrosion tests of 0.2 V
(SCE), the ratio of the nitrogen content eluted into the
solution calculated from weight loss measurements and the
nitrogen content calculated from stoichiometric NH4
þ
value
is almost 1, indicating that almost all the nitrogen in the
solution 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 localized
corrosion. The results in the present work coincide with the
report by Osozawa et al.2)
regarding the presence of NH4
þ
in
the solution in the vicinity of natural potential. According to
these results, the amount of NH4
þ
formed from Hþ
in the
pitting area and N counterbalances the amount of NH4
þ
as
nitrogen eluted from the steel. Results also coincide in that
the formation of NH4
þ
promotes a repassivation effect as it
raises the pH in the pitting area. After a potentiostatic crevice
corrosion test in the high potential region of 0.7 V (SCE),
small nitrogen content remained in existence as, apart from
NH4
þ
, NO3
À
and NO2
À
.
For the 0.9N-0Mo steel with a high nitrogen content, after
the potentiostatic crevice corrosion test at 0.2 V (SCE), the
ratio of the nitrogen value into the solution measured from
(a) (c)
(d)
0
0.5
1
1.5
0
0.5
1
1.5
0 0 0 0 0
0.7N-1Mo
0.2V(SCE) 72h
0.7N-1Mo
0.7V(SCE) 72h
Detectednitrogen/dissolvednitrogen
NH4
+
NO2
-
NO3
-
C
C(b)
0
0.5
1
1.5
0
0.5
1
1.5
0 0 0 0
0.8N-0Mo
0.2V(SCE) 72h
0.8N-0Mo
0.7V(SCE) 72h
NH4
+
NO2
-
NO3
-
(c) 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35o
C
C
Fig. 12 Ratio between the total amount of N from the steel and NH4
þ
, NO2
À
and NO3
À
dissolved after the crevice corrosion tests, (a):
0.7N-1Mo steel, 0.2 V (SCE); (b): 0.7N-1Mo steel, 0.7 V (SCE); (c): 0.8N-0Mo steel, 0.2 V (SCE); (d): 0.8N-0Mo steel, 0.7 V (SCE).
(a)
(b)
(c)
(d)
0
0.5
1
1.5
0
0.5
1
1.5
0 0 00 0
0.9N-0Mo
0.2V(SCE) 72h
0.9N-0Mo
0.7V(SCE) 72h
NH4
+
NO2
-
NO3
-
C
C
0
0.5
1
1.5
0
0.5
1
1.5
2
00
0.9N-1Mo
0.2V(SCE) 72h
0.9N-1Mo
0.7V(SCE) 72h
NH4
+
NO2
-
NO3
-
C
C
Fig. 13 Ratio between the total amount of N and NH4
þ
, NO2
À
and NO3
À
dissolved after the crevice corrosion tests, (a): 0.9N-0Mo steel,
0.2 V (SCE); (b): 0.9N-0Mo steel, 0.7 V (SCE); (c): 0.9N-1Mo steel, 0.2 V (SCE); (d): 0.9N-1Mo steel, 0.7 V (SCE).

weight loss to the stoichiometric nitrogen value calculated
from NH4
þ
in the solution was almost 1, conﬁrming that
almost all the nitrogen in the steel solid solution was present
in the solution as NH4
þ
after elution. In the case of the high
potential of 0.7 V (SCE), for 0.9N-0Mo steel and 0.9N-1Mo
steel, the nitrogen eluted forms besides NH4
þ
, NO3
À
and
NO2
À
. The amount of NO3
À
shows a tendency to increase,
as does the nitrogen content in the steel. As shown in Fig. 6
and 7, as the nitrogen content in solid solution in the steel
increases, the number of crevice corrosion spots and the
corrosion weight loss decrease, indicating that when NO3
À
was present in the solution, the number of crevice corrosion
spots in the high potential region was suppressed.
3.5 Repassivation characteristics after exposing new
surface by scratch test
Figure 14 shows the current density decay caused by the
appearance of a new surface after scratching of 0.2 cm with
the load of 200 g, which was made on a 0.7N-1Mo steel
immersed in a solution of 1 kmol/m3
NaCl + 0.1 kmol/m3
HCl at a potential of 0.2 V (SCE). There was an instantaneous
surge in current density when a new surface appeared after
the passivation ﬁlm was destroyed by scratching, and the
value of current went back to 0 mA/cm2
when repassivation
instantaneously occurred. This maximum value of current
density was the peak. After 3.6 seconds, the total quantity of
electricity corresponds to the sum of the current due to
repassivation and the current caused by dissolution of the
0.7N-1Mo steel. It can be observed that the lower the peak of
current density and total quantity of electricity, the easier
repassivation occurs.
Figure 15 (a) and (b) illustrate the relationship between
potentials and peaks of current density for 0.7N-1Mo steel
and 0.9N-1Mo steel immersed in a solution of 1 kmol/m3
NaCl + 0.1 kmol/m3
HCl, after using loads of 100 g and
200 g to make a 0.2 cm scratch respectively. For both steels,
the peak of current density increases as the potential
increases, and it also increase with the scratching load. On
the other hand, the peak is lower for the steel with the higher
nitrogen content, suggesting that the nitrogen in solid
solution in the steel promotes the repassivation.
Figure 16 illustrates the relationship between potential and
total quantity of electricity for repassivation for 0.7N-1Mo
steel and 0.9N-1Mo steel immersed in a solution of 1
kmol/m3
NaCl + 0.1 kmol/m3
HCl, after using a load of
200 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 to
increase as the potential increases. In the case of 0.9N-1Mo
steel, the electricity remains almost constant up to the
potential region of 0.7 V (SCE), but in the transpassivation
region there is a sudden increase in the value of total quantity
of electricity the same as in the case of 0.7N-1Mo. The high
nitrogen content in solid solution in the steel reduced the
value 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
Currentdensity,I/Am-2
0
0.01
0.02
0.03
0.04
0.05
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 0.2 V (SCE) in a 1 kmol mÀ3
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/m
3
NaCl+0.1kmol/m
3
HCl
200g scratch 2cm
0.7N-1Mo
0.9N-1Mo
Quantityofelectricity,Q/Cm
-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 kmol mÀ3
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/m
3
NaCl+0.1 kmol/m
3
HCl
100g scratch 0.2cm
0.7N-1Mo
0.9N-1Mo
Peakcurrentdensity,I/Am
-2
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
(b) 1 kmol/m
3
NaCl+0.1 kmol/m
3
HCl
200g scratch 0.2cm
0.7N-1Mo
0.9N-1Mo
Peakcurrentdensity,I/Am
-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 kmol mÀ3
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 with
modified take-off angles for a stainless steel with a nitrogen
content of approximately 1%, Sagara et al.13)
have suggested
the possibility that there is a high nitrogen concentration in
the inner layer of the passivation film, and that this nitrogen
concentration in the inner layer increases with the polar-
ization potential. From these observations, it is established
that the nitrogen concentration in the inner layer of the
passivation film has the effect of promoting repassivation.
On the other hand, it has been reported that in a NO3
À
solution at stable high potentials, NO3
À
tends to increase
with the potential, and the number of crevice corrosion spots
as well as weight loss due to corrosion are markedly
suppressed.18)
It was also reported that the presence of NO3
À
in the solution increases the resistance to pitting corrosion,21)
and that NO3
À
has a controlling effect on pitting corrosion at
the high potential region.22,23)
In the present research, when
NO3
À
is present in an acidic chloride aqueous solution, a
considerable decrease in electric current is observed at high
electric potential, corroborating that the crevice corrosion is
controlled. At relatively low potentials, solidified nitrogen
dissolves with crevice corrosion to produce NH4
þ
in the
solution and control the acidification inside the pit.2)
In the present research, increase of the nitrogen content in
nitrogen-bearing austenitic steel results in (1) decreased of
number of crevice corrosion spots and the corrosion weight
loss, with a tendency to further decrease as the polarization
potential reaches high values; (2) the amount of NO3
À
eluted
into the solution showed a tendency to increase, and at the
same time, the eluted NO3
À
was adsorbed onto the surface of
the passivation film, which had a inhibitor effect suppressing
the dissolution of the base metal; (3) the peak current density
and the total quantity of electricity for the repassivation
process decreased and indicated a high corrosion resistance
which promotes repassivation.
4. Conclusions
Austenitic stainless steels with a nitrogen content ranging
from 0.7 to 1 mass% were produced by the electroslag
remelting (ESR) method and the effect of nitrogen on the
crevice corrosion in an acidic chloride aqueous solution as
well as XPS analyses of the surface film after crevice
corrosion were carried out. At the same time, the repassiva-
tion process after scratching of the passivation film was
observed, 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 the
solution and was present as NO3
À
, The concentration of
nitrogen in the solution showed a tendency to increase
with the nitrogen content in the steel. The number of
crevice corrosion spots, the corrosion weight loss, and
the maximum depth of corrosion decreased with the
increase in nitrogen content, and further decreased with
the values of polarization potential.
(2) The NO3
À
eluted into the aqueous solution was
adsorbed onto the surface of the passivation film, and
acted as an inhibitor preventing the dissolution of the
base material.
(3) As the nitrogen content added to the steel and the
polarization potential increased, the current density
peak 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 in
the form of nitrides or as NH3 at the crevice corrosion
surface film and the inner layer of passivation film.
REFERENCES
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125–133.
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(Corros. Eng.) 24 (1975) 1–7.
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