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The effect of permeated hydrogen on the pitting of type 304 stainless steel
Corrosion Science, Vol. 40, No. 4/S, pp. 781-791, 1998 XJ 1998 Published by Elsevier Science Ltd. All rights reserved.
Pergamon
Printed in Great Britain. OOla-938X/98 $19.00+0.00
PII: SOOlO-938X(97)00178-9
THE EFFECT
OF PERMEATED HYDROGEN ON THE PITTING OF TYPE 304 STAINLESS STEEL
H. YASHIRO,” “Department
‘Presently,
B. POUND,h
N. KUMAGAI”
and K. TANNO”
of Molecular
Science and Applied Chemistry, Faculty of Engineering, Iwate University, Ueda, Morioka, 020 Japan %RI International, 333 Ravenswood Ave., Menlo Park, CA 94025 U.S.A. Center for Research and Development, Iwate Technology Foundation, 3-35-2, lokashinden, Morioka, 020 Japan
Abstract-The effect of permeated hydrogen on the pitting behavior of type 304 stainless steel was investigated using a Devanathan type cell. One side of the stainless steel sheet was galvanostatically charged with hydrogen, while the other side was subjected to pitting tests. The permeated hydrogen typically enhanced the pitting susceptibility of the stainless steel; the pitting potential during potentiodynamic polarization became less noble and the induction period for pitting became shorter. However, the permeated hydrogen did not change the critical pitting potential determined potentiostatically. The effect of permeated hydrogen on the pitting potential and the induction period diminished after hydrogen charging was stopped. Analysis by X-ray photoelectron spectroscopy showed that the cationic fraction of chromium in a hydrogen-charged passive film was lower than that in an uncharged film. Thus, the permeated hydrogen is thought to suppress the aging process of the passive film with respect to enrichment of chromium. 0 1998 Published by Elsevier Science Ltd. All rights reserved Key\cor&:
A. stainless
steel. B. hydrogen
permeation,
C. pitting corrosion.
INTRODUCTION A cathodic reaction commonly involved in the corrosion of metals is hydrogen evolution.’ It can be the cathodic reaction for not only general corrosion but also localized corrosion of stainless steel, as demonstrated by Yashiro and Tanno for high temperature acidic chloride solutions.’ Some of the atomic hydrogen generated on a corroding surface can enter the metal. Although the diffusivity of hydrogen in austenitic stainless steels is very low (N lo-l2 cm2 s-‘)~ because of the f.c.c. structure, it has been shown that the uptake of hydrogen is possible under open circuit conditions in deaerated 0.1 M Na2S04 solution at 368 K.4 Hydrogen entering a passive film could affect the structure or composition of the film. In spite of extensive work on hydrogen embrittlement of steels, limited data are available on hydrogen-induced changes in the resistance of passive films to localized corrosion. Hasegawa and 0sawasx6 showed that stainless steel containing hydrogen absorbed under high temperature and high pressure suffered anomalous corrosion, which might be caused by the formation of activated microdefects and destabilization of passive
Manuscript
received 4 July 1997; in amended
form 6 October 781
1997
182
H. Yashiro
rt cl/.
films. Armacanqui and Oriani.’ using a Devanathan type cell, found that passivated nickel became less resistant to pitting by permeated hydrogen. Pyun and Oriani8 studied the pitting behavior of iron using a gas phase hydrogen charging technique and proposed that passive films became less resistant to pitting because of the increase in hydrogen containing species, i.e., OH- or HzO, within them. Since hydrogen is known to cause damage (such as the formation of hydrides or a phase transition from austenite to martensite) on the input side of steel to a depth of 10 pm,” the exit side for hydrogen permeation is better suited to study the interaction between hydrogen and a passive film. Because the generation of pits is very sensitive to the nature of the passive film, it is expected that the interaction between hydrogen and the film will change the pitting susceptibility. In the present study. the effect of permeated hydrogen on the pitting behavior of type 304 stainless steel was investigated using the hydrogen permeation technique.
EXPERIMENTAL Commercially available type 304 stainless steel foil (50pm thick) was cut into a disk, electropolished in Cr03-H3P04H70 at 0.07 A cm-’ for 30 s at room temperature and then rinsed with pure water ultrasonically. The chemical composition (in wt.%) of the specimen was 18.24 Cr, 7.95 Ni, 0.16 MO, 0.75 Cu, 0.78 Mn, 0.076 C, 0.55 Si, 0.048 P, 0.0088 S. Both sides of the specimen were sealed with vinyl chloride adhesive tape such that the desired areas were left exposed on the respective sides for hydrogen charging and the pitting test (Fig. 1). Both sides of the specimen were again electropolished briefly and rinsed with pure water. The specimen was placed in a Devanathan type cell”’ that contained 260cm3 of deaerated 0.1 M Na,SO, in the hydrogen charging compartment (side A) and either NaCl (0.1 or 0.5 M) or (0.1 or 0.5 M) NaCl+O.l M Na$O, in the other compartment (side B). The diameter of the O-rings used for sealing was 36mm. Both solutions were deaerated with Ar throughout the experiment. All the experiments were conducted at 343 K because the diffusion of hydrogen through the specimen was too slow at lower temperatures. The potentials were measured with respect to a Ag/AgCl reference electrode. Side A of the foil was charged galvanostatically (- 1 mA cm-‘) with hydrogen for 24 h, while the corrosion potential of side B of the foil was monitored. A potential scan was then applied in the anodic direction to side B, beginning at the corrosion potential. For comparison, an anodic polarization curve for a specimen without hydrogen charging was measured after 24 h of exposure. In another set of experiments, the potential of side B of a specimen was stepped to a given value and the current was followed until pitting started. Thus, induction periods for pitting were measured as a function of applied potential for both charged and uncharged specimens. This test was performed at different applied potentials until the critical pitting potential was found. Hydrogen was charged generally using type (a) assembly (Fig. 1). The change in the corrosion potential of side B when hydrogen permeation occurs was clearest in this case. It was attempted to reduce the charging area, as shown in Fig. l(b), to minimize the effect of the crevice between the vinyl tape and stainless steel. The effect of permeated hydrogen on the pitting susceptibility could also be observed in this case, but the potential response of side B with hydrogen permeation was not as sharp as with the type (a) design and the difference in pitting potential between charged and uncharged specimens was less pronounced. The passive film was analyzed by using X-ray photoelectron spectroscopy (XPS). In the
The effect of permeated
hydrogen
on the pitting
of type 304 stainless
steel
783
Heater (343 K)
side B
side A
side B
side A
(c) Fig. 1. Assembly of the Devanathan type of cell and details of specimens for pitting tests (a,b) and for surface analysis by XPS (c).1: specimen (50pm thick), 2: Pt counter electrode, 3: Ag/AgCI reference electrode, 4: vinyl chloride adhesive tape (100 pm thick), 5: O-ring.
case of specimens to be used for analysis, side B in the type (c) design was charged locally with hydrogen for 24 h. The surface of side B was then analyzed to compare the composition of the permeated area (site a) with that of the unpermeated area (site b). A PHI 5600 XPS system was used with an X-ray source (MgKu) of 1253.6 eV, a pass energy of 93.9 eV, and an analysis diameter of 800pm. The atomic ratio of each element was calculated by using the application software available with the PHI 5600 system, after the manual subtraction of the background determined by integration.
RESULTS
AND
DISCUSSION
Permeation behavior of hydrogen Figure 2 shows the variation of the corrosion potential of side B with charging time. Also shown is the anodic current density for permeated hydrogen when side B was held
784
H. Yashiro
or ul.
9
E 8
-200 W 5
; . x
P > E
. Z g 5 a
-400
-600
-800 0”
0 20
40 Time/
Fig. 2.
Typical
variation
60
80
h
of the corrosion potential and oxidation current permeation. Solution: 0.1 M Na$O, (side A,B)
density
for hydrogen
at 400mV. The corrosion potential of side B fell sharply after about 10 h of charging and became almost steady after 20 h. The anodic current density for side B increased after several hours of charging, which corresponded to the fall in corrosion potential. The almost-steady value of the corrosion potential occurred when the anodic current density became greater than 0.3 ~Acm~‘. The further increase in permeation current density scarcely affected the corrosion potential of side B. Although the permeation current increased further for several days at the test temperature, pitting experiments were typically conducted after 24 h of charging when the corrosion potential was sufficiently stable. Figure 3 shows the pH dependence of the steady-state corrosion potential for side B after 24 h of charging. The corrosion potentials closely paralleled the respective hydrogen electrode potentials, although the correspondence was not exact. The deviation in the negative direction became prominent in acidic solutions, suggesting that corrosion of the specimen was accelerated on side B. qf‘ permeated hydrogen on pitting potentiul Figure 4 shows three typical anodic polarization curves in deaerated 0.5 M NaCl+O. 1 M Na,SO, solution. Curve (a) was measured in the absence of hydrogen charging just after the solution was heated to 343 K. Curve (b) was measured under similar conditions except that the specimen was left for 24 h in the open circuit condition at 343 K before the anodic scan. In case of curve (c), the foil was charged with hydrogen for 24 h and then the pitting test was started. The shape of the polarization curve was not affected by whether or not the charge was stopped just before the anodic scan. The pitting potential typically shifted to more noble values when the specimen was left &f&t
The effect of permeated
-200
hydrogen
on the pitting
\ 1
:
\
I
-
\ ‘CL
-600
-800
\
-
’
0
’ 2
steel
\ \
-400
of type 304 stainless
*
’ 4
’
’ 6
’
’ 8
\
\
’
10
pH(343K) Fig. 3.
Effect of pH on the corrosion
potential
of side B after 24 h of hydrogen
charging.
IO4 lo3 102 10’ loo 10-l
(cl
1o‘2 10”
I.
-600
-400
I.
-200 Potential
I.
0
I.
200
I.
400
600
/ mV vs SHE
Fig. 4. Effect of permeated hydrogen and aging of the passive film on anodic polarization curves for type 304 stainless steel.(a) after 24 h of hydrogen charging, (b) after 24 h of standing without hydrogen charging, (c) just after the solution was heated to 343 K. Solution: 0.5 M NaCI+O.l M Na,SO,, Scan rate: 10 mV min ’
785
786
H. Yashiro
el al
standing in solution, which suggested that the passive film became more stable. When the specimen was charged, however, this shift in pitting potential was not observed. Thus, hydrogen appears to impede the aging process of the passive film and thereby prevent the pitting susceptibility from becoming less pronounced with time. Alternatively, aging may occur, but hydrogen would then have to enhance the pitting susceptibility such that the pitting potential shows little change. Each specimen was examined after the pitting tests using scanning electron microscopy, which showed no apparent difference in pit morphology between charged and uncharged specimens.
.Fffkct
qf permeated
hydrogen
on criticul
pitting
potential
and
induction
period
Figure 5 shows the effect of applied potential on the induction period for pitting of charged and uncharged specimens. Each potential was applied after the specimens were left standing in 0.5 M NaCl (Fig. 5(a)) or 0.5 M NaClf0.1 M Na,SO, (Fig. 5(b)) for 24 h, during which one specimen was charged with hydrogen while the other remained uncharged. Apparently, the induction periods for pitting for charged specimens were shorter than those for uncharged specimens. In Fig. 5(a), the relationship between the logarithm of induction period and the applied potential is roughly linear above 200mV, whereas the induction period increased sharply at 180mV. The difference in induction period between the charged and uncharged specimens increased with potential. The critical pitting potential was regarded as the potential where the induction period increased sharply. The data in Fig. 5(b) showed similar tendency to those in Fig. 5(a). Thus, the permeated hydrogen appears to affect the structure of the passive film since the induction period is influenced by charging. However, the microstructure of the steel surface is evidently not changed because the critical pitting potential is unaffected by hydrogen. Figure 6 shows anodic polarization curves for charged specimens as a function of time after hydrogen charging is stopped. Each specimen was left in the cell in succession after the charging was stopped. After 24 h, hydrogen was still present to be anodically oxidized. After 96 h, the oxidation current of hydrogen diminished but the pitting potential was still low, which indicates that dissolved hydrogen ions formed at side B by the oxidation of permeated hydrogen are unimportant for promoting pitting. As seen after 216 and 288 h, the pitting potential became more noble with standing time. Thus, the effect of permeated hydrogen on the pitting susceptibility was shown to be reversible. Figure 7 shows another plot to confirm that there was not a significant difference in critical pitting potential between the charged and uncharged specimens. In this figure, dynamically determined pitting potentials. as shown in Fig. 3, are plotted as a function of square root of the potential sweep rate (v) with the critical pitting potentials (determined from Fig. 5) shown at v = 0. As found by Shibata and Takeyama,” the pitting potential depends linearly on the square root of the potential sweep rate, and extrapolation to zero gives the critical pitting potential. This dependence is clearly demonstrated in Fig. 7. The slope of the line was steeper for the hydrogen charged specimens, but the respective lines for the two solutions converged on an almost identical critical pitting potential in each case. Thus, the difference in pitting potential observed in dynamic polarization curves (Fig. 4) can be attributed to the difference in induction period, not in critical pitting potential.
The effect of permeated
hydrogen
on the pitting
of type 304 stainless
787
steel
lo4
‘Z . -0 .z?
lo3
lo*
x 5 ‘Z
10’
2 + loo
: with hydrogen A : without hydrogen
0
I
I ‘O“lo0
300
200
500
400
Applied potential / mV vs SHE
A : without hydrogen I
10-l 200
300
.I. 400
I.
I.
500
600
I 700
. 800
Applied potential / mV vs SHE Fig. 5.
Effect
Effect of permeated Solution:
hydrogen on the induction period for pitting of type 304 stainless 0.5 M NaCl (a) and 0.5 M NaCl +O. I M Na$O, (b)
steel.
of nitric acid treatment
It is well known that surface treatment of stainless steel in nitric acid results in a higher resistance to pitting because of modification of the passive film.r2 In the present study, a specimen was pretreated in boiling 2 M HN03 solution for 15 min. Specimen assembly (b) in Fig. 1 was used. To avoid interference with the entry of hydrogen by the passive film improved in HN03 solution, side A was electroplated with palladium
788
H. Yashiro
1
I
lo3
CI 01.
I
Time after the stop of hydrogen
j
charging:
24h
96h 216h288h
d
Potential / mV vs SHE Fig. 6.
Recovery of pitting potential with standing time after 24h of hydrogen - 1mAcm_‘. Solution: 0.5 M NaCI+O.l M Na$O,, scan rate: 100mVmin
charging ‘.
1000
0, 0
*
’ 2
.
’ 4
*
’ 6
.
’ 8
.
’ 10
*
12
Square root of sweep rate / (mV min-l)l’* Fig. 7. Pitting potentials as a function of sweep rate for hydrogen (0, 0) specimens. Solution: xM NaCI+O.l M Na?SO, (0.
l
charged (0, n ) and uncharged : x = 0.5. 0,m: x = 0.1)
at
The effect of permeated
hydrogen
on the pitting
of type 304 stainless
steel
789
for 100 s at - 10 mA/cm’ in PdCI,+NH,Cl solution. Figure 8 shows the polarization curves for charged and uncharged specimens with and without nitric acid treatment. The oxidation current density of permeated hydrogen is smaller than that shown in Fig. 4 because side A has a smaller area than side B. Although the oxidation current for permeated hydrogen through the HNO,-treated specimen (curve c) was only a little smaller than that through the untreated specimen (curve a), the pitting potential is shifted dramatically (almost 300mV) by the nitric acid treatment, indicating a large increase in stability of the passive film. Comparison of curves (b) and (c) shows that the effect of nitric acid treatment essentially overcame that of permeated hydrogen. Thus this fact suggests that hydrogen impedes stabilization of initially-formed passive films rather than degrading films that are already stable. On the other hand, curves (c) and (d) show that hydrogen still has a significant effect on the pitting potential of HNO,treated specimens with their stable films.
XPS analysis A specimen of type (c) shown in Fig. 1 was charged locally with hydrogen. After 24 h of charging, two sites were cut out of the specimen and analyzed by XPS. Table 1 summarizes the cationic fraction in the passive films. It can be seen that the hydrogen charged part (site a) is less enriched with chromium. Similar experiments for specimens treated with nitric acid showed almost no difference in cationic fraction between charged
-500
0 Potential
500
1000
/ mV vs SHE
Fig. 8. Effect of nitric acid treatment on the pitting behavior of type 304 stainless steel with (a,c) and without (b,d) permeated hydrogen. Specimens (c) and (d) were passivated in boiling 2 M HNO, for 15min in advance. Solution: 0.5 M NaCI for side B, Scan rate: 20 mV min’, Specimen assembly: type (b) in Fig. 1
790
H. Yashiro Table
I.
Effect of permeated
hydrogen
et ul.
and nitric acid treatment passive films
Specimen Sampling
site in Fig. l(c)
Cationic
Nitric acid treatment’ ___________
Site a (charged) Site b (uncharged) Site a (charged) Site b (uncharged) As mechanically polished ’ passivated
on cationic
no no yes yes
in boiling 2 M HNO,
solution
fraction
fraction
in the
(at “I&)
Fe
Cr
Ni
66.2 43.0 30.5 31.5 76.3
32.9 56.3 69.5 68.5 23.3
0.9 0.7 0.4
for I5 min
and uncharged parts. Thus, the modification process, as mentioned above, appears to be related to the enrichment of chromium in the passive film. The XPS data also showed a difference between sites a and b in the shape of the 0 1 s spectra. The 0 1 s spectrum for the charged part (site a) indicated a larger amount of OH-, while the ratio of 02-/OHwas higher for site b. These results are consistent with observations by Pyun et a1.13 who explained the poor resistance of hydrogen-permeated iron to pitting in terms of the fact that Cll is more likely to replace OH- than 02-. In case of stainless steels, the resistance to pitting is usually related to the cationic fraction of chromium rather than to the chemical state of oxygen. However, the chemical properties of chromium (III) hydroxide are changed considerably by the aging process; chromium (III) hydroxide, as precipitated, is dissolved easily by acid while the aged precipitate is sparingly soluble because of three-dimensional polymerization through olation or oxolation.‘4 This process should be facilitated if the hydrogen ion formed is removed from the film/solution interface. Permeated hydrogen should be oxidized after it arrives at the metal/oxide interface, but the exact events are unclear.” The hydrogen ion formed might be incorporated into the oxide film or migrate/diffuse to the film/solution interface. In either case, the presence of hydrogen ions should either enrich the hydrogen content of the passive film or retard the decrease in hydrogen content, both possibilities being consistent with the 0 1 s spectra in terms of hydrogen content. Finally, it should be noted that the XPS measurements showed no indication of film thinning in the hydrogen permeated part, although these measurements are performed es-situ. The possibility that permeated hydrogen increases the local concentration of hydrogen ions within a breakdown site of the passive film can be discounted, as discussed for Figs 6 and 8. Thus, the stability of the passive film is the most important factor in terms of the effect of permeated hydrogen on pitting. CONCLUSION The effect of hydrogen on the pitting investigated using a hydrogen permeation obtained: (1) Permeated
hydrogen
increased
the pitting
susceptibility technique. susceptibility
of type 304 stainless steel was The following conclusions were of the stainless
steel.
The effect of permeated
hydrogen
on the pitting
of type 304 stainless
steel
791
(2) The enhanced pitting susceptibility was associated with the aging process of passive films, the aging process being retarded by permeated hydrogen. (3) The permeated hydrogen caused a shortening of the induction period for pitting but had no effect on the critical pitting potential. (4) Aging of the passive film resulted in enrichment of chromium within the film. The chemical composition of a highly aged passive film, such as that produced by a nitric acid treatment, was not affected significantly by permeated hydrogen. Thus, the enhancement of pitting susceptibility by permeated hydrogen became less pronounced with a nitric acid treatment. Acknowledgements-The XPS analysis was carried out primarily at the Institute for Materials Research, Tohoku University under the inter-university cooperative program. The authors are grateful to Dr. A. Kawashima, Dr. K. Asami and Prof. K. Hashimoto for advice in the analysis of XPS data. The authors acknowledge Mr. S. Saito of Iwate University for his assistance in the experiment.
REFERENCES M.G., Corrosion Engineering. McGraw Hill. 1986, p. 16. Yashiro, H. and Tanno, K. Zairyo to Kankyo, 1994,43, 371. Zakroczymsky, T., Srklarska-Smialowska, Z. and Sialowski, M. Corrosion, 1983,39, 207. Ohnaka, N. and Furutani, Y. Corrosion, 1990, 46, 129. Osawa, M. and Hasegawa, M. Trans. ISIJ, 1981, 21,464. Hasegawa, M. and Osawa, M. Corrosion, 1980, 36, 67. Armacanqui, M.E. and Oriani, R.A. Corrosion, 1988, 44, 696. Pyun, Su-11, Oriani, R.A. Corros. Sci., 1989, 29, 485. Rozenak, P. and Eliezer, D. Acfa Metall., 1987, 35, 2329. Devanathan, M.A.V., Stachrski, 2. and Beck, W. .I. Electrochem. Sot., 1963, 110,886. Shibata, T. and Takeyama, T. Corrosion, 1977, 33, 243. Asami, K. and Hashimoto, K. Corros. Sci., 1979, 19, 1007. Pyun, S.u-11, Lim, C. and Oriani. R.A. Corros. Sci., 1992, 34, 437. Bailar, J.C.. Emeleus, H.J., Nyholm, R. and Trotman-Dickenson, A.F., Comprehensive Inorganic Chemistry, Vol. 3. Pergamon Press, New York, 1973, p. 666. 15. Casanova, T. and Crousier, J. Corros. Sci., 1996, 38. 1535.
1. Fontana,
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Pergamon
Printed in Great Britain. OOla-938X/98 $19.00+0.00
PII: SOOlO-938X(97)00178-9
THE EFFECT
OF PERMEATED HYDROGEN ON THE PITTING OF TYPE 304 STAINLESS STEEL
H. YASHIRO,” “Department
‘Presently,
B. POUND,h
N. KUMAGAI”
and K. TANNO”
of Molecular
Science and Applied Chemistry, Faculty of Engineering, Iwate University, Ueda, Morioka, 020 Japan %RI International, 333 Ravenswood Ave., Menlo Park, CA 94025 U.S.A. Center for Research and Development, Iwate Technology Foundation, 3-35-2, lokashinden, Morioka, 020 Japan
Abstract-The effect of permeated hydrogen on the pitting behavior of type 304 stainless steel was investigated using a Devanathan type cell. One side of the stainless steel sheet was galvanostatically charged with hydrogen, while the other side was subjected to pitting tests. The permeated hydrogen typically enhanced the pitting susceptibility of the stainless steel; the pitting potential during potentiodynamic polarization became less noble and the induction period for pitting became shorter. However, the permeated hydrogen did not change the critical pitting potential determined potentiostatically. The effect of permeated hydrogen on the pitting potential and the induction period diminished after hydrogen charging was stopped. Analysis by X-ray photoelectron spectroscopy showed that the cationic fraction of chromium in a hydrogen-charged passive film was lower than that in an uncharged film. Thus, the permeated hydrogen is thought to suppress the aging process of the passive film with respect to enrichment of chromium. 0 1998 Published by Elsevier Science Ltd. All rights reserved Key\cor&:
A. stainless
steel. B. hydrogen
permeation,
C. pitting corrosion.
INTRODUCTION A cathodic reaction commonly involved in the corrosion of metals is hydrogen evolution.’ It can be the cathodic reaction for not only general corrosion but also localized corrosion of stainless steel, as demonstrated by Yashiro and Tanno for high temperature acidic chloride solutions.’ Some of the atomic hydrogen generated on a corroding surface can enter the metal. Although the diffusivity of hydrogen in austenitic stainless steels is very low (N lo-l2 cm2 s-‘)~ because of the f.c.c. structure, it has been shown that the uptake of hydrogen is possible under open circuit conditions in deaerated 0.1 M Na2S04 solution at 368 K.4 Hydrogen entering a passive film could affect the structure or composition of the film. In spite of extensive work on hydrogen embrittlement of steels, limited data are available on hydrogen-induced changes in the resistance of passive films to localized corrosion. Hasegawa and 0sawasx6 showed that stainless steel containing hydrogen absorbed under high temperature and high pressure suffered anomalous corrosion, which might be caused by the formation of activated microdefects and destabilization of passive
Manuscript
received 4 July 1997; in amended
form 6 October 781
1997
182
H. Yashiro
rt cl/.
films. Armacanqui and Oriani.’ using a Devanathan type cell, found that passivated nickel became less resistant to pitting by permeated hydrogen. Pyun and Oriani8 studied the pitting behavior of iron using a gas phase hydrogen charging technique and proposed that passive films became less resistant to pitting because of the increase in hydrogen containing species, i.e., OH- or HzO, within them. Since hydrogen is known to cause damage (such as the formation of hydrides or a phase transition from austenite to martensite) on the input side of steel to a depth of 10 pm,” the exit side for hydrogen permeation is better suited to study the interaction between hydrogen and a passive film. Because the generation of pits is very sensitive to the nature of the passive film, it is expected that the interaction between hydrogen and the film will change the pitting susceptibility. In the present study. the effect of permeated hydrogen on the pitting behavior of type 304 stainless steel was investigated using the hydrogen permeation technique.
EXPERIMENTAL Commercially available type 304 stainless steel foil (50pm thick) was cut into a disk, electropolished in Cr03-H3P04H70 at 0.07 A cm-’ for 30 s at room temperature and then rinsed with pure water ultrasonically. The chemical composition (in wt.%) of the specimen was 18.24 Cr, 7.95 Ni, 0.16 MO, 0.75 Cu, 0.78 Mn, 0.076 C, 0.55 Si, 0.048 P, 0.0088 S. Both sides of the specimen were sealed with vinyl chloride adhesive tape such that the desired areas were left exposed on the respective sides for hydrogen charging and the pitting test (Fig. 1). Both sides of the specimen were again electropolished briefly and rinsed with pure water. The specimen was placed in a Devanathan type cell”’ that contained 260cm3 of deaerated 0.1 M Na,SO, in the hydrogen charging compartment (side A) and either NaCl (0.1 or 0.5 M) or (0.1 or 0.5 M) NaCl+O.l M Na$O, in the other compartment (side B). The diameter of the O-rings used for sealing was 36mm. Both solutions were deaerated with Ar throughout the experiment. All the experiments were conducted at 343 K because the diffusion of hydrogen through the specimen was too slow at lower temperatures. The potentials were measured with respect to a Ag/AgCl reference electrode. Side A of the foil was charged galvanostatically (- 1 mA cm-‘) with hydrogen for 24 h, while the corrosion potential of side B of the foil was monitored. A potential scan was then applied in the anodic direction to side B, beginning at the corrosion potential. For comparison, an anodic polarization curve for a specimen without hydrogen charging was measured after 24 h of exposure. In another set of experiments, the potential of side B of a specimen was stepped to a given value and the current was followed until pitting started. Thus, induction periods for pitting were measured as a function of applied potential for both charged and uncharged specimens. This test was performed at different applied potentials until the critical pitting potential was found. Hydrogen was charged generally using type (a) assembly (Fig. 1). The change in the corrosion potential of side B when hydrogen permeation occurs was clearest in this case. It was attempted to reduce the charging area, as shown in Fig. l(b), to minimize the effect of the crevice between the vinyl tape and stainless steel. The effect of permeated hydrogen on the pitting susceptibility could also be observed in this case, but the potential response of side B with hydrogen permeation was not as sharp as with the type (a) design and the difference in pitting potential between charged and uncharged specimens was less pronounced. The passive film was analyzed by using X-ray photoelectron spectroscopy (XPS). In the
The effect of permeated
hydrogen
on the pitting
of type 304 stainless
steel
783
Heater (343 K)
side B
side A
side B
side A
(c) Fig. 1. Assembly of the Devanathan type of cell and details of specimens for pitting tests (a,b) and for surface analysis by XPS (c).1: specimen (50pm thick), 2: Pt counter electrode, 3: Ag/AgCI reference electrode, 4: vinyl chloride adhesive tape (100 pm thick), 5: O-ring.
case of specimens to be used for analysis, side B in the type (c) design was charged locally with hydrogen for 24 h. The surface of side B was then analyzed to compare the composition of the permeated area (site a) with that of the unpermeated area (site b). A PHI 5600 XPS system was used with an X-ray source (MgKu) of 1253.6 eV, a pass energy of 93.9 eV, and an analysis diameter of 800pm. The atomic ratio of each element was calculated by using the application software available with the PHI 5600 system, after the manual subtraction of the background determined by integration.
RESULTS
AND
DISCUSSION
Permeation behavior of hydrogen Figure 2 shows the variation of the corrosion potential of side B with charging time. Also shown is the anodic current density for permeated hydrogen when side B was held
784
H. Yashiro
or ul.
9
E 8
-200 W 5
; . x
P > E
. Z g 5 a
-400
-600
-800 0”
0 20
40 Time/
Fig. 2.
Typical
variation
60
80
h
of the corrosion potential and oxidation current permeation. Solution: 0.1 M Na$O, (side A,B)
density
for hydrogen
at 400mV. The corrosion potential of side B fell sharply after about 10 h of charging and became almost steady after 20 h. The anodic current density for side B increased after several hours of charging, which corresponded to the fall in corrosion potential. The almost-steady value of the corrosion potential occurred when the anodic current density became greater than 0.3 ~Acm~‘. The further increase in permeation current density scarcely affected the corrosion potential of side B. Although the permeation current increased further for several days at the test temperature, pitting experiments were typically conducted after 24 h of charging when the corrosion potential was sufficiently stable. Figure 3 shows the pH dependence of the steady-state corrosion potential for side B after 24 h of charging. The corrosion potentials closely paralleled the respective hydrogen electrode potentials, although the correspondence was not exact. The deviation in the negative direction became prominent in acidic solutions, suggesting that corrosion of the specimen was accelerated on side B. qf‘ permeated hydrogen on pitting potentiul Figure 4 shows three typical anodic polarization curves in deaerated 0.5 M NaCl+O. 1 M Na,SO, solution. Curve (a) was measured in the absence of hydrogen charging just after the solution was heated to 343 K. Curve (b) was measured under similar conditions except that the specimen was left for 24 h in the open circuit condition at 343 K before the anodic scan. In case of curve (c), the foil was charged with hydrogen for 24 h and then the pitting test was started. The shape of the polarization curve was not affected by whether or not the charge was stopped just before the anodic scan. The pitting potential typically shifted to more noble values when the specimen was left &f&t
The effect of permeated
-200
hydrogen
on the pitting
\ 1
:
\
I
-
\ ‘CL
-600
-800
\
-
’
0
’ 2
steel
\ \
-400
of type 304 stainless
*
’ 4
’
’ 6
’
’ 8
\
\
’
10
pH(343K) Fig. 3.
Effect of pH on the corrosion
potential
of side B after 24 h of hydrogen
charging.
IO4 lo3 102 10’ loo 10-l
(cl
1o‘2 10”
I.
-600
-400
I.
-200 Potential
I.
0
I.
200
I.
400
600
/ mV vs SHE
Fig. 4. Effect of permeated hydrogen and aging of the passive film on anodic polarization curves for type 304 stainless steel.(a) after 24 h of hydrogen charging, (b) after 24 h of standing without hydrogen charging, (c) just after the solution was heated to 343 K. Solution: 0.5 M NaCI+O.l M Na,SO,, Scan rate: 10 mV min ’
785
786
H. Yashiro
el al
standing in solution, which suggested that the passive film became more stable. When the specimen was charged, however, this shift in pitting potential was not observed. Thus, hydrogen appears to impede the aging process of the passive film and thereby prevent the pitting susceptibility from becoming less pronounced with time. Alternatively, aging may occur, but hydrogen would then have to enhance the pitting susceptibility such that the pitting potential shows little change. Each specimen was examined after the pitting tests using scanning electron microscopy, which showed no apparent difference in pit morphology between charged and uncharged specimens.
.Fffkct
qf permeated
hydrogen
on criticul
pitting
potential
and
induction
period
Figure 5 shows the effect of applied potential on the induction period for pitting of charged and uncharged specimens. Each potential was applied after the specimens were left standing in 0.5 M NaCl (Fig. 5(a)) or 0.5 M NaClf0.1 M Na,SO, (Fig. 5(b)) for 24 h, during which one specimen was charged with hydrogen while the other remained uncharged. Apparently, the induction periods for pitting for charged specimens were shorter than those for uncharged specimens. In Fig. 5(a), the relationship between the logarithm of induction period and the applied potential is roughly linear above 200mV, whereas the induction period increased sharply at 180mV. The difference in induction period between the charged and uncharged specimens increased with potential. The critical pitting potential was regarded as the potential where the induction period increased sharply. The data in Fig. 5(b) showed similar tendency to those in Fig. 5(a). Thus, the permeated hydrogen appears to affect the structure of the passive film since the induction period is influenced by charging. However, the microstructure of the steel surface is evidently not changed because the critical pitting potential is unaffected by hydrogen. Figure 6 shows anodic polarization curves for charged specimens as a function of time after hydrogen charging is stopped. Each specimen was left in the cell in succession after the charging was stopped. After 24 h, hydrogen was still present to be anodically oxidized. After 96 h, the oxidation current of hydrogen diminished but the pitting potential was still low, which indicates that dissolved hydrogen ions formed at side B by the oxidation of permeated hydrogen are unimportant for promoting pitting. As seen after 216 and 288 h, the pitting potential became more noble with standing time. Thus, the effect of permeated hydrogen on the pitting susceptibility was shown to be reversible. Figure 7 shows another plot to confirm that there was not a significant difference in critical pitting potential between the charged and uncharged specimens. In this figure, dynamically determined pitting potentials. as shown in Fig. 3, are plotted as a function of square root of the potential sweep rate (v) with the critical pitting potentials (determined from Fig. 5) shown at v = 0. As found by Shibata and Takeyama,” the pitting potential depends linearly on the square root of the potential sweep rate, and extrapolation to zero gives the critical pitting potential. This dependence is clearly demonstrated in Fig. 7. The slope of the line was steeper for the hydrogen charged specimens, but the respective lines for the two solutions converged on an almost identical critical pitting potential in each case. Thus, the difference in pitting potential observed in dynamic polarization curves (Fig. 4) can be attributed to the difference in induction period, not in critical pitting potential.
The effect of permeated
hydrogen
on the pitting
of type 304 stainless
787
steel
lo4
‘Z . -0 .z?
lo3
lo*
x 5 ‘Z
10’
2 + loo
: with hydrogen A : without hydrogen
0
I
I ‘O“lo0
300
200
500
400
Applied potential / mV vs SHE
A : without hydrogen I
10-l 200
300
.I. 400
I.
I.
500
600
I 700
. 800
Applied potential / mV vs SHE Fig. 5.
Effect
Effect of permeated Solution:
hydrogen on the induction period for pitting of type 304 stainless 0.5 M NaCl (a) and 0.5 M NaCl +O. I M Na$O, (b)
steel.
of nitric acid treatment
It is well known that surface treatment of stainless steel in nitric acid results in a higher resistance to pitting because of modification of the passive film.r2 In the present study, a specimen was pretreated in boiling 2 M HN03 solution for 15 min. Specimen assembly (b) in Fig. 1 was used. To avoid interference with the entry of hydrogen by the passive film improved in HN03 solution, side A was electroplated with palladium
788
H. Yashiro
1
I
lo3
CI 01.
I
Time after the stop of hydrogen
j
charging:
24h
96h 216h288h
d
Potential / mV vs SHE Fig. 6.
Recovery of pitting potential with standing time after 24h of hydrogen - 1mAcm_‘. Solution: 0.5 M NaCI+O.l M Na$O,, scan rate: 100mVmin
charging ‘.
1000
0, 0
*
’ 2
.
’ 4
*
’ 6
.
’ 8
.
’ 10
*
12
Square root of sweep rate / (mV min-l)l’* Fig. 7. Pitting potentials as a function of sweep rate for hydrogen (0, 0) specimens. Solution: xM NaCI+O.l M Na?SO, (0.
l
charged (0, n ) and uncharged : x = 0.5. 0,m: x = 0.1)
at
The effect of permeated
hydrogen
on the pitting
of type 304 stainless
steel
789
for 100 s at - 10 mA/cm’ in PdCI,+NH,Cl solution. Figure 8 shows the polarization curves for charged and uncharged specimens with and without nitric acid treatment. The oxidation current density of permeated hydrogen is smaller than that shown in Fig. 4 because side A has a smaller area than side B. Although the oxidation current for permeated hydrogen through the HNO,-treated specimen (curve c) was only a little smaller than that through the untreated specimen (curve a), the pitting potential is shifted dramatically (almost 300mV) by the nitric acid treatment, indicating a large increase in stability of the passive film. Comparison of curves (b) and (c) shows that the effect of nitric acid treatment essentially overcame that of permeated hydrogen. Thus this fact suggests that hydrogen impedes stabilization of initially-formed passive films rather than degrading films that are already stable. On the other hand, curves (c) and (d) show that hydrogen still has a significant effect on the pitting potential of HNO,treated specimens with their stable films.
XPS analysis A specimen of type (c) shown in Fig. 1 was charged locally with hydrogen. After 24 h of charging, two sites were cut out of the specimen and analyzed by XPS. Table 1 summarizes the cationic fraction in the passive films. It can be seen that the hydrogen charged part (site a) is less enriched with chromium. Similar experiments for specimens treated with nitric acid showed almost no difference in cationic fraction between charged
-500
0 Potential
500
1000
/ mV vs SHE
Fig. 8. Effect of nitric acid treatment on the pitting behavior of type 304 stainless steel with (a,c) and without (b,d) permeated hydrogen. Specimens (c) and (d) were passivated in boiling 2 M HNO, for 15min in advance. Solution: 0.5 M NaCI for side B, Scan rate: 20 mV min’, Specimen assembly: type (b) in Fig. 1
790
H. Yashiro Table
I.
Effect of permeated
hydrogen
et ul.
and nitric acid treatment passive films
Specimen Sampling
site in Fig. l(c)
Cationic
Nitric acid treatment’ ___________
Site a (charged) Site b (uncharged) Site a (charged) Site b (uncharged) As mechanically polished ’ passivated
on cationic
no no yes yes
in boiling 2 M HNO,
solution
fraction
fraction
in the
(at “I&)
Fe
Cr
Ni
66.2 43.0 30.5 31.5 76.3
32.9 56.3 69.5 68.5 23.3
0.9 0.7 0.4
for I5 min
and uncharged parts. Thus, the modification process, as mentioned above, appears to be related to the enrichment of chromium in the passive film. The XPS data also showed a difference between sites a and b in the shape of the 0 1 s spectra. The 0 1 s spectrum for the charged part (site a) indicated a larger amount of OH-, while the ratio of 02-/OHwas higher for site b. These results are consistent with observations by Pyun et a1.13 who explained the poor resistance of hydrogen-permeated iron to pitting in terms of the fact that Cll is more likely to replace OH- than 02-. In case of stainless steels, the resistance to pitting is usually related to the cationic fraction of chromium rather than to the chemical state of oxygen. However, the chemical properties of chromium (III) hydroxide are changed considerably by the aging process; chromium (III) hydroxide, as precipitated, is dissolved easily by acid while the aged precipitate is sparingly soluble because of three-dimensional polymerization through olation or oxolation.‘4 This process should be facilitated if the hydrogen ion formed is removed from the film/solution interface. Permeated hydrogen should be oxidized after it arrives at the metal/oxide interface, but the exact events are unclear.” The hydrogen ion formed might be incorporated into the oxide film or migrate/diffuse to the film/solution interface. In either case, the presence of hydrogen ions should either enrich the hydrogen content of the passive film or retard the decrease in hydrogen content, both possibilities being consistent with the 0 1 s spectra in terms of hydrogen content. Finally, it should be noted that the XPS measurements showed no indication of film thinning in the hydrogen permeated part, although these measurements are performed es-situ. The possibility that permeated hydrogen increases the local concentration of hydrogen ions within a breakdown site of the passive film can be discounted, as discussed for Figs 6 and 8. Thus, the stability of the passive film is the most important factor in terms of the effect of permeated hydrogen on pitting. CONCLUSION The effect of hydrogen on the pitting investigated using a hydrogen permeation obtained: (1) Permeated
hydrogen
increased
the pitting
susceptibility technique. susceptibility
of type 304 stainless steel was The following conclusions were of the stainless
steel.
The effect of permeated
hydrogen
on the pitting
of type 304 stainless
steel
791
(2) The enhanced pitting susceptibility was associated with the aging process of passive films, the aging process being retarded by permeated hydrogen. (3) The permeated hydrogen caused a shortening of the induction period for pitting but had no effect on the critical pitting potential. (4) Aging of the passive film resulted in enrichment of chromium within the film. The chemical composition of a highly aged passive film, such as that produced by a nitric acid treatment, was not affected significantly by permeated hydrogen. Thus, the enhancement of pitting susceptibility by permeated hydrogen became less pronounced with a nitric acid treatment. Acknowledgements-The XPS analysis was carried out primarily at the Institute for Materials Research, Tohoku University under the inter-university cooperative program. The authors are grateful to Dr. A. Kawashima, Dr. K. Asami and Prof. K. Hashimoto for advice in the analysis of XPS data. The authors acknowledge Mr. S. Saito of Iwate University for his assistance in the experiment.
REFERENCES M.G., Corrosion Engineering. McGraw Hill. 1986, p. 16. Yashiro, H. and Tanno, K. Zairyo to Kankyo, 1994,43, 371. Zakroczymsky, T., Srklarska-Smialowska, Z. and Sialowski, M. Corrosion, 1983,39, 207. Ohnaka, N. and Furutani, Y. Corrosion, 1990, 46, 129. Osawa, M. and Hasegawa, M. Trans. ISIJ, 1981, 21,464. Hasegawa, M. and Osawa, M. Corrosion, 1980, 36, 67. Armacanqui, M.E. and Oriani, R.A. Corrosion, 1988, 44, 696. Pyun, Su-11, Oriani, R.A. Corros. Sci., 1989, 29, 485. Rozenak, P. and Eliezer, D. Acfa Metall., 1987, 35, 2329. Devanathan, M.A.V., Stachrski, 2. and Beck, W. .I. Electrochem. Sot., 1963, 110,886. Shibata, T. and Takeyama, T. Corrosion, 1977, 33, 243. Asami, K. and Hashimoto, K. Corros. Sci., 1979, 19, 1007. Pyun, S.u-11, Lim, C. and Oriani. R.A. Corros. Sci., 1992, 34, 437. Bailar, J.C.. Emeleus, H.J., Nyholm, R. and Trotman-Dickenson, A.F., Comprehensive Inorganic Chemistry, Vol. 3. Pergamon Press, New York, 1973, p. 666. 15. Casanova, T. and Crousier, J. Corros. Sci., 1996, 38. 1535.
1. Fontana,
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.