Corrosion Science, Vol. 35, Nos 1-4, pp. 89-96, 1993
0010-938X/93$6.00 + 0.00 Pergamon Press Ltd
Printed in Great Britain.
SYNERGISM OF ALLOYING CORROSION RESISTANCE
ELEMENTS AND PITTING OF STAINLESS STEELS
Y. C. Lu, M. B. IVES and C. R. CLAYTON* Institute for Material Research, McMaster University, 1280 Main St West, Hamilton, Ontario, Canada L8S 4M1 * Department of Materials Science & Engineering, State University of New York at Stony Brook, New York, U.S.A. Abstract--The anodic behaviour of the major pure metals of which stainless steels are composed has been studied in chloride-containing aqueous solutions at neutral and acidic pH values, both before and after a nitriding treatment. This suggests a mechanism by which molybdenum and nitrogen favourably influence the pitting resistance of austenitic stainless steels, and that the mechanisms are different at different stages of pit formation. It is shown that nitrogen is able to prevent the transpassive dissolution of molybdenum from the passive film of austenitic stainless steels. X-Ray photoelectron spectroscopy has shown also that the NH 3 ligand is formed in the anodically formed films on nitrided molybdenum as well as in passive films formed on nitrogen-bearing stainless steels. This raises the local pH in the film/substrate interface which leads the electrode reactions to favour the forming of molybdate, a beneficial species which enhances the passivity of stainless steels, instead of forming M o O 3, the loose transpassive product. INTRODUCTION MOLYBDENUM has b e e n f o u n d to be very effective in improving the pitting resistance of stainless steels in chloride containing media. S u g i m o t o and Sawada 1 have shown that the addition of m o l y b d e n u m to stainless steels decreases the current density in the active region two or three orders of m a g n i t u d e in acidic solutions. Recently, new alloys containing increasing a m o u n t s of alloyed nitrogen have led to i m p r o v e m e n t s in passivation and pitting resistance. 2-4 T h e greatest effect of nitrogen has b e e n observed in m o l y b d e n u m - b e a r i n g steels, suggesting a possible synergism b e t w e e n m o l y b d e n u m and nitrogen. 5-7 M o l y b d e n u m alone, it was suggested, l could form M o 6÷ oxide in the passive film thereby blocking the penetration of C1- attack, or alternatively to decrease the rate of dissolution by the f o r m a t i o n and retention of m o l y b d e n u m oxyhydroxide or m o l y b d a t e at active surface sites, s Sakashita and Sato 9 have suggested that when cation-selective m o l y b d a t e ions adsorb o n t o relatively thick m e m b r a n e s of an anionselective h y d r a t e d iron oxide, they can alter ionic transport t h r o u g h the film by inducing ionic rectification. T h e evidence of a bipolar structure of the passive film of stainless steels was o b s e r v e d by Clayton and Lu.1°-12 The results suggested that m o l y b d e n u m improves the passivity of stainless steels by the f o r m a t i o n of molybdate. A variety of opinions exist on the m e c h a n i s m by which nitrogen improves localized corrosion resistance. T o m a s h o v et al. 13 suggested that nitrogen structurally h o m o g e n i z e s the alloy. F r o m solution analysis, O s o z a w a and O k a t o 3 detected a m m o n i u m in the pitting solution and p r o p o s e d that nitrogen buffers the local p H t h r o u g h the f o r m a t i o n of a m m o n i u m ions. N e w m a n and Shahrabi 14 have suggested that due to the sluggish reaction of nitrogen with protons during anodic dissolution, 89
Y. C. Lu, M. B. IVESand C. R. CLAYTON TABLE 1.
TRACE METALLIC IMPURITY IN METAL SAMPLES
Trace elements (in ppm) Sample
Mo Cr Ni Fe
410 3.1 ---
-3.4 20 60
1 0.4 -1
4 0.5 0.4 0.7
-14 9 --
-A1 8, Ni 1.1 Se 80, Mo 25, Zr 8, Na 9 Co 100, Cr 60, Zn 7
e l e m e n t a l n i t r o g e n enriches in the surface, which inhibits the a n o d i c dissolution at less t h a n m o n o l a y e r coverage by b l o c k i n g the kinks a n d steps in the surfaces. F r o m studies of the cathodic r e a c t i o n kinetics occurring in a c h l o r i n a t e d s i m u l a t e d seawater, Ives et al. 15 suggested that n i t r o g e n m a y f o r m a m m o n i u m which could c o m b i n e with active o x i d a n t s such as free chlorine or b r o m i n e to form less active c h l o r a m i n e or b r o m o - a m i n e , t h e r e b y i n h i b i t i n g pit growth. Surface analysis has p r o v i d e d e v i d e n c e of n i t r o g e n - e n h a n c e d surface s e g r e g a t i o n , resulting in surfaces e n r i c h e d in beneficial e l e m e n t s . 16,17 I n a n i t r o g e n - b e a r i n g stainless steel, the nitrog e n c o n c e n t r a t i o n is limited by its m a x i m u m solubility in solid solution. F o r austenitic stainless steels p r o d u c e d using c o n v e n t i o n a l m e t h o d s a n d alloy c o n t e n t s , this value is in the r a n g e 0.1-0.45 w t % . H o w e v e r , after c o n t i n u e d a n o d i c polarization, A u g e r e l e c t r o n spectroscopy has s h o w n 16 that the surface c o n c e n t r a t i o n of n i t r o g e n can be e n r i c h e d as m u c h as s e v e n times the b u l k c o n c e n t r a t i o n . A t such c o n c e n t r a t i o n s , relatively stable interstitial nitrides are possible. C o n s e q u e n t l y , in discussing the role of n i t r o g e n in i m p r o v i n g the pitting resistance of stainless steels, the role of surface nitrides m u s t be investigated. I n this study, the a n o d i c b e h a v i o u r of the m a j o r p u r e metals of which stainless steels are c o m p o s e d has b e e n c o m p a r e d b e f o r e a n d after n i t r i d i n g t r e a t m e n t in c h l o r i d e - c o n t a i n i n g a q u e o u s solutions at n e u t r a l a n d acidic p H values. T h e results have p r o v i d e d clues for the u n d e r s t a n d i n g of the synergism of the alloying e l e m e n t s in i m p r o v i n g the c o r r o s i o n resistance of stainless steels at distinct stages of pit d e v e l o p m e n t . E X P E R I M E N T A L METHOD Samples were prepared from 1 mm thick iron, nickel, molybdenum and chromium coupons. The analytical data of the trace metallic impurities are listed in Table 1. Samples were wet-ground to 600 grit emery paper (for electrochemical tests) or polished to 0.25/~m diamond (for surface analysis) and cleaned with acetone in an ultrasonic cleaner. Samples were nitrided in a quartz tube in ammonia at 600°Cfor 48 h. X-Ray diffractometry was used to identify the phases present after nitriding. Solutions were made from distilled water and analytical grade reagents. The samples were tested in 0.05 M sodium chloride, and in 0.1 and 1 M hydrochloric acid solutions respectively. The solutions were purged with nitrogen for at least 2 h before the tests and continued during the tests. The solution temperature was kept at 22 _+I°C in all experiments. During the electrochemical tests, the solutions were mechanically stirred. Potentiodynamic measurements were performed in a conventional electrochemical cell. All potentials were measured against a saturated calomel electrode (SCE). Specimens were immersed in the test solution for 30 min until a stable corrosion potential was reached. They were then potentiodynamically polarized in the anodic direction at 0.33 mV s -1. All XPS measurements were performed using a V.G. ScientificESCA 3 MKII spectrometer controlled by a V.G. 1000data system. An AI Ka X-ray (1486.6 eV) source and a 20 eV pass energy were used for all analyses, employing the FWHM for the Au 4f7/2singlet of 1.25 eV as a reference, the binding energy of the
Pitting corrosion resistance of stainless steels
Au 4f7/2 electron was found to be 83.8 eV. By taking the C ls spectra from the adventitious carbon at 284.6 eV, all binding energies were corrected for charge shifts. The XPS measurements were carried out at 5°, 20° and 50° photoelectron take-off angles, measured with respect to the sample surface. Experimental details of the XPS study are discussed elsewhere, is
EXPERIMENTAL RESULTS AND DISCUSSION From the X-ray diffraction spectra obtained on the nitrided chromium, molybdenum and nickel it was found that nitriding produces CrN and Mo2N on chromium and m o l y b d e n u m samples respectively. However, for the nitrided iron, both Fe4 N and Fe2N were found, with a preponderance of the latter. Nitriding does not produce nitrides on pure nickel and there are no significant effects of nitriding on the anodic kinetics on nickel. As discussed previously 19 and seen in Fig. l(a), the anodic current for nitrided iron is more than two orders of magnitude smaller than that for pure iron in a neutral chloride solution. This suggests that the anodically segregated nitrogen in the surface of the nitrogen-bearing stainless steels may form surface iron nitride which can inhibit the dissolution of iron, providing the opportunity for a pit to heal rather than develop an autocatalytic local chemistry. As shown by Luo et al. 2o pit electrolytes eventually become very aggressive, with a p H value close to zero and the activity of chloride ions increasing to 5.5. When samples of the metals before and after nitriding are exposed to solutions which simulate the local p H at various stages of pit development, some indication of the behaviour of high-nitrogen surfaces can be obtained. At low p H values, in solutions similar to those found within pits, the polarization curves for iron and nitrided iron obtained in 0.1 M (Fig. lb) and 1 M hydrochloric acid (Fig. lc) show a different behaviour to that in a neutral solution. The rate of anodic dissolution for nitrided iron is higher than pure iron until the mass transport limited current is reached. This indicates that once a pit is formed and the p H of the pit solution has decreased to a low value, iron nitride can no longer survive and dissolves away. The healing of the pit from this point on then relies on the local enrichment of m o l y b d e n u m and chromium. It was found that in a neutral 0.5 M NaCI solution nitriding does not have strong impact on the anodic behaviour of chromium. However, when the solution becomes acidic, one can see a clear inhibition of the anodic electrode reaction of chromium by nitriding. Figure 2 shows the anodic polarization behaviour of chromium and nitrided chromium in de-aerated (a) 0.1 M HCI and (b) 1 M HC1 solutions. The corrosion potential for nitrided chromium is shifted in the noble direction by 300 mV and 600 m V respectively. In 1 M HCI, the passive current density for the nitrided chromium is also observed to be at least one order of magnitude smaller than for pure chromium in the same solution. Contrary to the behaviour of nitrided iron, chromium nitride is more stable in acidic solution. This is consistent with Auger • "~1 analysis, ~ which suggests that when passivity of the stainless steel breaks down, nitrogen inhibits the anodic dissolution by enhancing the surface enrichment of beneficial elements, principally chromium• Of particular significance is the inhibition of the transpassive reactions of m o l y b d e n u m by nitriding, Figure 3(a) shows the anodic polarization behaviour of m o l y b d e n u m and nitrided molybdenum in de-aerated 0.1 M HCI solution. The corrosion potential for nitrided m o l y b d e n u m is shifted in the noble direction by
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Pitting corrosion resistance of stainless steels
230 mV. The transpassive potential is about 115 mV(SCE) for pure molybdenum. Above that potential, loose black transpassive products are formed which are essentially Mo 5+ and Mo 6+ oxide22 which offer no protection to the molybdenum substrate. However, the transpassive reactions are inhibited greatly by nitriding. Nitrided molybdenum breaks down at 360 mV(SCE) at several weak points while the majority of the sample surface remains passive without any transpassive product formation. In 1 M HC1 solution, nitriding has a similar effect (Fig. 3b). X-Ray photoelectron spectroscopy was also employed to study the influence of nitrogen on the passive and transpassive behaviour of molybdenum in 0.1 M HC1. t8 The XPS spectrum in Fig. 4 is the Mo 3d peak obtained from the anodic film formed at 250 mV in de-aerated 0.1 M HC1 for 1 h. The main peak is contributed by the Mo2N substrate and the passive film comprises two surface species. The doublet having binding energies of 229.1 and 232.3 eV are identified as MOO2. The second doublet (labelled 5 and 6) was previously detected 22'23 as hydrated M o 4+ but according to Kim and Clayton's latest work, 24it may be contributed by Mo 5+ . These results indicate that although 250 mV is beyond the transpassive potential of pure Mo, the surface film is still a typical passive film. The surface is completely different from that on pure molybdenum electrochemically treated at the same condition. 22 At 250 mV pure molybdenum is covered with a thick brown transpassive product which is a mixture of M o 3+, M o 4+, M o 5+ and Mo 6+ oxides. Nitrogen completely inhibits the formation of molybdenum trioxide in the transpassive potential range. Also, the Mo 3+ peak, usually detected on pure molybdenum polarized at 180250 mV, was never detected on nitrided molybdenum samples anodically polarized in the same potential region. Figure 5 is the N is spectrum observed from the anodic film formed on nitrided molybdenum at 250 mV, The N ls peaks overlap the Mo 3p photoelectron peaks (labelled 1, 2, 3), Peak 4 is the nitrogen ls peak contributed by Mo2N. A fifth peak with a binding energy of 399.6 eV is also detected. This peak has the binding energy value for nitrogen in NH s. It is difficult to confirm by XPS whether this NH 3 ligand is
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Y, C. Lu, M. B. IVES and C. R. CLAYTON 1.000
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present in the form of a complex or not. However, it may suggest a local buffering effect of nitrogen at the film/substrate interface. The significance of this observation is that it may explain the synergistic effect between nitrogen and molybdenum in improving pitting corrosion resistance. In previous work, 1°'11 MoO 2- was detected in all passive films formed on molybdenum-bearing alloys in acidic solutions. It was suggested that MoO42- anions are responsible for producing, in 0.1 M HC1, a bipolar film consisting of a cation selective outer layer and an intrinsically anion selective inner layer. The bipolarity of the duplex film was considered to be largely responsible for the development of an
o 4J ,ft C
. . . . . . . . . . .
288 236 234 232 230 BSncl£ng Energy
FiG. 4. Mo 3d photoelectron spectra recorded at 5 ° take-off angle from nitrided molybdenum surface after anodic polarization in de-aerated 0.1 M HCI at 250 mV: (1) Mo 3dmz 228.2 eV; (2) Mo 3d3/2 231.4 eV Mo2N; (3) Mo 3,/5/2 229.1 eV; (4) Mo 3d3/2 232.3 eV MoO 2 ; (5) Mo 3d5/2 230.7 eV; (6) Mo 3d3/2 233.9 eV Mo 5+.
Pitting corrosion resistance of stainless steels
B:).r~cI:I n g
3 8 ca
FIG. 5. Mo 3p and N l s photoelectron spectra recorded at 5° take-off angle from nitrided molybdenum surface after anodic polarization in de-aerated 0.1M HCI at 250 mV: (1) Mo 3p 394.3 eV Mo2N;(2) Mo 3p 395.5 eV MOO2;(3) Mo 3p 397.1 eV Mo5+; (4) N ls 397.3 eV MozN; (5) N ls 399.6 eV NH3.
interracial barrier layer composed mainly of Cr203. This was achieved by enhancing the deprotonation of Cr(OH)3 and resisting C1- and O H - ingress. Both of these properties should provide greater resistance to the breakdown of passivity in chloride-containing media. The same effect was observed by introducing M o O 2- to the passive film formed on stainless steels from solution. 25 According to Pourbaix, 26 molybdate ions are thermodynamically unstable in acidic solution. The thermodynamically stable species, MOO3, at high potential is the transpassive product of molybdenum and this makes no beneficial contribution to the passivity of stainless steels. Nitrogen effectively buffers the local p H at the film substrate interface by forming species with N H 3 ligands. This may shift the electrode reaction to a higher pH level than that of the bulk solution and thereby favouring the formation of molybdate instead of Mo203 and MOO3. This agrees with the observation that no M o 3+ a n d M o 6+ oxide has been found in anodic films formed on nitrided Mo. CONCLUSIONS (1) In a neutral solution, nitrides restrain the anodic dissolution of iron and slow down the acidification in pit sites, thereby hindering the self-catalytic process of pit development. (2) In acidic solutions, nitrided iron dissolves at a higher rate than pure iron. However, chromium and molybdenum nitrides are more stable in acidic than in neutral solutions• This p H dependence of the anodic kinetics of the nitrides may accelerate the anodic segregation of beneficial elements, such as chromium and molybdenum, during localized corrosion, therefore building up a more resistive surface layer at the pit site. (3) The formation of surface nitrides on the surface of anodically polarized stainless steel inhibits the transpassive dissolution of molybdenum• This effectively retains molybdenum in the passive surface, and thereby improves the localized corrosion resistance of stainless steels.
Y.C. Lu, M. B. IvEs and C. R. CLAYTON
Acknowledgements--This work was supported by grants from Chemetics International Company Ltd, The University Research Incentive Fund (Province of Ontario), and the Natural Sciences and Engineering Research Council of Canada. The work done in SUNY/Stony Brook was supported by a grant from U.S.O.N.R. (Dr A. J, Sedriks, Contract Officer), contract number: N0001485K0437. REFERENCES 1. K. SUGIMOTOand Y. SAWADA,Corros. Sci. 17,425 (1979). 2. J. ECKENRODand C. W. KOVACK,A S T M STP679, 17 (1977). 3. K. OSOZAWAand N. OKATO, Passivity and its Breakdown on Iron and Iron Based Alloys (U.S.A.Japan Seminar, Honolulu), p. 135. NACE, Houston, TX (1976). 4. J. E. TRUMAN, M. J. COLEMANand K. R. PIRT, Br. Corros. J. 12, 236 (1977). 5. A. J. SEDPOKS,Int. Metal. Rev. 28,306 (1983). 6. R. BANDYand D. VAN ROOYEN, Corrosion 39, 227 (1983). 7. O. I. LUKIN et al. Z. Metall. 115,545 (1979). 8. K. HASHIMOTO, K. ASAMIand K. TERAMOTO,Corros. Sci. 19, 3 (1979). 9. M. SAKASHITAand N. SATO, in Passivity of Metals (eds R. P. FRANKENTHALand J. KRUGER),p. 479. The Electrochemical Society, Princeton, NJ (1978). 10. Y. C. Lu and C. R. CLAYTON,J. electrochem. Soc. 132, 2517 (1985). 11. C. R. CLAYTONand Y. C. Lu, J. electrochem. Soc. 133, 2465 (1986). 12. Y. C. Lu, C. R. CLAYTONand A. R. BROOKS, Corros. Sci. 29, 863 (1989). 13. G. P. CHERNOVA,L. A. CHIGIRINSKAYAand N. TOMASHOV,Prot. Metals 16, l (1980). 14. R. C. NEWMANand SHAHRABI, Corros. Sci. 27,827 (1987). 15. M. B. IVES, Y. C. Lu and J. L. Luo, Corros. Sci. 32, 91 (1991). 16. Y. C. Lu, R. BANDY,C. R. CLAYTONand R. C. NEWMAN,J. electrochem. Soc. 130, 1774 (1983). 17. C. R. CLAYTON,L. ROSENZWEIG,M. OVERSLUIZEN and Y. C. Lu, in Surfaces, Inhibition and Passivation (eds E. MCCAFFERTYand R. J. BRODD), p. 323. Electrochemical Society, Pennington, NJ (1986). 18. Y. C. Lu, M. B. IvEs, C. R. CLAYTONand D. KIM, XPS studies of the influence of nitrogen on the anodic behaviour of molybdenum in 0.1 M HCI (in preparation). 19. Y. C. Lu, J. L. Luo and M. B. IVES, Corrosion 47,835 (1991). 20. J. L. Luo, Y. C. Lu and M. B. IVES, Corrosion~92, Paper #233. NACE, Houston (1992). 21. Y. C. Lu, J. L. Luo and M. B. IVES, Proc. N A C E Canadian Region Western Conf., pp. 271-277, Saskatoon (1991). 22. Y. C. Lu and C. R. CLAYTON,Corros. Sci. 28,927 (1989) 23. C. R. CLAYTONand Y. C. Lu, Surf. Interface Anal. 14, 66 (1989). 24. D. KIM and C. R. CLAYTON,private communication. 25. Y. C. Lu, C. R. CLAYTONand A. R. BROOKS, Corros. Sci. 29, 863 (1989). 26. M. POURBAIX, Atlas o f Electrochemical Equilibria in Aqueous Solution, p. 272. Pergamon Press, Oxford (1966).