Combining the Ohmic Drop and Critical Crevice Solution Approaches to Rationalize Intermediate Attack in Crevice Corrosion. R.G. Kelly, K. C.

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1 Combining the Ohmic Drop and Critical Crevice Solution Approaches to Rationalize Intermediate Attack in Crevice Corrosion R.G. Kelly, K. C. Stewart Abstract Crevice corrosion material loss is often most severe at intermediate distances into the crevice. This intermediate attack can be rationalized using the ohmic drop model of Pickering if and only if the material in question exhibits active/passive behavior in the crevice environment. Conventional theories that ascribe control of crevice corrosion to chemical changes exclusively cannot rationalize intermediate attack. The present work merges these two approaches, allowing rationalization of intermediate attack, even for materials that undergo active dissolution as might be expected in real crevices. INTRODUCTION Several competing frameworks have been proposed to explain the phenomenology of crevice corrosion [1-8]. Recent debate has focused on the relative contributions of ohmic drop [3,7,8] and occluded chemistry change [5,6]. Pickering [3] has demonstrated the importance of ohmic drop in systems with active-passive transitions by performing experiments in which the ph of the occluded solution chemistry was experimentally forced to remain constant. Polarizing the external portion of the sample into the passive region resulted in preferential attack within the crevice, thus showing the viability of IR-induced crevice corrosion. This observation is identical to the dangers of poor current distribution in anodic protection of stainless steels in acid solutions [9] in which shielded areas suffer increased attack rather than protection. For decades the importance of changes in the corrosivity of the solution within occluded regions has shaped thought concerning crevice corrosion, pitting, and environment-assisted cracking. In the late 1960 s Brown et al. demonstrated that low ph solutions can indeed develop in cracks in stainless steels as well as aluminum and titanium alloys [10]. Oldfield and Sutton later [11] put the conceptual framework of Fontana and Greene [1] into a computational model for crevice corrosion initiation by invoking the idea of the formation of a critical crevice solution (CCS) as the ratedetermining step. Later work has extended the idea to consider the effects of alloying elements, temperature, and bulk environment [11]. Additional measurements of aggressive occluded chemistries have also been made [12-15]. One aspect of crevice corrosion phenomenology that is often overlooked is that of intermediate attack. Figure 1 shows an example of such attack in Type 304 stainless steel after exposure within a crevice in neutral chloride solution [6]. This stainless steel is spontaneously passive in the bulk solution and was held several hundred millivolts below the pitting potential. After an incubation period, crevice corrosion initiated. The area immediately inside the crevice was unattacked (as was that portion of the sample that was boldly exposed outside the crevice). Deeper into the crevice the attack becomes increasingly severe for approximately a millimeter before decreasing in severity with further depth. This intermediate attack (IA) is often observed in crevice corrosion of

2 many alloys. Any complete description of crevice corrosion must therefore be able to rationalize such behavior. Pickering [16-18] has demonstrated both experimentally and computationally that for systems which meet the criteria of the IR* theory, intermediate attack is predicted. The amount of potential drop increases as one moves into the crevice due to the current leaving the crevice. If the geometry, solution conductivity, and passive current density of the material in the environment conspire to create sufficient ohmic drop, then the potential of some portion of the material within the crevice falls to the primary passive potential. Under these circumstances the passive film is not stable and active dissolution occurs. The potential difference between the applied potential and the primary passivation potential is referred to as IR*. Deeper still into the crevice the ohmic drop leads to decreased dissolution as the overpotential for the anodic reaction decreases. Thus, ohmic drop is responsible for the initiation and stabilization of crevice corrosion according to this model. Note that the IR* model applies strictly only to materials which undergo an active-passive transition in the crevice solution which is maintained at the same composition as the bulk solution. In systems in which the occluded solution has been shown to differ from the bulk, the theory must be modified to quantitatively predict the position of the IA, but as long as the material undergoes an active-passive transition, the basic description remains intact. In many systems the occluded solution leads to active behavior only or unstable passivation [19,20]. For these systems ohmic drop still occurs due to the current emanating from the crevice, but it acts to destabilize, not stabilize crevice corrosion. The CCS theory of crevice corrosion cannot predict intermediate attack. The CCS theory bases its predictions on the predominance of occluded chemistry changes in determining crevice corrosion susceptibility. As shown by Watson and Postelwaite [21], the CCS theory predicts that the most severe attack will occur at the deepest part of the crevice, i.e., the most occluded portion. At this most occluded point, the chemistry will be the most altered, and thus the attack will be expected to be most severe there. A paradox thus exists in crevice corrosion. The theory that can explain one of the most commonly observed phenomena (IA) is of restricted applicability, whereas the theory that cannot rationalize IA is thought to occur more widely. The current work was undertaken to attempt to resolve this paradox by considering both ohmic drop and chemical changes. A set of boundary conditions was selected for which neither the CCS nor the IR* model would predict IA. The electrochemical boundary conditions were based upon measurements for stainless steel in solutions simulating occluded conditions [6]. MODELING APPROACH We have used computational modeling to address this paradox. The geometry used was that of a one-dimensional crevice (i.e., a slot) with a 4 micron gap and a 1 cm depth (L:g = 2,500). The initial solution in the crevice was neutral 0.3 M NaCl. Each segment of the crevice was assumed to have the electrochemical behavior shown in Figure 2. The polarization behavior of the material was allowed to vary with the local ph as has been shown to be the case for stainless steel [6]. The mouth of the crevice was held at 0.05V(SCE) and the chemistry at the boundary of the first element was fixed to the initial solution composition.

3 A two-dimensional representation of the flux equation considering diffusion and migration, but not convection (Eqn 1) and the conservation of mass (Eqn 2), J i D i C i z i Fu i C i (1) C i t J i R i (2) where: J i = flux of species i (mol/m 2 -s) D i = diffusion coefficient of species i (m 2 /s) C i = concentration of species i (mol/m 3 ) z i = valence of species i (equiv/mol) F = Faraday s Constant (96, 487 C/equiv) u i = mobility of species i (m 2 -mol/j-s) R i = production/consumption of species i by chemical reactions (mol/m 3 -s) All modeling has inherent limitations and uses assumptions to make the calculations more tractable. The modeling described in the present work does not consider several phenomena. These include precipitation of solid corrosion products, convection (including natural convection), and the effects of concentrated solutions on transport. The modeling also assumes that the polarization behavior in Figure 2 applies for all times (i.e., the electrochemical steady state is established immediately). Chemical reactions were assumed to attain equilibrium instantaneously. Hydrolysis was considered only for Cr 3+, with data taken from the standard literature [22]. Electroneutrality was maintained when necessary by the addition or subtraction of chloride ion from the element of interest. RESULTS Figure 3 shows the evolution of the ph profile within the crevice. Due to the lack of buffering, within two seconds of the start of the simulation, the ph has dropped to 6 throughout the crevice. As the simulation continues the ph continues to fall due to the Cr 3+ hydrolysis. The decrease in ph is more marked at positions just in from the mouth. The position of minimum ph continues to move deeper into the crevice with time, but the differences in ph between the mouth, the minimum point and the base of the crevice are small (< 0.5 ph units). Figure 4 shows the evolution of the potential profile within the crevice. Initially the profile is flat. All points within the crevice are polarized to the value at the mouth. Within a short time, the currents increase due to the ph changes and larger ohmic drop occurs. After 82 seconds the potential at the base of the crevice is more than 300 mv below that at the mouth. Over the next 480 sec the potential profile flattens somewhat, and the maximum potential difference is slightly more than 250 mv. In addition to this decrease in potential drop, an inflection point can be observed in the profile at a distance of approximately 0.15 cm from the mouth. Figure 5 shows the evolution of the current density distribution within the crevice. If these curves were integrated with respect to time, then the amount of metal dissolved could be calculated. The peak in the dissolution current density at an intermediate point

4 within the crevice can be clearly observed. This intermediate attack develops very quickly (< 8 s). The point of maximum attack moves slowly deeper into the crevice until it stabilizes at 0.07 cm for these boundary conditions. DISCUSSION A complete description of any physical process must be able to quantitatively rationalize the key aspects of the phenomenology. In the case of crevice corrosion such a description must therefore be able to predict the common observation of intermediate attack. Pickering et al. [3,16-18] has demonstrated that the IR* model can meet this requirement. The IR* model requires that the material of interest exhibit active/passive behavior in the occluded solution. Many engineering materials do not meet this requirement of the model [6, 19, 20]. In such cases, the ohmic drop would act to limit the extent of crevice corrosion according to mixed potential theory by reducing the overpotential for the dissolution reaction. Thus, although the IR* model can predict IA in some systems, its applicability is somewhat limited. It should be noted that due to the general lack of information concerning occluded chemistries, the relative proportions of systems exhibiting active/passive vs. active behavior has not been determined. Whereas the CCS model has long dominated the discussion of crevice corrosion, its limitations have recently become more clearly defined [6-8]. A direct consequence of the CCS model is the prediction of the most severe attack occurring at the deepest point in the crevice. This behavior results from the increasing occlusion with depth into the crevice, and the premise that only chemical composition controls crevice corrosion. Observations of maximum attack at the deepest point in the crevice have been reported [11]. Again, no comprehensive study of the relative abundance of such behavior and IA has been conducted. In engineering systems it is likely that CCS and IR interact to focus attack at intermediate positions within the crevice. The modeling work shown above demonstrates the close interaction between the two aspects of crevice corrosion. Neither can predict IA alone with the boundary conditions selected. The IR* model would predict a maximum of attack at the mouth, and the attack would be minimal due to the absence of any change in chemistry. The CCS model would predict maximum attack at the base of the crevice where the solution is most occluded [23]. The approach presented here demonstrates that for reasonable boundary conditions (similar to those measured for Type 304SS in solutions based on occluded solution analyses) IA can be predicted if and only if both chemical changes and ohmic potential drop are considered. These two phenomena are closely linked. The chemical changes lead to increased currents in some areas that lead to increased potential drop. The increased potential drop mitigates the attack deeper in the crevice. This mitigation of the attack deep in the crevice leads to a slightly less aggressive environment because mass transport out of that portion overwhelms the production of Cr 3+. The attack is also mitigated at positions very close to the mouth because of the ability of mass transport to prevent the change in the chemistry. The result of these two opposing forces is IA. SUMMARY Combining the effects of ohmic drop and hydrolysis reactions allows the rationalization of intermediate attack in crevice corrosion for systems without active/passive behavior in the crevice solution. Both chemistry changes and potential

5 drop are important in such cases, as neither alone can explain the experimental observations. ACKNOWLEDGMENTS The financial support of the National Science Foundation (DMR and DMR ) and the Alcoa Foundation is gratefully acknowledged. The provision of computer equipment through the IBM Shared University Resources program is also gratefully acknowledged. REFERENCES 1. M. G. Fontana, N. D. Greene, Corrosion Engineering, p. 41, McGraw-Hill, New York (1967). 2. J. W. Oldfield, W. H. Sutton, Brit. Corros. J., 13, 13 (1978). 3. H.W. Pickering, in Advances in Localized Corrosion, H.S. Isaacs, U. Bertocci, J. Kruger and S. Smialowska, Editors, p. 77, NACE, Houston, TX (1990). 4. R. J. Brigham, Corros. Sci., 33, 799 (1992). 5. N. Sridhar and D. S. Dunn, J. Electrochem. Soc., 140, 643 (1997). 6. C. S. Brossia, R. G. Kelly, Corros. Sci., accepted for publication (1999). 7. B. A. Shaw, P. J. Moran, P. O. Gartland, Corros. Sci., 32, 707 (1991). 8. R. S. Lillard and J. R. Scully, J. Electrochem. Soc., 141, 3006 (1994). 9. D.A. Jones, Principles and Prevention of Corrosion, p. 134, Macmillan, New York (1992). 10. B. F. Brown, C. T. Fujii, and E. P. Dahlberg, J. Electrochem. Soc., 116, 218 (1969). 11. J. W. Oldfield, W. H. Sutton, Brit. Corros. J., 13, 104 (1978). 12. T. Suzuki, M. Yamabe, Y. Kitamura, Corrosion, 29, 18 (1973). 13. J. L. Luo, Y. C. Lu and M. B. Ives, Mater. Performance, 31, 44 (1992). 14. J. A. Davis, in Localized Corrosion, R. W. Staehle, B. F. Brown, J. Kruger, A. K. Agrawal, Editors, p.168, Houston, NACE (1974). 15. C. S. Brossia, R. G. Kelly, Corrosion, 54, p. 145 (1998). 16. H. W. Pickering, Materials Science and Engineering. A. 198, 213 (1995). 17. H. W. Pickering, K. Cho, and E. Nystrom, Corrosion Science, 35, 775 (1993). 18. Y. Xu, M. Wang, and H. W. Pickering, J. Electrochem. Soc., 140, 3448 (1993). 19. G. Salamat, G. A. Juhl, and R. G. Kelly, Corrosion, 51, 826 (1995). 20. T. Hakkarainen, in Corrosion Chemistry within Pits, Crevices and Cracks, A. Turnbull, Editor, p. 17, Her Majesty's Stationery Office, London (1984). 21. M. Watson, J. Postelwaite, Corrosion, 46, 522 (1990). 22. C. F. Baes, R. E. Mesmer, The Hydrolysis of Cations, p. 211, Krieger, Malabar, FL, (1986) 23. R. A. H. Edwards, in Advances in Localized Corrosion, H.S. Isaacs, Editor, NACE, Houston, 381 (1990).

6 1 mm 1 m m Figure 1 - Surface of Type 304SS after removal from crevice corrosion test at 0.05 V(SCE) in 17 mm NaCl. The edge of the crevice former is indicated by the solid white line whereas the edge of the region of severe attack is indicated by the dotted white line. 0.1 ph ph 1 E [V SCE ] Current Density [A/cm 2 ] Figure 2 - Assumed polarization behavior for the material within the crevice as a function of ph. The cathodic reactions are assumed to produce charge, but no ph-altering species.

7 8 7 ph t = 2 s t = 4 s t = 8 s t = 18 s t = 82 s t = 332 s t = 662 s Depth into Crevice (cm) Figure 3 - ph as a function of depth inside the crevice and time. Although the ph falls throughout the crevice, it does so essentially uniformly for the boundary conditions selected t = 2 s E (V vs. SCE) t = 8 s t = 662 s t = 42 s t = 82 s Depth inside the Crevice (cm) t = 4 s Figure 4 - Potential of material as a function of depth into the crevice and time. Note the increase in the potential (decrease in the ohmic drop) between 82 and 662 s that results from increased solution conductivity within the crevice.

8 1.6 I diss (ma/cm 2 ) t = 662 s 162 s 42 s 8 s 2 s Depth into Crevice (cm) Figure 5 - Anodic dissolution current as a function of depth into crevice and time. Note the development of a peak in the profile indicating the position of maximum attack.