Combining the Ohmic Drop and Critical Crevice Solution Approaches to Rationalize

8/11/2019 Combining the Ohmic Drop and Critical Crevice Solution Approaches to Rationalize

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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 ofPickering 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 workmerges 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 thephenomenology 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 transitionsby 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 theviability 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] inwhich shielded areas suffer increased attack rather than protection.

For decades the importance of changes in the corrosivity of the solution withinoccluded regions has shaped thought concerning crevice corrosion, pitting, andenvironment-assisted cracking . In the late 1960s 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 byinvoking the idea of the formation of a critical crevice solution (CCS) as the rate-

determining step. Later work has extended the idea to consider the effects of alloying

elements, temperature, and bulk environment [11]. Additional measurements ofaggressive 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 steelafter 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 wasboldly 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


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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 thatfor 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 thepotential 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 primarypassivation 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 corrosionaccording 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 samecomposition 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, thebasic description remains intact. In many systems the occluded solution leads to activebehavior 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 willbe 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 themost commonly observed phenomena (IA) is of restricted applicability, whereas thetheory 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 werebased upon measurements for stainless steel in solutions simulating occluded conditions

[6].

MODELING APPROACH

We have used computational modeling to address this paradox. The geometryused 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 pHas 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.


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Ji

Di

Ci

zi

F ui

Ci

A two-dimensional representation of the flux equation considering diffusion and

migration, but not convection (Eqn 1) and the conservation of mass (Eqn 2),

(1)

(2)

where: Ji= flux of species i (mol/m2-s)

Di= diffusion coefficient of species i (m2/s)

Ci= concentration of species i (mol/m3)

zi= valence of species i (equiv/mol)

F = Faradays Constant (96, 487 C/equiv)

ui= mobility of species i (m2-mol/J-s)

Ri= production/consumption of species i by chemical reactions (mol/m3-s)

All modeling has inherent limitations and uses assumptions to make thecalculations more tractable. The modeling described in the present work does notconsider 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 appliesfor all times (i.e., the electrochemical steady state is established immediately). Chemical

reactions were assumed to attain equilibrium instantaneously. Hydrolysis was considered

only for Cr3+

, with data taken from the standard literature [22]. Electroneutrality wasmaintained 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

Cr3+

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 dropoccurs. 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 distanceof 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 dissolvedcould be calculated. The peak in the dissolution current density at an intermediate point

Ci

t

J i R i


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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 quantitativelyrationalize 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 thisrequirement. The IR* model requiresthat 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 theextent 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 thegeneral 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 ofthe 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 demonstratesthe 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 maximumof attack at the mouth, and the attack would be minimal due to the absence of any changein 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 bothchemical 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 inthe 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 Cr3+. The attack is also mitigated at positions very close to the mouthbecause 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 withoutactive/passive behavior in the crevice solution. Both chemistry changes and potential


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drop are important in such cases, as neither alone can explain the experimental

observations.

ACKNOWLEDGMENTS

The financial support of the National Science Foundation (DMR-9119304 and

DMR-9505760) and the Alcoa Foundation is gratefully acknowledged. The provision ofcomputer 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, inAdvances 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, inLocalized 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, inAdvances in Localized Corrosion, H.S. Isaacs, Editor, NACE,Houston, 381 (1990).


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Figure 1 - Surface of Type 304SS after removal from crevice corrosion test at 0.05V(SCE) in 17 mM NaCl. The edge of the crevice former is indicated by the solid whiteline whereas the edge of the region of severe attack is indicated by the dotted white line.

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.

Current Density [A/cm2]

10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1

E[V

SCE

]

-0.4

-0.3

-0.2

-0.1

0.0

0.1

pH 7

pH 1

1

m

m

1 mm


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Figure 3 - pH as a function of depth inside the crevice and time. Although the pH fallsthroughout the crevice, it does so essentially uniformly for the boundary conditions

selected.

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 resultsfrom increased solution conductivity within the crevice.

Depth into Crevice (cm)

0.0 0.2 0.4 0.6 0.8 1.0

pH

1

2

3

4

5

6

7

8

t = 2 s

t = 4 s

t = 8 st = 18 st = 82 st = 332 st = 662 s

Depth inside the Crevice (cm)

0.0 0.2 0.4 0.6 0.8 1.0

E(V

vs.

SCE)

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

t = 2 s

t = 4 s

t = 8 s

t = 662 st = 42 st = 82 s


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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.

Depth into Crevice (cm)

0.0 0.2 0.4 0.6 0.8 1.0

Idiss

(mA/cm

2)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

t = 662 s

162 s

42 s

8 s

2 s

Combining the Ohmic Drop and Critical Crevice Solution Approaches to Rationalize

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