Artificial Pit Electrodes Research Articles

Critical Crevice Temperature – an Electrolyte Modeling Perspective

Critical pitting and crevice temperatures have been measured for many alloys using open-circuit potential tests in standard electrolytes, such as the 10 % FeCl3 or mixtures of NaCl with FeCl3 and CuCl2 for many years. Although these measurements are primarily used for alloy ranking purposes, there is interest in using them for applications with different solution chemistries. On the other hand, critical pitting temperature has been modeled in terms of the threshold condition for the formation of metal-chloride salt film or the maintenance of a critical metal-chloride concentration in the pit environment, using artificial pit electrodes and relatively simple solutions, such as NaCl. This paper addresses a modeling approach to bridge the gap between the immersion tests, electrochemical measurements, and theory. An irreversible thermodynamics model is combined with concentrated electrolyte speciation model to predict the critical crevice temperature of several Ni-Fe-Cr-Mo-W alloys. The model computes the repassivation and corrosion potentials of alloys as functions of solution composition and temperature. The superposition of these parameters provide a conservative estimate of the critical crevice corrosion temperature. The predicted values are validated against critical crevice temperature data reported in the literature. The effect of aqueous electrolyte composition on critical crevice temperature is calculated and compared to field experience. The model is further extended to mixed solvent systems, where the solvent is a non-aqueous environment. The implications of the model are discussed.

(Olin Palladium Award Address of the Electrochemical Society) Pitting Corrosion Retrospective

Pitting corrosion has captured the attention of corrosion scientists for many decades because it embodies so many interesting aspects and phenomena. Passive film breakdown and pit growth stability are the two critical phenomena in pitting, either one of which can be controlling. The pitting processes involve reaction and transport on the passive film surface, inside the passive film, on the actively dissolving pit surface and inside the pit electrolyte. This talk will summarize different studies that have shed insight into pitting corrosion. The analysis of metastable pits, very small pits that initiate, grow for a period, and then repassivate at potentials below the pitting potential or at times before the onset of stable pitting, has illuminated pit growth kinetics and the conditions for pit stability. So called 2-D pitting, the pitting of thin metallic films on inert substrates clarified the behavior of very small pits because 2-D pits have depths that are fixed at the depth of the film thickness, unlike pits in bulk materials, which deepen very quickly. Specimens with another configuration and dimensionality, artificial pit electrodes or 1-D pits, provide insight on the behavior of deeper pits including pit growth kinetics and pit growth stability. Finally, the application of a technique that makes use of the lateral sensitivity of atomic force microscopy, Scanning Kelvin Probe Force Microscopy, provides visualization of the microstructural heterogeneities that drive pitting corrosion and other forms of localized corrosion, particularly in Al alloys, which are controlled by the behavior of intermetallic particles.

Localized corrosion: Passive film breakdown vs. Pit growth stability, Part VI: Pit dissolution kinetics of different alloys and a model for pitting and repassivation potentials

The pitting resistance of SS316L, SS304, SS430, and pure Fe was studied in chloride-containing solution. Among these alloys, SS316L exhibits the highest pit initiation rate, but also the highest pitting potential. To explain this phenomenon, the maximum pit dissolution current density (idiss,max) of the alloys in saturated pit solution was measured using one-dimensional artificial pit electrodes. A lower idiss,max was found to correlate with higher pitting and repassivation potentials. Based on a recently proposed pitting framework, a conceptual model for pitting and repassivation potentials was established, based on which the effect of idiss,max on pitting resistance was discussed.

(Invited) Cryo-Based FIB/SEM Characterization of Salt Film on Metal

Metal salt films play a critical role in high rate dissolution processes such as within an active pit.1-5 To better understand the film growth mechanism, the salt film structure and composition must be characterized with high resolution, which however, is challenging owing to the dynamic nature of the dissolving interface. In this work, a novel cryogenic specimen preparation approach to preserve the pit solution and salt film was introduced, allowing ex situ characterization of the native structure formed on the dissolving metal surface. The salt film formed on a 304 stainless steel one-dimensional artificial pit electrode was preserved by flash freezing in liquid nitrogen while under an external driving potential, processed using cryo-based focused ion beam, and investigated with cryo-based scanning electron microscopy cross sectional imaging. The salt film exhibits a porous structure, with many voids and gaps between salt crystallite clusters, forming channels to provide ionic connection between the metal/film and film/solution interfaces. A spatial separation of Cr and Fe was observed in the salt film using energy dispersive X-ray spectroscopy, while Ni was found to follow the distribution of Fe, indicating the existence of co-precipitated salt containing Fe2+ and Ni2+. Acknowledgments: This work was supported as part of the Center for Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584.Reference G. S. Frankel, T. Li, and J. R. Scully, Journal of the Electrochemical Society, 164 (4), C180-C181 (2017).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 165 (9), C484-C491 (2018).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 165 (11), C762-C770 (2018).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 166 (6), C115-C124 (2019).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 166 (11), C3341-C3354 (2019).

Corrosion and repassivation of Super 13Cr stainless steel in artificial 1D pit electrodes at elevated temperature

Repassivation behavior of Super 13Cr martensitic stainless steel was investigated using a one-dimensional artificial pit electrode in deaerated NaCl solutions at 85 °C. Critical potentials for pit repassivation (Ecrit) were measured by downward potential scans. Ecrit and the final current density (if) at the end of the scan can be correlated to the pit repassivation, showing that repassivation is more probable with smaller if and higher Ecrit. Pit repassivation at a constant potential was correlated with if and average current density (iavg). A combination of lower if and iavg suggests a higher probability for pit repassivation.

Review of Simplified Models for the Pitting Potential and the Critical Pitting Temperature, Taking into Account Recent Observations

From the 1980s through the early 2000s, building on pioneering work by Galvele and others, we developed some simplified models for pitting corrosion using the artificial pit electrode (pencil electrode) as the main experimental tool. Some of the main conclusions of this research are listed below, using CPT to denote the critical pitting temperature – - The critical pit chemistry for simple stainless steels, at room temperature, is not lower than 60% of saturation in FeCl2, and may be considerably higher, as the artificial pit surface tends to split into active and passive areas [1]. Very careful experimentation is required to detect this splitting phenomenon, and to reject conclusions that do not take it into account. - The variation of the pitting potential with log [Cl-], as discussed by Galvele, can be rationalized rather exactly by plotting a quantity called the transition potential (ET) against log [Cl-]. One can dial-in a limiting current density (dependent on pit depth) to harmonize both potentials – pitting potential and transition potential [2]. A similar procedure works for alloying and (low) temperature effects. - The CPT is a kind of active-passive transition potential where (essentially) a developing pit repassivates, no matter how concentrated the local solution [3-5], but with many small complexities. - The complex morphology of real pits involves active-passive transitions within a developing 3D cavity, leading to lacy metal covers and other observations [6-8]. This topic has recently become quite fashionable after a period of some years. In this presentation, the original observations and simplified models will be reiterated and defended.

A New Framework for Pit Growth Stability and Its Experimental Validation

A new framework has been developed in recent works to describe pit growth stability 1-4. In this framework, the concept of maximum pit dissolution current density, i diss,max, was proposed to consider the competition between the dissolution and diffusion processes within a pit 2, 3, an issue that has not been satisfactorily addressed previously. The quantity i diss,max is the rate of dissolution in a pit in the absence of a salt film. According to this new framework, the pit stability and salt film formation can be defined by the critical conditions of i diss,max = i diff,crit and i diss,max = i lim, respectively, where i diff,crit and i lim are diffusion current densities related to maintain critical and saturated pit concentration 3. Unifying sets of principal parameters for pit stability and salt film formation were developed based on these critical conditions: (T crit, E crit, r crit) and (T sat, E sat, r sat), respectively 3. This talk described experiments using 1D artificial pit electrodes that validate this new framework for pit stability. According to the model prediction, the critical potential for salt film formation (E sat) should decrease with the log of pit depth. The results from experiments on 1D pits followed this relationship exactly, which provides strong support of the validity of the new model. Additionally, a critical pit concentration equal to 43% of the saturation concentration, C sat, was determined, which is much lower than the critical concentration determined by others. Furthermore, i diss,max values at fixed pit surface solution concentrations (100%C sat and 70%C sat) were determined using pits of varying depth. The Tafel slope was found to be about 116 mV in both environments. This is the first ever determination of pit dissolution kinetics in pit solutions of fixed concentration. This new framework provides an opportunity to revisit several key topics of this field and generates new insights to interpret previous observations in a self-consistent way, such as to understand the effects of alloy composition, bulk chloride concentration, ohmic potential drop of the solution, etc. Acknowledgments: This work was supported as part of the Center for Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584. Reference G. S. Frankel, T. Li and J. R. Scully, Journal of the Electrochemical Society 164, C180-C181 (2017).T. Li, J. R. Scully and G. S. Frankel, Journal of The Electrochemical Society 165, C484-C491 (2018).T. Li, J. R. Scully and G. S. Frankel, Journal of The Electrochemical Society 165, C762-C770 (2018).T. Li, J. R. Scully and G. S. Frankel, Journal of The Electrochemical Society 166, C115-C124 (2019).

Localized Corrosion: Passive Film Breakdown vs Pit Growth Stability: Part V. Validation of a New Framework for Pit Growth Stability Using One-Dimensional Artificial Pit Electrodes

Experimental evidence is provided to support the validity of a new framework for pit growth stability established in previous papers in this series. Downward potential scans from the diffusion-controlled condition were performed on SS316L one-dimensional (1D) artificial pit electrodes in 0.6 M NaCl. The variation of the critical pit surface potential for salt film formation with pit depth was in agreement with the theoretical prediction of the new framework. Using pits of different depths, the maximum pit dissolution current density, idiss,max, was obtained as a function of maximum pit surface potential, Emax, at fixed pit surface metal cation concentrations, Csurf, equal to the saturation concentration, Csat, and 70%Csat. The apparent Tafel slope of the pit dissolution kinetics determined using the downward potential scan of a 1D pit in the charge-transfer-controlled region, where Csurf changes continuously during the scan, was smaller than the Tafel slope for idiss,max(Emax) at a fixed Csurf, indicating that Csurf is a key parameter for pit dissolution. The critical concentration for pit stability/repassivation was found to be approximately 43% of Csat. Charge-transfer-controlled 1D pit growth was shown to be a transient state, which will spontaneously transition to diffusion-controlled growth in accordance with the predictions of the new framework.

Experimental Considerations for Modeling Galvanic Corrosion in Aluminum and Its Alloys

In this paper, experimental data are compared for an aluminum/copper galvanic system with model predictions from a commercially available software package. The experimental data consisted of optical profilometry measurements of damage accumulation in a bimetallic Al-Cu strip as a function of immersion time in a chloride environment. Model predictions of corrosion rate were accomplished with software that used a solution of the Laplace equation to determine the potential distribution for a mesh of the system. Current density and, ultimately, corrosion rates were determined with this model by correlating the potential at a mesh point with a current density from the experimental potentiodynamic polarization curves for the materials. Although this is a proven method for systems where the anode material is under activation control, this methodology presents a problem for the case of pitting corrosion, such as Al and its alloys in chloride solution. In the case of pitting corrosion, the surface average current density from the polarization curve (i.e., the couple current density) is not equal to the current density at the base of the pit resulting in a model prediction that underestimates the corrosion rate. To overcome this shortcoming, an alternate method for determining the polarization curve for Al is offered from artificial pit electrodes (pencil electrodes) and model predictions using that method are presented.

Differences in pitting growth kinetics between Zr60Ni25Al15 and Zr60Cu25Al15 metallic glasses exposed to a 0.6 M NaCl aqueous solution

By the artificial pit electrode approach, growth behaviours of stable pits for melt-spun Zr60Ni25Al15 and Zr60Cu25Al15 metallic glasses (MGs) exposed to a 0.6M NaCl solution are investigated. By characterising the current transient events and growth kinetics parameters, the effect of selective dissolution caused by the late transition metals on the pitting growth kinetics is revealed. The pit solution acidity is estimated with local acidification model. It is shown that the stable pitting growth for Zr60Cu25Al15 MG is considerably more favourable compared to Zr60Ni25Al15, because of its more rapid pit growth rate and more aggressive pit solution.

On a Recent Quantitative Framework Examining the Critical Factors for Localized Corrosion and Its Impact on the Galvele Pit Stability Criterion

A quantitative framework examining the Galvele pit stability criterion in the context of repassivation using information from experiments on 1D stainless steel artificial pit electrodes and mass transport modeling is discussed. The framework is built principally upon three studies that have systematically evaluated the critical electrochemical conditions describing the transition of a corroding surface from active dissolution to repassivation. First, the applicability of the Galvele pit stability product for the interpretation of the results of artificial pit experiments was assessed by analyzing the 1D flux as a function of pit geometry, which delineated the parameter space for appropriate data collection and analysis. Mass transport modeling of the 1D pit in combination with experimental measurements of the pit stability product and the repassivation potential provided an estimate of the critical concentration of metal ions at the corroding surface of the pit as it transitioned from stability to repassi...

Geometric Evolution of Flux from a Corroding One-Dimensional Pit and Its Implications on the Evaluation of Kinetic Parameters for Pit Stability

The flux from a one-dimensional (1-D) artificial pit electrode corroding under a salt film was examined using experimental and modeling techniques. Finite element simulations showed that the flux at shallow depths was consistently lower than analytically determined calculations for 1-D Fickian diffusion. This deviation was due to a substantial contribution of the external hemispherical boundary layer to the overall diffusion length. Increasing the pit diameter resulted in a larger boundary layer, which in turn affected the flux characteristics to greater depths. Data from experiments and simulation converged with the theoretical 1-D calculations only when pit depths approached nearly ten times the pit diameter. The experimental data from this artificial pit study as well as data from related published work were observed to span the range bounded by the numerically simulated and the analytically determined flux predictions. Comparison with published pit stability phenomenology showed that only deep pits provided kinetic data based on the cation concentration gradient unadulterated by bulk chloride effects. Finally, this work also provided insight into the origin of the dependence of the measured repassivation potential with pit depth, contributing towards a quantitative framework relating the various critical factors governing pitting.

One-Dimensional Pit Experiments and Modeling to Determine Critical Factors for Pit Stability and Repassivation

Experiments on stainless steel artificial pit electrodes in sodium chloride were used to inform a diffusion model developed based on the mass transport behavior within a one-dimensional corroding pit. Measurable estimates of the dissolution flux as well as the potential describing the conditions of interest were obtained from experiment as the one-dimensional pit stability product under a salt film (i·x)saltfilm and the repassivation potential Erp, respectively. These parameter estimates were acquired as a function of pit depth and were related to the concentration of the metal cation at the corroding surface at each depth via a one-dimensional mass transport model. These results allowed for the construction of a quantitative framework relating the various electrochemical parameters describing the transition from pit stability to repassivation. Such an analysis permitted the straightforward estimate of the critical surface concentration associated with this transition, which resulted in a single conservative lower bound of 50% of the saturation concentration for the minimum aggressive chemistry to sustain stable pitting and prevent repassivation. Along with published data, these results were utilized to advance the idea that the critical pit solution chemistry is independent of bulk chloride concentration up to 4 M, a range frequently encountered in atmospheric conditions.

Evaluating the Critical Chemistry for Repassivation at the Corroding Surface Using Mass Transport Model-Based Artificial Pit Experiments

One-dimensional mass transport model calculations were utilized to design experiments with stainless steel artificial pit electrodes to determine the critical concentration of cations at the corroding surface representing the transition between stable pitting and repassivation. Rapid polarization scans following salt film precipitation and consequent open circuit dilution to different surface concentrations permitted the evaluation of kinetics at various degrees of saturation. These experiments showed a distinct change in kinetics as the surface concentration decreased, thus identifying the critical pit chemistry for the onset of repassivation. Chemical modeling of oxide precipitation from solution permitted the investigation of oxide nucleation across varying surface concentration as a function of pH as the cause of repassivation. This analysis enabled the estimation of the pH at which the first oxide would precipitate in the critical pit solution chemistry, which when combined with cation hydrolysis calculations, resulted in the evaluation of the critical pH at the transition between pit stability and repassivation. A mixed potential theory-based analysis of these results was utilized to provide mechanistic support to the quantitative framework describing critical factors for pitting stability and repassivation.

Study of Single Al Corrosion Pit Growth By Electrochemical Techniques

Electrochemical techniques are commonly used for the study of localized corrosion growth [1]. However, pit growth at open circuit is hard to study by electrochemical techniques because of its difficulty of measuring current at open circuit while pit growth under potentiostatic or galvanostatic condition may be totally different from the open circuit. For stable Al pit growth, it is widely accepted that local pit chemistry and environment are critical; or a critical value of the stability product (x.i), where x the pit depth, and i the current density, must be maintained for pit survival [2-6]. Several techniques have been proposed for studying pit growth rates, including (1) Use of artificial pit (“lead-in-pencil”) electrode[7]; (2) Single pit formed by implantation[8]; laser irradiation[9]; (3) Think film electrode [10] ; (4) The foil penetration technique [11]. In this work, we propose to employ a single Al wire (~100um dia.) mounted in epoxy with different initial pit depth d, which is produced by anodic dissolution, for singe Al corrosion pit growth by coupling with a large Al foil ring as its remote cathode through a zero resistance ammeter (ZRA), as schematically shown in Fig.1. The purpose of this study is, in combined with in-situ scanning vibrating electrode technique (SVET, Fig. 2), to determine quantitatively whether the single Al corrosion pit growth follows a depth-dependent growth model as often predicted. Acknowledgements This work was performed under United Technologies Corporate R&D funding. Fig. 1 Schematic geometry of single al corrosion pit formed by an Al wire mounted in epoxy, by coupling with a large Al foil ring as its remote cathode through a zero resistance ammeter (ZRA). Fig. 2 In-situ SVE images of single Al corrosion pit growth, showing transition from (a) open circuit condition (before coupled) to (b) galvanically coupled condition. References G. S. Frankel, Corros. Sci., 30, 1203 (1990). J. R. Galvele, J. Electrochem. Soc., 123,464 (1976). J. R. Galvele, Corros. Set, 21, 551 (1981). Streblow H.H., Ives M.B., Corr.Sci. 16, 317 (1976). T.R. Beck, Electrochimica Acta, 29, 485 (1984). S.T. Pride, J.R. Scully J.R., Hudson J.L., J. Electrochem. Soc. 141, 3028(1994). H. S. Isaacs and R. C. Newman, Corrosion Chemistry within Pits, Crevices and Cracks, A. Turnbull, Editor, HMSO Books, London (1987). K. P. Wong and R. C. Alkire, J. Electrochem. Soc., 137, 3010 (1990). D. W. Buzza and R. C. Alkire, J. Electrochem. Soc., 142, 1104 (1995). G. S. Frankel, Mat. Sci. For., 247, 1 (1997). F. Hunkeler and H. Bohni, Corrosion (Houston), 37, 645 (1981). Figure 1

Effects of environmental factors on key kinetic parameters relevant to pitting corrosion

AbstractExperiments using stainless steel artificial pit (lead-in-pencil) electrodes in ferric chloride and lithium chloride solutions were performed in order to determine the effects of key environmental factors such as chloride concentration and pH of the bulk solution on the central parameters utilized to characterize the pitting phenomenon—the repassivation potential Erp and the pit stability product under a salt film (i · x)saltfilm. For all the stainless steel alloys studied, a relative independence of the Erp to the pit depth was observed once sufficient anodic charge had been passed. The pit stability product under a salt film was seen to be largely insensitive to the pH of the bulk solution. Erp, on the other hand, was fairly independent of bulk pH only at the lower chloride concentrations of both lithium chloride and ferric chloride solutions. The two parameters were affected differently by variation in the chloride concentration of the bulk solutions. Increasing the chloride concentration resulted in a decrease in the value of (i · x)saltfilm for all alloys in both solutions. In ferric chloride, the value of Erp increased with increasing chloride concentration for Custom 465 and the austenitic steels, whereas it decreased across the same range for 17–4 pH. These trends were explained qualitatively using solution conductivity and alloying composition arguments. Finally, the results obtained from this study allowed for a rationalization of the phenomenology, enabling a method of measurement of the diffusion coefficient and the concentration at saturation of the “stainless steel cation” within the pit, both of which agreed well with values obtained from the existing literature.

A High-Throughput Artificial Pit Technique to Measure Kinetic Parameters for Pitting Stability

A cyclic electrochemical technique was developed which enables rapid, high-throughput extraction of important kinetic parameters from one-dimensional artificial pit (lead-in-pencil) electrode experiments. This approach combined methods for pit initiation and propagation with kinetic measurements at a series of pit depths to measure two key kinetic parameters – the pit stability product under a salt film (i⋅x)saltfilm and the repassivation potential Erp – from a single experiment. The entire sequence was cycled and automated for efficient, high-volume data collection and analysis for the case of stainless steel in neutral chloride solution. This method offered advantages in the extraction and interpretation of data from a wide range of pit depths, permitting statistical analysis of the kinetic parameters required for determining the critical conditions for pit stability and repassivation.

Experimental and Modeling Studies on Mass Transport and Electrochemical Factors Influencing Stainless Steel Pitting and Repassivation

Studies on the critical conditions describing pitting stability and repassivation have focused on either determining the dissolution flux and solution chemistry or the potential. Experimental and modeling techniques toward building a quantitative relationship among the critical conditions were performed in this study. The artificial pit (lead-in-pencil) electrode constructed from stainless steel wires of small diameter was used as the tool to obtain measurable estimates of the critical dissolution flux and potential in aqueous sodium chloride (NaCl) solution. Both estimates were obtained using a single experimental technique. Experimental results so obtained were utilized to develop a one-dimensional diffusional model with representative boundary conditions. Results from these analyses indicated that the critical surface concentration required for stable pitting (or to prevent repassivation) may be more dilute than previously accepted in the literature.

The Artificial Pit Electrode as a Technique to Evaluate Critical Mass Transport and Electrochemical Parameters in Stainless Steel Pitting and Repassivation

The critical conditions of mass transport and localized chemistry required for a 1-D pit to grow stably were first examined by Galvele (1), resulting in the pit stability product (i*x) as the key parameter of interest. Experimentally determining this parameter requires a quasi-steady-state configuration (2) such as that under a salt film (3, 4). The lowest potential at which this film is stable has been termed the transition potential Et (5). However, pitting is known to proceed stably at potentials lower than Et (6, 7), and hence Et cannot be used as a conservative lower bound. This means that a salt film is not critical to stability and the critical pit stability product (i*x)crit = f*(i*x)salt film, where f is some fraction of saturation. The repassivation potential Erp, the potential below which all existing pits passivate, has been proposed as an alternative critical potential (8). In this regard, Erp has to be examined in terms of diffusive transport in order to relate it to pitting stability. Moreover, studying the effects of experimental variables on (i*x) and Erp enables understanding their influence on pitting stability, providing information for corrosion mitigation strategies. A cyclic artificial pit technique was developed to enable high-efficiency, automated data collection and analysis. This approach utilized an artificial pit constructed from stainless steel wire (diameter 50-100 μm), grown to different depths applying a high potential in neutral chloride solution for different time periods, after which a cathodic potentiodynamic sequence was employed, a variation of the method of Sridhar and Cragnolino (8). The entire sequence was looped multiple times. The pit stability product under a salt film, (i*x)salt film was obtained from the diffusion-limited dissolution current density at different depths (5), and Erp was noted as the potential at which a significant number of current density data points were lower than 30 μA/cm2 (8, 9). This approach enabled the measurement of both critical parameters from the same experiment and the direct correlation of Erp to pit depth and diffusive transport. The high level of automation permitted sampling a wide region of parameter space. The large volume of data from this technique facilitated replication and a statistical treatment of (i*x) and Erp. Data obtained were also utilized to develop a mass-transport-based model relating Erp and the surface concentration at the edge of pit stability, based on which an estimate of the critical degree of saturation, f was obtained. Simultaneous examination of the effects of different variables on Erp, salt film solubility, and kinetics was possible. As shown in Figure 1, (i*x)salt film values of three stainless steels tested were the same within the limits of experimental scatter. Erp for 17-4 PH was seen to be slightly higher than for 316L. Erp for 17-7 PH was much lower, leading to the hypothesis that electroactive copper in 17-4 PH may contribute to interfering redox reactions (10), yielding an artificially high Erp. Current-peak analysis (11) using data from the same experiment allowed this hypothesis to be tested. Studies have focused on the inhibitory effects of oxyanions on the pitting potential and therefore, pit initiation (12). However, examining the effects of these anions on pit repassivation and stability is important to understand their influence on pit growth. As shown in Figure 2, salt film solubility and repassivation potential are affected differently by the oxyanions. Preliminary analysis shows that the oxyanions may enhance the cathodic reaction in the pit solution, resulting in a higher measured Erp. Real-time ohmic compensation could also be included in the polarization sequence inside the loop to examine the approach to Erp, which may help understand the inhibitory mechanisms better.

Mass Transport and Electrochemical Phenomena Influencing the Pitting and Repassivation of Stainless Steels in Neutral Chloride Media

Mass transport and electrochemical parameters affecting pitting stability and repassivation in neutral chloride media were measured using the artificial pit electrode technique and the effect of different variables was studied. Sensitivity analysis showed that accurate estimation of the critical pit stability product, (i*x)crit is necessary for correctly estimating the maximum pit size that can be attained on a stainless steel surface under a thin electrolyte film. The value of (i*x)crit is directly related to the surface concentration and is some fraction of the value when a salt film is present (i*x)sf. A graphical method to determine the surface concentration at the edge of pit stability was developed in terms of diffusive transport by relating repassivation potential (Erp) to pit stability. It was seen that the value of 60-80% commonly accepted in the literature as the minimum degree of saturation for pitting stability may in fact overestimate the actual value. Further, it was noted that redox reactions involving dissolved minor alloying elements may significantly affect the measurement of Erp in some systems.