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A combined modeling and experimental study was performed to understand the dependence of the cathodic current delivery capacity on the electrolyte film thickness and cathode size in a galvanic couple. The appropriate cathodic kinetics for the modeling were generated by use of a rotating disk electrode to simulate the electrolyte film thickness. These results provided boundary conditions for a finite element model which calculated the potential distribution along a metallic surface and the associated cathodic current supplied for electrolyte layers of varying thickness. The total cathodic current was then calculated through integration of the current density across the surface. Electrolyte layer domains were delineated by three limits which described, in order of decreasing film thickness, i) transition in exposure condition from full immersion to thick film, ii) the hydrodynamic boundary layer due to natural convection which defined the upper limit of the thin film regime, and iii) the relative dominance of ohmic resistance over mass transport in determining the total current output. This study also showed that for sufficiently thin films, this total current was independent of the size of the cathode and the nature of kinetics at the electrochemical interface, being solely driven by the ohmic resistance in solution. Dissimilar engineering alloys in close proximity and in electrical contact are frequently encountered in the architecture of high-value structural assets in the transportation, aerospace, and marine industries. In service, these structures are often exposed to atmospheric environments, such as sea spray, that result in the formation of a thin electrolyte layer on the surface sufficient to allow the dissimilar alloys to form galvanic couples. Under atmospheric conditions, the electrolyte can also form via deliquescence of salt as either a droplet or a thin film on the alloy surface, leading to the establishment of a corrosion cell. The extent of galvanic corrosion on the anode that results depends on a number of environmental, physicochemical, and geometric variables, which include relative humidity, temperature, electrolyte conductivity, electrolyte film thickness, in addition to the electrochemical kinetics on the alloy surface.1-7 Different exposure conditions can be modeled by varying the thickness of the electrolyte film (also termed the water layer, WL), which in turn affects solution resistance as well as cathodic kinetics, thus having a direct effect on the total cathodic current available to support corrosion of the galvanic system.The effects of the electrolyte film thickness on corrosion rate were recognized initially by Tomashov, 2 who qualitatively identified four regions of WL corresponding to different types of reaction control. As film thickness decreased, a transition from a plateau in corrosion rate when a constant diffusion layer was attained (conditions of full immersion) to cathodically controlled corrosion limited by diffusion of dissolved oxygen to the reaction surface (the corr...

Experiments using stainless steel artificial pit (leadin-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 E rp and the pit stability product under a salt film (i· x) saltfilm . For all the stainless steel alloys studied, a relative independence of the E rp 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. E rp , 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 E rp 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 Bstainless steel cation^within the pit, both of which agreed well with values obtained from the existing literature.

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 E rp -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.

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 E rp , 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. © The Author Many authors have separately reported the critical electrochemical conditions necessary for stable pitting and repassivation, focusing on dissolution flux, 1-8 pit solution chemistry, 9-16 or potential. 17-31The existence of these critical parameters is predicated upon the steady-state relationship that emerges between two competing processes. These processes are i) metal dissolution and hydrolysis that results in a local aggressive chemistry inside the pit, and ii) the dilution of this chemistry by diffusion out of the pit that contributes to repassivation. [32][33][34] The mathematical description of this relationship for a one-dimensional pit was framed by Galvele 1 in terms of the product of the current density and the pit depth, (i · x), which was termed the pit stability product in later studies. [35][36][37][38] Once this product decreased below a critical value, (i · x) crit , the conditions in the pit would no longer be able to sustain the local aggressive chemistry necessary for active dissolution. Galvele's formulation, originally intended to describe the conditions leading to pit initiation, has also been successfully extended to pit propagation. 4,7,[35][36][37] Experimental assessment of the Galvele pit model is typically performed using the artificial pit or lead-in-pencil electrode, 4,7,[39][40][41][42] which consists of a metal wire embedded in epoxy. The lead-in-pencil electrode is particularly useful because it closely represents Galvele's pit model configuration, i.e. a single activated surface and inert walls. 7,8 Additionally, the precipitation of a salt film 40,[43][44][45][46][47][48] at high anodic po...

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. The stable growth of corrosion pits requires the presence of an aggressive chemistry at the corroding surface as characterized by a high metal chloride concentration and low pH.1,2 The conditions leading to the maintenance of such chemistry were mathematically considered by Galvele 3 through an investigation of the steady state relationship between metal dissolution and mass transport 4,5 out of a one-dimensional (1-D) pit. For the case of a 1-D pit, it was theoretically demonstrated that a minimum critical value of cation flux -expressed as the product of the current density and the pit depth, (i·x) -was necessary for the pit to maintain a critical chemistry and thus stably corrode. As such, should the product of the current density and the pit depth fall below this critical value, repassivation would set in due to the loss of the aggressive chemistry. Subsequent studies on stainless steel pitting referred to this parameter and equivalent relationships for other pit geometries as the pit stability product. 6,7 This critical pit stability product (denoted (i·x) crit ) has been employed as the anodic stability parameter to determine the maximum pit size that can be attained on a particular metal surface in a given corrosive environment. 8-11Experimental results obtained using the artificial pit or lead-inpencil electrode 12-17 can be directly used to quantify 1-D dissolution kinetics because the construction of this electrode results in inert walls surrounding a single active surface 13,14,16 corroding under a precipitated salt film 18-24 upon the application of high anodic potential in corrosive solution. The presence of the salt film results in diffusionlimited dissolution conditi...

The effect of relative humidity (RH) on the corrosion of coarse-ground 304 stainless steel exposed for one year under sea salt particles was investigated. Total corrosion damage accumulation was higher at 40% RH than at 76% RH. At 40% RH, pits were numerous and irregularly shaped with a rough, cross-hatched structure. At 76% RH, pits were much fewer in number and ellipsoidal with crystallographically faceted surfaces. Higher E pit resulting from lower [Cl − ] impeded initiation at 76% RH. Cathodic resource competition likely limited growth and resulted in lower total volume loss. At 40% RH, lower E pit due to higher [Cl − ] led to initiation of multiple pits supported by discrete cathodes under individual droplets. Despite more cathodic current available at 76% RH, higher damage accumulation at 40% RH was due to lower anodic stability requirements resulting from higher [Cl − ]. At 76% RH, pitting proceeded with increasing active area at conditions above critical stability, leading to ellipsoidal pits with facets. The cross-hatched morphology at 40% RH was ascribed to growth at the critical stability conditions, driven by constant current through a fixed active area. Small cracks at the 40% RH pits might have been caused by hydrogen environment assisted cracking.

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.