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.

An experimental and modeling study of factors influencing intergranular corrosion (IGC) penetration depth into an Al-Mg alloy, AA5083 (UNS A95083), is presented. Potentiostatic tests over a range of potentials in 0.6 M sodium chloride (NaCl) solution were conducted on longitudinal by long transverse (LT), long transverse by short transverse (ST), and longitudinal by short transverse (LS) surfaces of sensitized AA5083. The IGC penetration rate was found to depend on applied potential, degree of sensitization (DoS), exposure time, and propagation direction relative to the rolling direction. Statistical IGC depth distributions were analyzed and served as inputs for a phenomenological model to estimate IGC damage progression for AA5083 during exposure to 0.6 M NaCl solution at pH 8.3. This model was validated by comparing the model predicted and experimental depths for the 100°C sensitization condition. Extension of this combined experimental and modeling approach to the 80°C sensitization demonstrated that sensitization temperature is another significant factor in IGC penetration depths.

Experiments 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 potentialE_rpand the pit stability product under a salt film (i · x)_saltfilm. For all the stainless steel alloys studied, a relative independence of theE_rpto 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)_saltfilmfor all alloys in both solutions. In ferric chloride, the value ofE_rpincreased 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.

Calculated maximum pit sizes for stainless steel 304L (SS304L) are presented for atmospheric conditions with elevated chloride concentrations and temperatures. These calculations were carried out by experimentally determining the pit stability product ((i · x)_sf), repassivation potential (E_rp), and cathodic kinetics. These results were aided by determination of a solubility curve as a function of temperature for a stoichiometric metal salt mixture of iron chloride (FeCl₂), chromium chloride (CrCl₃), and nickel chloride (NiCl₂). Based on these newly measured saturation concentrations, metal ion diffusivities were calculated. Overall, it was found that (i · x)_sf decreases with increasing chloride concentration and increases with increasing temperature. E_rp was determined to decrease with increasing concentration and increase with increasing temperature. When incorporated into the maximum pit size model, these experimental results yielded a small increase in maximum pit size with increasing bulk chloride concentrations and/or increasing temperature.

In this work, a rotating disk electrode was used to measure the cathodic kinetics on stainless steel as a function of diffusion layer thickness (6 to 60 μm) and chloride concentration (0.6 to 5.3 M NaCl). It was found that, while the cathodic kinetics followed the Levich equation for large diffusion layer thicknesses, the Levich equation overpredicts the mass-transfer limited current density for diffusion layer thicknesses less than 20 μm. Also, an unusual transitory response between the activation and mass-transfer controlled regions was observed for small diffusion layer thicknesses that was more apparent in lower concentration solutions. The presence and reduction of an oxide film and a transition in the oxygen reduction mechanism were identified as possible reasons for this response. The implications of these results on atmospheric corrosion kinetics under thin electrolyte layers is discussed.

In a two-part series of papers, the galvanic corrosion behavior of sensitized aluminum alloy (AA)5083-H131 (UNS A95083) when coupled with AISI 4340 (UNS G43400) steel was studied experimentally and computationally, with an emphasis on atmospheric conditions. In Part 1, the electrochemical kinetics of AA5083-H131 and steel in salt solutions of relevance to atmospheric corrosion were determined, including the influence of the degree of sensitization (DoS), potential, and sodium chloride (NaCl) concentration in the context of intergranular corrosion (IGC) propagation. More severe IGC was seen on materials with higher degrees of DoS, with damage depths of 1 mm or more developing in 100 h. The extent of IGC damage over 100 h was proportional to potential for a wide range of NaCl concentrations. These results from full immersion were compared with exposure testing and scanning Kelvin probe (SKP) measurements conducted under atmospheric conditions. It was found that the extent and spatial distribution of IGC damage on the atmospherically exposed material corresponded well to the IGC damage distribution expected from the SKP potential measurements and the damage results from the full immersion experiments. These findings are used as the input data for the computational model to predict the IGC damage distribution presented in the second paper.