A computational mass transport framework for occluded region corrosion was extended to study alloys exposed to the thin electrolytes characteristic of atmospheric corrosion. Of interest were the interactions of an aluminum-clad layer with its Al-substrate in the vicinity of a scratch. The model focuses on open circuit conditions and local galvanic couples. Accurate electrochemical kinetics for metallic clad and bare AA2024-T3 are used, including the observed decrease in the pitting potential with increasing chloride concentration. The model has been applied to studies of the parameters that affect the throwing power of metallic claddings (e.g., scratch size, [Cl-], ipass, idl). Two geometries were investigated: a 1-D model and a square sample (two spatial dimensions) with a scribed cross, the latter simulating to what is usually done experimentally in exposure testing. A quantitative computation of the throwing power of metallic cladding on AA2024-T3 in thin electrolytes was achieved, including the effect of the criterion for scratch protection. For the base case of electrochemical behavior studied, a loss of protection ability was observed between a scratch size of 50 μm and 500 μm for the most conservative protection criterion (i.e., protection is achieved when the potential of all points in the scratch is at or below the pitting potential of the cladding). When a less conservative protection criterion is used, the size of the scratch that can be protected increased to between 500 μm and 3500 μm.
The metallic cladding modeling also demonstrated the equivalence of reducing the ipass of the clad or increasing the idl for oxygen reduction on the AA2024-T3 on the size of the scratch that can be protected. A less than linear dependence was found between these parameters and the scratch size that can be protected. An order of magnitude reduction in ipass (or increase in idl) led to a decrease of anywhere from 40% to 90% in the scratch size that was protected. Two competing effects of increasing the chloride concentration in the thin electrolyte were observed. Higher chloride concentrations increased the throwing power (due to increased conductivity), but they also lowered the pitting potential of both the cladding and the base material. In our simulations, the former dominated the latter, indicating better protection of scratches at higher chloride concentrations, with the extent of the effect depending on the protection criterion used.