Finite element modeling of the galvanic corrosion of aluminum at engineered copper particles

Finite element modeling based on solving the Nernst-Planck equation was used to describe the evolution of current densities and pH distribution at the surface of a bimetallic system. This system consisted of five single copper particles fabricated on isolated, thin-film aluminum electrodes exposed to dilute aqueous chloride solutions. Excess anodic and cathodic currents flowing during exposure were used to validate the model. The corrosion of the bimetallic system exhibited a passive and an active stage. The model was used firstly to quantify the influence of pH (modified by alkalization by O ₂ reduction at the cathodes and acidification by Al ³⁺ hydrolysis) on the passive-active transition and secondly to verify the anodic control of the active corrosion. EQCM (Electrochemical Quartz Crystal Microbalance) was used to obtain an experimental relationship between Al dissolution rate and pH. In the conditions of the model, Al ³⁺ hydrolysis was found to activate Al dissolution in the early instants of exposure (≈10 s) whereas OH ^- would have an effect only at longer times (hundreds of s). The pH-related destabilization appeared not to be sufficient enough to trigger active dissolution and it was proved that this stage was controlled by the anodic activity and not O ₂ diffusion.