Probing the degradation mechanism of a Cr(VI) coating/aluminum alloy 2024-T3 system based on dynamic mechanisms and a 2D deterministic-probabilistic approach

Electrochemical impedance spectroscopy (EIS) has been experimentally used to characterize the degradation mechanisms of a multilayered Cr(VI) coating/aluminum alloy 2024-T3 substrate system in 3.5% NaCl solution at low pH. A 2D damage evolution model is proposed to understand the influence of defined elements into the multilayered coating/substrate degradation process. The model was developed assuming steady state conditions, while discretization of the model included multilayered stacks such as a topcoat, a primer, and a chromate conversion layer as a pretreatment on an aluminum substrate. The modeling technique was deterministic-stochastic in nature. The deterministic approach was based on a transmission line model that was developed to represent the local impedance for small segments of the system to characterize physicochemical properties inherent to the organic coatings and the transport mechanisms. The randomly generated structures including coating pore distributions are illustrated by stochastic modeling with probability density functions. The quantitative understanding of the degradation mechanism from this 2D model is achieved, and the 2D analysis shows consistent results with experimental conditions over various immersion times. The degradation mechanism for the multilayered coating system at low pH can be described as follows: intact condition, electrolyte uptake, lixiviation of Cr(VI) ions/diffusion, and self-healing process that provide repassivation of the Cr(III) oxide layer.