Degradation kinetics and critical failure transition of polyurethane/epoxy aircraft coatings under accelerated cyclic aging conditions

Abstract

The long-term durability of aircraft exterior coatings is fundamental to ensuring structural integrity and corrosion resistance under aggressive marine and industrial conditions. This study elucidates the degradation mechanism of a commercial aluminum alloy coating system subjected to an advanced cyclic aging protocol that alternates between humid-hot, dry, and acidic salt-spray environments. Electrochemical impedance spectroscopy (EIS) revealed a three-stage failure process: barrier protection, corrosion initiation, and diffusion-controlled deterioration, with a critical transition at around 20 cycles. Chemical analyses identified oxidation- and hydrolysis-induced polymer chain scission, creating hydrophilic functional groups that facilitated electrolyte adsorption and transport. This hydrophobic-to-hydrophilic transition accelerated ionic ingress, leading to the accumulation of voluminous aluminum corrosion products at the interface. The associated internal stresses disrupted interfacial adhesion, transforming the failure mode from cohesive to adhesive and triggering delamination and underfilm corrosion. The results establish a mechanistic linkage between polymer chemistry, electrochemical kinetics, and interfacial stress evolution, providing a robust framework for lifetime prediction and the rational design of next-generation aerospace coatings.