Localised atmospheric corrosion under aerosol droplets: Influence of electrolyte geometry, kinetics, and alloy composition in pure Zn, AlZn, and AlZnFe systems

Introduction: Atmospheric corrosion of zinc and Zn-based coatings in marine environments is highly dependent on environmental factors and microstructural features. Whereas pure Zn corrosion under aerosol droplets has been extensively studied, engineering alloys such as AlZn and AlZnFe present heterogeneous microstructures with intermetallic particles (IMPs) that significantly alter local electrochemistry. Although individual aspects such as droplet geometry and Zn hydrolysis have been modelled previously, a unified mechanistic framework coupling droplet-scale physics with phase-specific alloy electrochemistry is still lacking.

Methods: This work simulates anodic and cathodic differentiation under a droplet driven by oxygen diffusion, coupled with Zn hydrolysis, Zn(OH)₂ precipitation and porous ZnO evolution. A 2D axisymmetric reaction–transport model was developed for pure Zn under a 0.6 M NaCl droplet, based on the Nernst–Planck equations. The model resolves the spatial distribution of anodic and cathodic zones as a function of local oxygen concentrations, redox kinetics, pH and evolving corrosion products. This framework is extended to Al and Al alloys to investigate the role of IMPs on local anodic activation in the Al matrix and evolution of porous oxides under varying droplet geometries and environmental compositions.

Results: The effect of droplet geometry and redox kinetics on anodic–cathodic separation and passivation behaviour is investigated. Increasing the droplet radius and contact angle both enhance oxygen availability at the droplet edge, promoting oxygen reduction. While a higher cathodic exchange current density amplified cathodic localisation, a higher exchange current density for oxidation increased the overall dissolution rate. The model predicts the protective and non-protective zones governed by the corrosion products under varying geometric and kinetic conditions and highlights how IMPs modify these regimes.

Conclusions: This methodology integrates droplet-scale oxygen diffusion, alloy-specific phase electrochemistry, and corrosion-product evolution into a unified framework. It provides mechanistic insight into how droplet geometry and reaction kinetics jointly control localised corrosion of pure Zn and Al alloys.