Corrosion morphology in Al-Zn-Mg-Si alloy coatings: Interdependence of microstructure heterogeneity and microstructure-induced local pH variations

Corrosion morphology in multi-phase Al-Zn-based alloy coatings stems from the interplay between microstructure heterogeneity and microstructure-induced variations in local pH. Here, we investigate the corrosion morphology of the α-Al dendrite in both Mg-containing (Al-Zn-Mg-Si) and Mg-free (Al-Zn-Si) alloy coatings. In Al-Zn-Mg-Si coatings, α-Al dendrites typically exhibit two features: a core-to-edge corrosion morphology and discrete Zn-rich clusters at the advancing corrosion front. The spatial distribution and connectivity of Zn particles within the α-Al dendrites govern the core-to-edge corrosion morphology. Multi-scale electron microscopy analysis reveals a dense dispersion of relatively fine Zn particles in the dendrite core is contrasted by a discontinuous distribution of coarse Zn particles at the dendrite edge. This disparity in cathode-to-anode connectivity concentrates galvanic currents at the core, intensifies local Al dissolution and drives corrosion outward. Furthermore, Mg dealloying from Mg-rich intermetallics elevates local pH, accelerating Al dissolution relative to Zn, and resulting in Zn-rich cluster formation. Similar Zn clustering in Mg-free Al-Zn-Si coatings exposed to alkaline environments confirms that pH shifts reverse Zn and Al selective dissolution. Together, these findings support a mechanistic framework in which the heterogenous distribution of Zn and the microstructure-induced variations in local pH synergistically modulate dissolution rates in galvanic coupling and corrosion morphologies. This offers a blueprint for designing advanced sacrificial coatings with optimised performance. • Core-to-edge corrosion linked to Zn particle distribution in α-Al dendrites • Mg dealloying raises local pH, reversing Al-Zn galvanic coupling • Zn-rich clusters form at corrosion fronts under elevated pH conditions • Corrosion directionality driven by Zn network connectivity, not just content