Corrosion performance divergence and its microstructure origin in T′/η' dual-phase Al-Zn-Mg-Cu alloys with varying Zn/Mg ratios

Abstract

Al-Zn-Mg-Cu alloys are widely used in aerospace and marine engineering, with service reliability jointly affected by general and localized corrosion. For emerging T′/η' dual-phase alloys, it remains unclear how Zn/Mg ratio modulates microstructure-corrosion correlations across different corrosion conditions, restricting targeted alloy design. Five peak-aged alloys with Zn/Mg ratios of 1.50–10.00 are systematically investigated via electrochemical, intergranular corrosion (IGC) and exfoliation corrosion (EXCO) tests combined with microstructure characterization. A notable performance divergence between general and localized corrosion resistance is observed. The T′-dominated alloy (Zn/Mg = 1.50) shows a low corrosion current density of 0.78 × 10⁻⁶ A/cm² but suffers the severest IGC with 347 μm maximum depth. The alloy with balanced T′/η' fraction (Zn/Mg = 4.44) exhibits the best localized-corrosion resistance (minimum IGC depth of 85 μm) with moderate general-corrosion activity. With rising Zn/Mg ratio, precipitate-free zone (PFZ) width decreases from 193 nm to 47 nm then rebounds slightly. Grain boundary precipitates (GBPs) refine from continuous to discontinuous distribution, with Cu content of GBPs peaking at Zn/Mg = 4.44. This divergence stems from distinct dominant corrosion mechanisms. General corrosion is governed by matrix anodic activity, whereas localized corrosion is controlled by grain boundary galvanic coupling. The balanced dual-phase structure suppresses intergranular corrosion via optimized grain boundary features, and this work demonstrates that single electrochemical metrics cannot evaluate localized-corrosion risk for multi-phase alloys.