Influence of Zn/Mg ratio on cathodic corrosion and hydrogen embrittlement in crossover Al-Zn-Mg alloys

Crossover Al-Zn-Mg alloys combine the strength of both 5xxx and 7xxx Al alloys, offering a desirable combination of strength, ductility, and recyclability. The exceptional performance of these alloys is primarily due to the coexistence of η′ and T′ phases; however, an influence of the Zn/Mg ratio on hydrogen embrittlement (HE), particularly when η′ and T′ phases phases coexist, remains unclear. This study systematically investigates the resistance to cathodic corrosion and HE in three Al-Zn-Mg alloys with varying Zn/Mg mass ratios below 2.2. Hydrogen was introduced through the electrochemical charging method, followed by ex-situ tensile tests to examine HE resistance. Microstructural analyses confirmed the coexistence of coherent η′ and T′ phases within both the matrix and grain boundaries (GBs). Thermal desorption analysis identified the η′ and T′ phases as reversible hydrogen trap sites, emphasizing their important role in HE behavior. A key point of this study lies in demonstrating that an increase of the Zn/Mg ratio leads to significant microstructural changes, including reduced continuity of grain boundary precipitates and an increase in grain size. These changes collectively enhance resistance to both cathodic corrosion and HE. Reduced continuity of grain boundary precipitates impedes intergranular corrosion, suppressing hydrogen diffusion. Additionally, larger grain size hinders the repartitioning of diffusible hydrogen to the GBs, rendering hydrogen-induced crack propagation along the GBs more challenging. This work provides valuable insights for developing high-performance, hydrogen-resistant Al alloys through optimizing the Zn/Mg ratio in crossover Al-Zn-Mg alloys.