Tailoring the grain boundary precipitates and precipitate-free zone to mitigate corrosion and stress corrosion cracking in Al-Zn-Mg-Cu alloy

The static corrosion and stress corrosion cracking (SCC) behaviors of peak-aged Al-Zn-Mg-Cu alloys were systematically investigated as a function of Zn/Mg ratio and Si microalloying, while maintaining constant total Zn and Mg contents. Within the peak-aging regime, reducing the Zn/Mg ratio and adding Si both refine the grain structure. Concurrently, a lower Zn/Mg ratio coarsens grain boundary precipitates (GBPs), whereas Si addition renders them more discontinuous and narrows the precipitate-free zone (PFZ). These modifications promote the formation of an Al2O3-rich oxide corrosion product film, which improves the global corrosion resistance by suppressing Cl penetration. Moreover, the combination of coarse and discontinuous GBPs with a narrowed PFZ blocks intergranular corrosion pathways and diminishes hydrogen enrichment, decreasing the SCC susceptibility index (ISCC) from 9.5% to 3.6%. Collectively, these findings provide a systematic understanding of how tailoring GBPs and PFZ can mitigate corrosion and SCC in Al-Zn-Mg-Cu alloys.