Unexpected Stress Corrosion Cracking Improvement Achieved by Recrystallized Layer in Al-Zn-Mg Alloy

To explore the correlation between the stress corrosion sensitivity and formation processing for high-strength Al-Zn-Mg alloy, extrusion surface and the matrix were investigated by microstructural characterizations, electrochemistry, and stress corrosion cracking (SCC) tests. Larger, but with low density and discontinuity, grain boundary precipitates (GBPs, − 60 nm and − 3.7% area fraction) and intermetallic particles (IMPs, − 0.5% area fraction) were achieved in the secondary recrystallized (SR) coarse-grained layer compared with the non-recrystallized matrix. The maximum Volta potential differences between the IMPs and Al matrix were about 373 mV, causing the pitting initiation by micro-galvanic effect. The decreased pitting density and the low residual stress in the SR layer reduced the available initiation sites for SCC. Thus, the SR layer acted as a barrier layer to reduce the SCC susceptibility of the extruded Al-Zn-Mg alloy in Cl⁻-containing environments.