Correlations between intergranular stress corrosion cracking, grain-boundary microchemistry, and grain-boundary electrochemistry for Al–Zn–Mg–Cu alloys

There is little or no correlation between grain-boundary, or matrix, microstructures and stress corrosion cracking (SCC) susceptibility. Grain-boundary microchemistry, especially the copper content of grain-boundary precipitates (GBP) is the most important factor. Further evidence for this is a correlation between the initial open-circuit potential of ‘fresh’ brittle intergranular fracture surfaces, the copper content of GBP, and the plateau SCC velocities of overaged 7079 and 7075 alloys. Preliminary comparisons of overaged (T7) and peak-aged (T651) material have also been made, and support the view that beneficial effects of overaging on SCC resistance is associated with increasing copper content of GBPs.

Research highlights
► There is a good correlation between the intergranular SCC plateau velocities and the initial open-circuit potential of brittle intergranular fracture surfaces.
► The initial OCP and SCC velocity of overaged alloys appeared to be mainly controlled by the copper content of grain-boundary precipitates.
► Increases in OCP with time of immersion are due to de-alloying of grain-boundary precipitates and film formation.
► Measurements of OCP of ‘fresh’ brittle intergranular fracture surfaces give a good global indication of the electrochemical reactivity of grain-boundaries.