Localized Corrosion Susceptibility of AA7050 Produced by Controlled Diffusion Solidification

Controlled diffusion solidification (CDS) is a process designed to enable net-shape casting of high-strength AA7xxx alloys by mitigating hot tearing during solidification. The objective of this work was to benchmark the localized corrosion susceptibility of AA7050 produced by CDS against the conventional wrought rolled plate counterpart (both in the peak-aged T6 temper). Relative localized corrosion susceptibility was determined by electrochemical polarization measurements and cyclic corrosion testing (ASTM G85 - Annex 2) and rationalized by differences in the microstructure, as revealed by light optical and scanning electron microscopy. The localized corrosion mode was a combination of intergranular corrosion and selective grain attack, with the dominant mode depending on material: selective grain attack for the CDS material (equiaxed grain structure) and intergranular (exfoliation) corrosion for wrought material (elongated grain structure). The presence of large intermetallic particles that decorate grain boundaries, which are cathodic relative to the Al matrix phase, combined with a Cu-depleted zone adjacent to grain boundaries, were implicated as critical factors driving selective grain attack of the CDS material, as observed in the through-thickness direction.