Localized Corrosion of 7xx.x Al Alloy Castings Grain-Refined with Al-5Ti-1B Master Alloy

The objective of this work was to determine the localized corrosion susceptibility of two artificially-aged 7xx.x Al alloy near net-shaped solidified castings (with a low and high Zn/Mg ratio) that were grain-refined with an Al-5Ti-1B master alloy to reduce hot tearing susceptibility. This was achieved using electrochemical polarization and bulk immersion measurements, coupled with electron microscopy techniques to reveal controlling microstructure aspects associated with corrosion. Both alloys, regardless of temper, exhibited the same corrosion mode: one that involved pit-like corrosion (selective grain attack) that occurred in sequence with the grain boundary region dissolving first, followed by the interdendritic region and then by the dendrite core of the equiaxed dendritic grain structure. The over-aged temper yielded the lowest corrosion susceptibility in terms of the depth of attack observed in cross-section. Of the various coarse eutectic phase particles present, Al2CuMg phase particles were particularly problematic for corrosion.