Pit Initiation on Partially Amorphous Alloys with Solute-Lean Nanocrystals

Abstract

The good global pitting resistance of amorphous Al90Fe5Gd5 in 0.6 M NaCl solution is maintained upon partial devitrification to form solute lean crystals in an amorphous matrix. The focus of this paper is to distinguish whether this can be explained by decreased pit initiation or hindered pit growth and stabilization. The metastable pitting properties of Al90Fe5Gd5 were investigated at the nanometer (nm) and micrometer (μm) scales. The fully amorphous alloy, versions of the alloy containing a high volume fraction of isolated 5-15 nm diameter Al-rich crystals embedded within the amorphous matrix, and versions containing ~500 nm transitional phases in a remaining amorphous-nanocrystalline matrix were compared to a fully crystallized condition and pure polycrystalline Al. Nanocrystals and transitional phases embedded in an amorphous Al-Fe-Gd matrix do not promote pit initiation at the nm-scale in NaCl solution under the same electrochemical conditions where the bulk equivalent of Al-rich f.c.c. nanocrystals do initiate sub-μm sized pits. The number and size of sub-micrometer pit initiation events is similar for the fully amorphous and amorphous-nanocrystalline alloys under the same conditions.