Pitting initiation in Fe-based amorphous coatings

Amorphous metallic coatings, especially Fe-based amorphous coatings, are thought to have promising potential for industrial applications in aggressive environments due to their excellent corrosion resistance. The intersplat regions in the coating have been suspected to be the preferential locations for pitting initiation. However, no direct evidence has been provided so far for pitting initiation at the regions. In this study, the pitting behavior of a high-velocity oxy-fuel thermally sprayed Fe-based amorphous coating of composition Fe48Cr15Mo14C15B6Y2 in a 6 M NaCl solution was studied in detail via high-resolution transmission electron microscopy (TEM) coupled with nanobeam energy-dispersive X-ray spectroscopy (EDX). It was found that pitting was always initiated in a narrow region ∼100 nm wide near the intersplat regions, but not exactly at the expected regions. Nanobeam EDX indicated that a Cr-depleted zone exists near the intersplat due to the oxidation effect. More interestingly, pitting was found to occur only on one side although Cr depletion is equal on the two sides of the intersplat. This can be well explained in terms of the galvanic effect between the Cr-depleted zone and the Cr-rich intersplat regions. The findings provide a new insight into the corrosion mechanism of Fe-based amorphous coatings.