Experimental evidence for the critical size of heterogeneity areas for pitting corrosion of Cr-Zr alloys in 6 M HCl

Sputter-deposited amorphous Cr-60Zr and Cr-67Zr alloys are spontaneously passive in 6 M HCl but suffer pitting by anodic polarization. The change in pitting susceptibility by the introduction of heterogeneity to the homogeneous amorphous alloys by crystallization heat treatment is studied. Heat treatment results in two-stage crystallization: (i) precipitation of hcp zirconium ranging in average size from 8 to 20 nm and (ii) massive transformation from amorphous phase to intermetallic compound, Cr₂Zr. The heat treatment gives rise to the ennoblement of pitting potential in spite of the formation of nanocrystalline phases. This is due to the fact that the formation of the hep zirconium phase leads to an increase in the chromium content of the matrix phase which is able to form thin, protective chromium-rich passive films covering the entire heterogenous alloy surface. When the average size of the less corrosion-resistant hep zirconium precipitates exceeds a critical size, 20 nm, the protective chromium-rich passive films cannot completely cover the precipitates and the pitting resistance decreases.