Change in pitting susceptibility with alloy heterogeneity induced by crystallization of amorphous alloys

Sputter-deposited amorphous Cr-60Zr and Cr-67Zr alloys are spontaneously passive in 6 M HCl and sputter-deposited amorphous Al-Cr alloys are spontaneously passive in 0.1 M HCl. They suffer pitting by anodic polarization. The change in the pitting susceptibility by introduction of heterogeneity to the homogeneous amorphous alloys by crystallization heat treatment at 400-700°C for 30 min is studied. Heat treatment of the Cr-Zr alloys results in two stage crystallization, (i) precipitation of hcpzirconium phase and (ii) massive transformation from amorphous phase to intermetallic compound, Cr₂Zr. Since the precipitation of the hep zirconium phase depends on the zirconium content of alloys, the average size of the hep zirconium phase in the Cr-60Zr alloy specimens heated to 700°C is less than 20 nm, while heating of the Cr-67Zr alloy to 600°C results in precipitation of the hep zirconium phase larger than 20 nm in diameter. Heat treatment of Al-43Cr alloy leads to formation of a two phase mixture of Al₁₇Cr₉ and bcc Cr(Al). The size of nanocrystalline two phase mixture is about 15 nm when heated at 550°C but becomes 20-30 nm by heating at 600°C 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 or Al₁₇Cr₉ 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 entire heterogenous alloy surfaces. However, when the average size of the less corrosion-resistant phases exceeds 20 nm, the protective chromium-rich passive films cannot completely cover the less corrosion-resistant phases and the pitting resistance decreases.