Localized Corrosion Behavior of Zr-based Bulk Metallic Glasses in Neutral NaCl Electrolytes

The electrochemical behaviors the Zr₅₀Cu₄₀Al₁₀, Zr₅₀Cu₃₃Al₁₀Pd₇, and Zr_52.5Cu_17.9Al₁₀Ni_14.6Ti_5.0 BMGs were investigated in NaCl solutions. Both Zr₅₀Cu₄₀Al₁₀ and Zr₅₀Cu₃₃Al₁₀Pd₇ exhibited poor corrosion behaviors. Zr₅₀Cu₄₀Al₁₀ was vulnerable to localized corrosion at the open-circuit conditions (OCC). Zr₅₀Cu₃₃Al₁₀Pd₇ experienced localized corrosion at OCC or at slightly higher potentials. Anodic-polarization of both alloys demonstrated that small increases in the applied potential resulted in significant increases in the current density. Scanning electron microscopy examination of the corroded surfaces of both the Zr₅₀Cu₄₀Al₁₀ and Zr₅₀Cu₃₃Al₁₀Pd₇ BMGs revealed that an oxide layer formed on the surfaces after the exposure to the chloride solutions. In both cases, the growth pattern of the oxide seemed to be related to a dendritic structure. Scanning Auger microanalysis of the heavily corroded areas on Zr₅₀Cu₄₀Al₁₀ showed that the attacked regions were enriched with Cu, Cl, and O. Higher concentrations of Pd and Cu were observed in pits as opposed to the surface of Zr₅₀Cu₃₃Al₁₀Pd₇. A corrosion mechanism related to a galvanic electrode effect and the formation of CuCl and CuO was proposed.

The anodic-polarization behavior of Zr_52.5Cu_17.9Al₁₀Ni_14.6Ti_5.0 (Vitreloy 105) was improved over the behaviors of Zr₅₀Cu₄₀Al₁₀ and Zr₅₀Cu₃₃Al₁₀Pd₇. Corrosion investigations were performed on samples obtained from regions transverse to and along the long axis of Vitreloy 105 ingots. “Swirl” features, usually thought to be areas of crystallinity, were visible in the longitudinal samples. Some longitudinal samples were treated with a laser in attempts to induce crystallinity. The cross-sectional samples were passivated at OCC; however, pits did initiate after significant overpotentials were applied. Some regions of the pits had a porous morphology that was enriched with Cu. The longitudinal samples also exhibited passive behavior at OCC. However, despite the presence of the “swirl” regions, the longitudinal samples were immune to pitting corrosion for the tested potential range. In-situ AFM experiments were performed on longitudinal samples in 0.6 M NaCl at an applied potential of -190 mV, SCE. Generally, the height of the surface increased with time, which was thought to be related to the thickening of the passive film. This observation may be associated with the improved pitting resistance of the longitudinal samples.