Date of Award

12-2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Roberto Benson, Hahn Choo, Chain-Tsuan Liu, David C. Joy

Abstract

The electrochemical behaviors the Zr50Cu40Al10, Zr50Cu33Al10Pd7, and Zr52.5Cu17.9Al10Ni14.6Ti5.0 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 Zr52.5Cu17.9Al10Ni14.6Ti5.0 (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.

Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS