Date of Award

3-1980

Degree Type

Thesis

Degree Name

Master of Engineering (ME)

Department

Metallurgy and Materials Science

Supervisor

W.W. Smeltzer

Co-Supervisor

G.R. Purdy

Abstract

The investigation concentrates on the morphology of the oxidation products produced in Ag-1.8, 7.5, 10.9, 12.9 and 18.7 wt % Zn alloys at 550°C under an oxygen pressure of 1 atm. Particular attention is given to the structure of the internal ZnO produced and the morphological development of the internal precipitation zone in the alloys containing Ag-1.8, 7.5 and 10.9 wt % Zn. Penetration depth of the internal ZnO was measured using a light microscope, and was observed to obey the parabolic law. A model based on volume diffusion control of oxygen has been proposed for the velocity of the internal ZnO precipitates.

The Ag-12.9 wt % Zn alloy was observed to form an internal ZnO scale in combination with an external ZnO scale. An exclusive protective external scale of ZnO is formed on the Ag-18.7 wt % Zn alloy. The growth of the scale was found to obey parabolic kinetics. The morphological development of the external ZnO on the above alloys has also been studied.

The theoretical basis for the transition from internal to external scale formation has been given according to the Wagner(²⁶) criteria for binary alloys.

It was necessary to extend the Wagner binary treatment for the transition into a ternary thermodynamic and diffusion analysis. A portion of the Ag-Zn-O ternary isotherm has been proposed, thus making possible the placement of representative diffusion paths.


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