The transition from internal oxidation to external oxidation is silver-zinc alloys

Abstract

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