Selective External Oxidation of the Intermetallic Compound, BaAg5

Abstract

The selective oxidation of has been examined at 650–680°C in flowing . Under these conditions, a continuous external barium oxide scale formed. Depletion of Ba from the underlying led to the formation of a continuous Ag layer between the oxide scale and the . Ba was only detected along grain boundaries in the continuous Ag layer, which was consistent with the negligible solubility reported for Ba in bulk Ag. The local thickness of the continuous Ag layer was inversely correlated to the local Ag grain size. Subsequent experiments with Ag‐clad revealed that surface oxide formation commenced at exposed Ag grain boundaries. specimens clad with fine grained Ag foil exhibited more extensive oxide formation in a given time than specimens clad with coarse grained Ag foil. These observations confirmed that outward Ba migration through the continuous Ag layer occurred preferentially along Ag grain boundaries. This work demonstrates that an intermetallic compound may undergo external oxidation even when a continuous metallic (or intermetallic) layer, that possesses a low solubility for the oxidizable element, forms under the oxide scale. © 2000 The Electrochemical Society. All rights reserved.