Selective External Oxidation of the Intermetallic Compound, BaAg_5

The selective oxidation of BaAg_5 has been examined at 650–680°C in flowing 3%H2/Ar (po2 ≤ 1.1×x×10^–19atm). Under these conditions, a continuous external barium oxide scale formed. Depletion of Ba from the underlying BaAg_5 led to the formation of a continuous Ag layer between the oxide scale and the BaAg_5. 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 BaAg_5 revealed that surface oxide formation commenced at exposed Ag grain boundaries. BaAg_5 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