Selective external oxidation of the intermetallic compound, BaAg5

The selective oxidation of BaAg5 has been examined at 650-680 degrees C in flowing 3%H-z/Ar (Po-2 less than or equal to 1.1 X 10(-19) atm). Under thes e conditions, a continuous external barium oxide scale formed. Depletion of Ba from the underlying BaAg5 led to the formation of a continuous AS layer between the oxide scale and the BaAg5. Ba was only detected along grain bo undaries in the continuous Ag layer, which was consistent with the negligib le solubility reported fur Ba in bulk Ag. The local thickness of the contin uous Ag layer was inversely correlated to the local Ag grain size. Subseque nt experiments with Ag-clad BaAg5 revealed that surface oxide formation com menced at exposed Ag grain boundaries. BaAg5 specimens clad with fine grain ed Ag foil exhibited more extensive oxide formation in a given time than sp ecimens clad with coarse grained Ag foil. These observations confirmed that outward Ba migration through the continuous Ag layer occurred preferential ly 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 e lement, forms under the oxide scale. (C) 2000 The Electrochemical Society. S0013-4651(99)05-076-4. All rights reserved.