Abstract
Two nanocystalline two-phase Ag-Cr alloys prepared bq: mechanical alloying and containing approximately 30 and 50 wt.% Cr were oxidized in I atm OZ at 700 and 800 degreesC. Under all conditions, a continuous layer of chromia formed at the surface of the alloys, in spite of the very low solubility of Cr in Ag. A laver of AgCrO2 also formed externally to the chromia layer. In the case of the Ag-30 Cr alloy, some Ag particles were also present on the scale, directly in contact with the gas phase. Moreover, Cr particles dissolved in the subsurface region of the alloy, while internal oxidation of Cr, was absent. Ag-Cr alloys prepared by powder metallurgy with coarse grain si-es were able to form an irregular thin chromia layer only at a Cr content of 69 wt.%, while an alloy containing 35 wt.% Cr corroded much more rapidly than the nanocrystalline Ag-30Cr alloy. This difference in the scaling behavior is attributed to the large reduction in the alloy grain size, which favors the dissolution of the Cr-rich particles in a Cr-depleted silver matrix and thus provides a faster supply of chromium from the alloy to the scale.
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