High-Temperature Oxidation of Two-Phase Nanocrystalline Ag–Cr Alloys in 1 atm O2

Abstract

Two nanocystalline two-phase Ag–Cr alloys prepared by mechanical alloying and containing approximately 30 and 50 wt.% Cr were oxidized in 1 atm O₂ at 700 and 800°C. 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 layer of AgCrO₂ also formed externally to the chromia layer. In the case of the Ag–30Cr 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 sizes 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.