Oxidation of Tin + 1AlXn ( n = 1 ­ 3 and X = C , N): II. Experimental Results

Abstract

In this, Part II of a two-part study, the oxidation kinetics in air of the ternary compounds and are reported. For the first two compounds, in the 1000-1100°C temperature range and for short times (≈20 h) the oxidation kinetics are parabolic. The parabolic rate constants are for and for At 900°C, the kinetics are quasi-linear, and up to 100 h the outermost layers that form are almost pure rutile, dense, and protective. For the second pair, at short times (<10 h) the oxidation kinetics are parabolic at all temperatures examined (800-1100°C), but become linear at longer times. The values are for and for In all cases, the scales that form are comprised mainly of a rutile-based solid solution, where and some The oxidation occurs by the inward diffusion of oxygen and the outward diffusion of Al and Ti. The C and N atoms are presumed to also diffuse outward through the oxide layer. At the low oxygen partial pressure side, the ions dissolve in and diffuse through the layer and react with oxygen to form at the high oxygen pressure side. This demixing results in the formation of pores that concentrate along planes, especially at longer times and higher temperatures. These layers of porosity impede the diffusion of Al, but not those of Ti and oxygen, which results in the formation of highly striated scales where three layers, an -rich, a -rich, and a porous layer repeat multiple (>10) times. The presence of oxygen also reduces the decomposition (into and Al) temperatures of and from a to one less than 1100°C. © 2001 The Electrochemical Society. All rights reserved.