Stoichiometry Effects on Paralinear Oxidation

Abstract

Paralinear oxidation models provide a description of parabolic scale growth combined with linear loss, as might occur for scales forming volatile oxide, hydroxide, chloride, or fluoride scales. Classic weight change exhibits an initial parabolic oxygen gain, a maximum (ΔW_max at t_max), then a linear loss. The magnitude of these features is determined by the parabolic growth rate, k_p, the linear volatility rate, k_v, and the stoichiometric constant of the reaction, S (fixed by the atomic weights and stoichiometry of the reaction). Model curves were generated (at constant k_p and k_v) to show that, for typical oxides, increases in S only moderately decrease ΔW_max and t_max, but directly increase the rate of mass loss. Universal oxidative behavior can be produced using normalized ½ k_p/k_v weight and ½ k_p/k_v² time constants. Furthermore, it is shown that, on average, k_p ≈ 4.1 (ΔW_max)²/t_max and k_v ≈ 1.2 (ΔW_max)/t_max. These relations apply for a broad spectrum of scale molecular weights, ranging from low mass SiO₂ to high mass Ta₂O₅ oxides. Oxidation of carbides and nitrides may release C and N elements and thus increase the effective S_eff, with concomitant effects on the paralinear curves.