Stoichiometry Effects on Paralinear Oxidation

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 (ΔWmax at tmax), then a linear loss. The magnitude of these features is determined by the parabolic growth rate, kp, the linear volatility rate, kv, and the stoichiometric constant of the reaction, S (fixed by the atomic weights and stoichiometry of the reaction). Model curves were generated (at constant kp and kv) to show that, for typical oxides, increases in S only moderately decrease ΔWmax and tmax, but directly increase the rate of mass loss. Universal oxidative behavior can be produced using normalized ½ kp/kv weight and ½ kp/kv2 time constants. Furthermore, it is shown that, on average, kp ≈ 4.1 (ΔWmax)2/tmax and kv ≈ 1.2 (ΔWmax)/tmax. 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 Seff, with concomitant effects on the paralinear curves.