Short-time Oxidation Behavior of Low-carbon, Low-silicon Steel in Air at 850-1,180 °C-III: Mixed Linear-and-Parabolic to Parabolic Transition Determined Using Local Mass-Transport Theories

摘要

Under certain assumptions, Levich derived an equation for calculating gas-phase mass transport rate. This equation is commonly used to calculate the linear oxidation rate constant when the oxidation kinetics is controlled by gas phase transport and has been used in this study to calculate oxidation kinetics of steel at 850-1,180 °C in flowing air. In theory, at the early stage, two kinetics components, one linear and another parabolic, simultaneously operate at different locations of the sample, contributing to the overall kinetics and therefore, the oxidation kinetics should show an initial mixed linear-and-parabolic pattern, followed by a purely parabolic stage. However, the actual gas flow pattern in a laboratory tube furnace does not meet the requirements for deriving the Levich's equation and as a result, significant discrepancies are found between the calculated and experimental results. To resolve the discrepancy, experimental conditions should be modified to meet the gas flow conditions required for deriving the Levich's equation. Alternatively, a more appropriate equation under the current gas flow conditions in a laboratory tube furnace should be derived.