Determination of oxygen diffusion coefficient in oxidation films of the AISI 439 ferritic stainless steel

摘要

In order to investigate the role of oxygen diffusion in the oxidation process of the AISI 439 ferritic stainless steel, oxygen ion diffusion coefficients were determined, for the first time, in oxide films formed by the oxidation of this steel. Steel samples were firstly oxidized from 750°C to 900°C, in synthetic air, in order to grow oxide films mainly made up of chrome oxide; The oxygen diffusion experiments were then performed using the stable isotope ~(18)O as oxygen tracer. The introduction of the ~(18)O in the oxide film was performed by means of the gas-solid isotopic exchange method, in the temperature range of 750-900°C, in Ar+21% ~(18)(O_2) atmosphere. The ~(18)O diffusion profiles were established by secondary ion mass spectrometry (SIMS). Parabolic oxidation constants calculated by means of Wagner's theory, using the oxygen ion diffusion coefficient determined by our experimental process, are greater than oxidation constants previously determined in oxidation experiments from 850 to 950°C, in air, which indicates that the oxygen ion diffusion is large enough to assure the growth rate of the oxide film formed by the oxidation of the AISI 439 steel in these temperatures.