Early stage oxidation of AISI 304 stainless steel: Role of temperature and oxygen pressure

We address the nucleation and growth features of transient films produced in the initial stages of austenitic stainless steel oxidation, prior to the establishment of the steady-state oxide, as well as the effect of temperature and atmosphere composition on the oxidation process. Specimens of cold rolled AISI 304 strips were isothermally oxidised in the temperature range between 700 and 1200°C under controlled atmosphere conditions (air or O2/N2 gas mixtures) for time intervals ranging between a few seconds and a maximum of 1200 seconds. Thermogravimetric experiments were conducted only for the higher temperatures (≥1000°C) where mass gain can be appreciated for such short oxidation times. The thickness, composition and morphological characterisation of the oxide films present on the oxidised specimens were carried out by using glow discharge optical emission spectroscopy (GDOES) and by scanning electron microscopy (SEM). The experimental results indicate the occurrence of different oxidation behaviour for AISI 304 which depend on temperature and on oxygen pressure. AISI 304 is able to form a protective oxide layer rich in Cr2O3 upon isothermal oxidation when the atmosphere is rich in oxygen (air or O2-N2 gas mixture with 3% or 9% O2) and for temperatures higher than 1000°C. The role of the oxygen concentration seems to be negligible in the low temperature range 700-900°C.