Mechanism of High Temperature Oxidation of Austenitic Stainless Steels with High Silicon

Abstract

It was published in the previous paper that a small amount of sulfur existing as an impurity in the steels has the harmful effect on the high temperature oxidation resistance of 19Cr-13Ni-3.5Si austenitic stainless steels.
In the present study, the high temperature oxidation behaviors of 19Cr-13Ni-3.5Si steels containing extra low sulfur (0.0004%S) and high sulfur (0.012%S) have been observed from the initial oxidation state in the temperature range of 800°to 1 200°C, and the oxide scales formed on these steels have been studied by means of microscope, EPMA, fluorescent X-ray analyzer, scanning Auger analyzer, IMMA and so on.
The steel containing extra low sulfur uniformly forms the protective oxide scales consisting of corundumtype oxide (mainly Cr₂O₃) and SiO₂ which is accumulated at the metal-Cr₂O₃ interface by heating at high temperatures, and shows the excellent oxidation resistance. On the other hand, in the steel containing high sulfur, MnS compounds near the surface dissolve during heating at higher temperatures than 800°C change into Cr-Mn-O compounds. Therefore, the chromium-depleted zone is formed around the compounds, and sulfur concentrates around the compounds on the steel surface. These portions form the nonprotective spinel-type oxide containing mainly iron, and the other portions form the protective corundum-type oxide. However, the corundum-type oxide is attacked by the rapid growth of the spineltype oxide formed around MnS compounds, and the oxidation resistance of the steel containing high sulfur is extremely lowered.
From the above results, the mechanism of high temperature oxidation of 19Cr-13Ni-3.5Si steels with extra low sulfur and high sulfur has been discussed.