CYCLIC OXIDATION AND BREAKAWAY CORROSION OF STAINLESS STEELS IN SO2 +AIR ATMOSPHERE

Abstract

In sulfuric acid production plants, stainless steels used in the hot gas converters are subject to both thermal cycling (temperature fluctuation and shutdown) and SO2+air or SO3 +air gas mixture. In this study, a discontinuous cyclic oxidation test was designed to simulate the corrosion of stainless steels in SO2+air gas mixture that is often encountered in the plants. Two commercial stainless steels, 304H (UNS. S30409)(common construction material used in hot gas converter, low Silicon content) and A611 (UNS. S30601) (high Silicon content) were used for testing, and 24-hour cyclic oxidation was conducted in 7%SO2+air gas mixture at 420°C, 620°C, and 720°C. The exposed samples were examined by SEM (Scanning Electron Microscope) and XRD (X-Ray Diffraction). At all test temperatures, in both air and SO2 containing atmosphere, A611 showed a good corrosion resistance and no breakaway corrosion. Silicon was found to be concentrated at the scale/metal interface that is assumed to be in the form of a layer of silica. The good corrosion resistance of A611 is attributed to the silica layer at the interface that both improves the adherence of scale and acts as a barrier to diffusion. However, for 304H alloy, although at 420°C the alloy was protected under thermal cyclic condition, at both 620°C and 720°C, breakaway corrosion occurred after only two 24hour-thermal cycles. The partial spallation of the scale contributes to the higher weight gain at 620 °C while complete spallation results in a lower weight gain at 720 °C. Due to the partial spallation and therefore the penetration of molecular SO2, a rapid interface recession was observed at 620 °C leaving no obvious internal oxidation & sulfidation occurring within the alloy; a clear internal oxidation&sulfidation region was found close to the interface at 720 °C. A possible spalling & breakaway corrosion mechanism of 304H is proposed. Also the oxide-metal interface adherence of 304H and A611 is discussed.