Area specific resistance of the scale formed on Crofer 22APU ferritic steel in atmospheres containing SO2

Abstract

The effect of the presence of sulfide in scales grown on the Crofer 22APU steel in the air+1% SO2 gas mixture at 1073K on their area specific resistance has been investigated. Three types of samples were studied: “non-pre-oxidized” steel exposed to oxidation in the gas mixture of air+1% SO2 for 300h, “pre-oxidized” steel oxidized in air for 200h and then for 100h in the gas mixture of air+1% SO2, and steel oxidized in pure air for 300h at 1073K as reference. The structure and microstructure of the scales were characterized by means of XRD, SEM-EDS and TEM-EDX. Electrochemical impedance spectroscopy (EIS) and dc 2-probe methods were used to determine electrical resistance. TEM-EDX analyses indicate that the scales grown in the mixture of air+1% SO2 are two-layered and consist of an outer MnCr2O4 spinel containing a small amount of sulfide, probably in the form of MnS, and an inner chromium oxide layer, which is the main oxidation product formed on the Crofer 22APU steel. Since the thickness of both scales is approximately equal, the ASR difference may be attributed to the manganese sulfide content in the studied scales. The reference scale formed on the studied steel in pure air after 300h of oxidation at 1073K is thinner than the above scales, and subsequently exhibits a lower resistance. The long-term test of ASR for scales formed on the Crofer 22APU steel in the atmosphere containing SO2 showed that they meet the requirements set for interconnect materials in SOFCs.