Transport of sulfur through preformed spinel films on low alloy Fe-Cr steels

The formation of iron sulfide (Fe sub 1-x S) corrosion product on steels in the range, 250-500 deg C, during the processing of crudes is a problem of concern in the petrochemical industry. The purpose of this work is to report on the nature of the preformed oxide layer and elucidate the mechanism of sulfur transport through this oxide layer. The transport path of sulfur through the spinel layer was investigated by transmission microscopy of a sample of Fe-7Cr alloy with the spinel layer which was exposed to Ar+0.5% CH sub 3 SH gas mixture. The sulfur is found to be located predominantly in the grain boundary region and not in the grains. Therefore it is proposed that the transport of sulfur through the spinel layer occurs via grain boundaries.