The effect of chlorine on mixed oxidant corrosion of stainless steels

Previous work has shown that mixed oxidant corrosion below 600 degrees C is characterized by the formation of outward growing Fe and Ni rich sulfide scales and inward growing Cr oxide and sulfide precipitation zones and/or scales. In syngas without HCl a somewhat protective FeCr2S4 scale is formed below the outward growing sulfide scale. When the oxygen pressure of the gas is increased this layer becomes gradually enriched in oxygen, but corrosion losses only drop significantly,when the scale consists of FeCr2O4 only. In this paper we will review the effect of HCl on the corrosion process. In addition, the effect of deposits containing 5-15% chlorides and exposure to high humidity conditions during downtime will be discussed. The data indicate that the presence of HCl or chlorides will cause accelerated corrosion, once a threshold chloride level is reached. For many alloys the threshold level is very high and is only reached in the presence of chloride rich deposits. High humidity conditions during downtime can cause the formation of chloride rich liquids in deposits, which may concentrate at the scale-metal interface, thus causing accelerated corrosion during subsequent exposures. In the worst case, aqueous corrosion products can form below the high temperature protective scale, thus causing scale spallation, which will further increase corrosion losses. Fitting during downtime corrosion is also frequently observed.