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 FeCr2 S4 scale is formed below the outward growing sulfide scale. When the o xygen pressure of the gas is increased this layer becomes gradually en riched in oxygen, but corrosion losses only drop significantly,when th e scale consists of FeCr2O4 only. In this paper we will review the eff ect of HCl on the corrosion process. In addition, the effect of deposi ts containing 5-15% chlorides and exposure to high humidity conditions during downtime will be discussed. The data indicate that the presenc e of HCl or chlorides will cause accelerated corrosion, once a thresho ld chloride level is reached. For many alloys the threshold level is v ery 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 belo w the high temperature protective scale, thus causing scale spallation , which will further increase corrosion losses. Fitting during downtim e corrosion is also frequently observed.