The effect of chlorine on mixed oxidant corrosion

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. Depending on the PS2/PO2 ratio of the gas and the alloy composition, this can lead to the formation of semiprotective FeCr2S4 scales, accelerated corrosion through non protective internal oxidation of Cr or the formation of a highly protective dense oxide precipitation zone in the alloy surface. These results were obtained in gases containing 400 ppm HCl. In the present work the HCl content of the gas was varied from 0 - 1200 ppm. In addition, the effect of deposits containing 5-15% chlorides and exposure to high humidity conditions during downtime was studied. 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 liquid chlorides 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.