The Influence of KCl and HCl on the High-Temperature Oxidation of a Fe-2.25Cr-1Mo Steel at 400 °C

The influence of alkali- and chlorine-containing compounds on the corrosion of superheater alloys has been studied extensively. The current paper instead investigates the corrosive effects of KCl and HCl under conditions relevant to waterwall conditions. A low-alloy (Fe-2.25Cr-1Mo) steel was exposed to KCl(s), 500 vppm HCl(g) and (KCl + HCl) in the presence of 5%O2 and 20% H2O at 400 °C. The results indicate that alloy chlorination by KCl occurs by an electrochemical process, involving cathodic formation of chemisorbed KOH on the scale surface and anodic formation of solid FeCl2 at the bottom of the scale. The process is accompanied by extensive cracking and delamination of the iron oxide scale, resulting in a complex, convoluted scale morphology. Adding 500 vppm HCl to the experimental environment (KCl + HCl) initially greatly accelerated the formation of FeCl2 at the scale/alloy interface. The accelerated alloy chlorination is attributed to HCl reacting with KOH at the scale surface, causing the cathodic process to be depolarized. A rapid slowing down of the rate of chlorination and corrosion in KCl + HCl environment was observed which was attributed to the electronically insulating nature of the FeCl2 layer which forms at the bottom of the scale, disconnecting the anodic and cathodic regions.