Abstract
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.
Highlights
To raise power production from biomass- and waste-fired boilers, it is necessary to increase the steam data of the plants and the temperature of both the steam superheaters and the waterwalls
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
Jonsson et al [17] and Folkeson et al [18] reported that the corrosion of steel in KCl + O2 + H2O environment starts at about 350 °C, being accompanied by consumption of KCl and formation of iron oxide on the KCl crystallites
Summary
To raise power production from biomass- and waste-fired boilers, it is necessary to increase the steam data of the plants and the temperature of both the steam superheaters and the waterwalls. The corrosivity of alkali chlorides toward high-temperature alloys is often explained by a mechanism termed “active oxidation” or the “chlorine cycle” [10,11,12,13,14,15,16]. It involves the oxidation of alkali chlorides and/or HCl by O2 to form molecular chlorine on the scale surface. The present study is intended to help fill in this apparent knowledge gap by investigating the corrosive effects of the combination of KCl(s) and HCl(g) on a lowalloyed steel at 400 °C
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