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

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%O 2 and 20% H 2 O 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 FeCl 2 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 FeCl 2 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 FeCl 2 layer which forms at the bottom of the scale, disconnecting the anodic and cathodic regions.

The Long-Term Corrosion Behavior of Fecral(Si) Alloys after Breakaway Oxidation at 600 °C

“…The presence of water vapor and KCl is generally understood to significantly increase the corrosion rate. This has especially been shown to be the case at lower temperature (400-500 • C) [3,54,55]. However, a study by Folkeson et al indicated that the influence of KCl in the presence of water vapor on the growth rate of fast-growing iron-rich oxide scales became less significant with temperature (the accelerating effect was not as prominent at 500 • C as at 400 • C) [3].…”

Corrosion of Heat Transfer Materials by Potassium-Contaminated Ilmenite Bed Particles in Chemical-Looping Combustion of Biomass

“…Gaseous chlorine-induced corrosion, where gaseous HCl or Cl 2 diffuses to and reacts with a metal surface forming iron chloride (FeCl x ). These chlorides may vaporize and diffuse to the surface towards a higher oxygen partial pressure, where it oxidizes forming a Fe-rich oxide layer at the metal-gas interface, releasing chlorine, which could react again with iron and thus, act as an intermediate or a catalyst in the corrosion process [26,27].…”

“…Another alternative for chlorine transportation is an electrochemical mechanism, which suggests that chloride ions diffuse to the metal-oxide interface instead of a gas transport of chlorine through the oxide scale [26,32].…”