High Temperature Corrosion of FeCrAl Alloys: The influence of water vapour and alkali salt

Licentiate thesis, 2013

The corrosive environment in biomass- and waste-fired boilers has been known for several years. The traditionally used FeCr and FeNiCr steels show limited performance in the aggressive environment that contains more H2O and alkali salts than traditional fuel. A more sustainable material is needed to satisfy the constant strive for higher operating temperatures. FeCrAl alloys are well-known for their good oxidation resistance at elevated temperatures and might be a suitable candidate. In this thesis, the KCl induced corrosion of FeCrAl alloys is investigated at 600 °C. The chosen temperature is higher than today’s operating temperatures in the superheater region (̴ 350-550 °C) and was chosen to meet the increasing demand from industry. In the laboratory, the effect of adding 0.5-0.1 mg/cm2 KCl was investigated on polished samples in O2 or O2 + H2O for up to 168 hours at 600 °C. Samples were exposed to the same environment in the absence of KCl as reference. Pre-oxidised samples were also exposed to O2 + H2O + KCl in order to investigate the interaction of alkali salts with an alumina scale. The samples were analysed using TGA, SEM/EDX, XRD, IC, AES, SIMS and BIB cross sections. The two investigated FeCrAl alloys form a protective oxide layer in O2 and O2 + H2O which is rich in alumina with minor amounts of chromium and iron. Water vapour accelerates the oxidation and a slightly thicker oxide scale is formed in O2 + H2O. No evidence of chromium evaporation was recorded, as can be seen, on stainless steels. KCl accelerates the corrosion and a rapidly growing iron-chromium rich oxide forms in humid and dry environments. Chromate formation and alloy chlorination are found to initiate the formation of the non-protective oxide scales. Pre-oxidation of the alloy inhibits the severe corrosion attack in O2 + H2O + KCl. The oxide from the pre-oxidation at 700 °C is not completely inert to KCl but is nevertheless considered protective in the present environment. Keywords: FeCrAl, oxidation, water vapour, alumina, high temperature corrosion, KCl, pre-oxidation, kinetics.