CORROSION MECHANISM OF FE(NI)25CRAIX TYPE ALLOYS IN A BIOXIDANT ENVIRONMENT AT ELEVATED-TEMPERATURE

The corrosion performance of several Fe(Ni)25CrAlX (X = e.g. Zr and Hf) type alloys has been examined in a bioxidant, oxygen/chlorine, environment (pO2 approximately 10(-15) Pa and pCl2 approximately 10(-5) Pa) at 800-degrees-C. Whilst oxide scales (Cr2O3 or Cr2O3 and Al2O3) usually formed on the exposed alloys, chlorides (FeCl2 and CrCl2) were confirmed to develop at the oxide/alloy interface, particularly after longer term exposure. The corrosion products deposited OD the reaction tube were found to contain mainly FeCl2 with some CrCl2. The experimental results also indicate that Ni-based alloys exhibited superior corrosion resistance to Fe-based alloys mainly due to the high stability of nickel in the present oxychlorine environment. Based on these experimental results, a possible corrosion mechanism has been proposed for the Fe(Ni)25CrAlX type alloys in this bioxidant environment: the corrosion resistance of this group of alloys was mainly associated with the formation of an external, adhesive oxide scale which, however, was broken down due to the formation and growth of chlorides (FeCl2 and CrCl2) at the oxide/alloy interface via diffusion of chlorine species through the external oxide scale.