Hot Corrosion Behaviour of Protective Oxide Scales with Sodium Chloride in Chlorine Gas Environment

Sodium chloride (NaC1) is one of the prominent hot corrosion inducing constituents of salt deposits that accumulate on the alloy surface in the field. In a marine environment NaC1 is ingested into a gas turbine as an aerosol of sea salt along with intake air. At high temperatures, conventional alloys are severely attacked by NaC1 and protective oxide scales like Cr₂O₃, Al₂O₃ and NiO invariably deteriorate. This paper deals with the high temperature reaction kinetics of such metal oxides with NaC1 in a C1₂ gas environment at atmospheric pressure. The kinetics of the reactions between NaC1 and metal oxides were studied by monitoring them by thermo-gravimetric analysis. Reaction products were identified by XRD analysis and SEM studies. Water soluble metal species in the mixture were estimated with the help of atomic absorption spectroscopy and pH of aqueous solution was taken into account to establish the reaction mechanism. The result showed that the mixed oxide of the type (Na₂O.M₂Ox) and metal chlorides were the usual reaction products. Water soluble reaction products indicated the presence of metal oxy anions (CrO₂^-; A1O₂>^-; and NiO₂>^-;) in the mixture. The aqueous solution of NaC1 and Al203 in the system was found basic whereas for NiO and Cr₂O₃ it was acidic.