Corrosion Behaviour of Hastelloy C22 in Alkali Salt Environment

Chloride-induced corrosion at elevated temperature is the major drawback of commercialization of biomass fired power generation. The incapability of the biomass-fired boilers to increase the temperature has resulted in a lower efficiency of the power plant. Hastelloy C22 is one of the potential materials for superheater due to the high content of Ni-Cr which provides an excellent corrosion resistance. The aim of this work is to obtain more information on corrosion behaviour of Hastelloy C22 at high temperature in alkali salt environment with the influence of air humidity, hence to justify the suitability of this alloy as a superheater material for higher operating temperature. Corrosion tests were performed in the synthetic salt containing 80 wt% KCl and 20 wt% K₂SO₄ under normal and dry air conditions. The test was conducted at temperature 650°C for 218 hours. The surface morphology, elemental distribution and the corrosion products were characterized using Field Emission Scanning Electron Microscope (FESEM) equipped with Energy Dispersive X-ray (EDX). The phase identification was carried out using X-ray Diffraction (XRD) analysis. The results revealed a significant effect of humidity on the corrosion behaviour of the alloy where the presence of air moisture reduces the protective capability of the chromium oxide (Cr₂O₃) scale. A formation of unstable chromium (IV) hydroxide as well as the volatilization of Cr₂O₃ led to the oxide spallation, thus reducing the corrosion resistance of the superalloy. (paper)