High temperature corrosion of coatings and boiler steels below chlorine-containing salt deposits

High temperature corrosion tests were performed on low-alloy ferritic steel and austenitic stainless steel, five high velocity oxy-fuel (HVOF) coatings, a laser cladding, and a diffusion chromized steel. Test conditions simulated superheater conditions of biofuel-fired boiler. The samples were exposed to synthetic salt containing 40 wt% K₂SO₄, 40 wt% Na₂SO₄, 10 wt% KCl, and 10 wt% NaCl. Exposures were carried out in oxidizing and in reducing atmospheres. The test temperature was 550 °C and the test duration was 100 h. Corrosion was extremely severe in oxidizing conditions because of active oxidation. In reducing atmosphere corrosion was retarded due to depletion of chlorine in the scales by evaporation of metal chlorides, and formation of a layer rich in chromium, sodium, sulfur, and oxygen adjacent to the metal surface. The corrosion resistance of coatings was determined by composition and microstructure. Oxides at splat boundaries were attacked by chlorine, and chlorine was able to penetrate through the coatings along splat boundaries.