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

High temperature corrosion tests were performed on low-alloyferritic steel and austenitic stainless steel, five high velocityoxy -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 byevaporation 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 bycomposition and microstructure. Oxides at splat boundaries were attacked bychlorine, and chlorine was able to penetrate through the coatings along splat boundaries. � 2003 Elsevier Ltd. All rights reserved.

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