Corrosion of CrN and CrN/TiN coated heat-resistant steels in molten A356 aluminum alloy

The components of the equipment for processing aluminum melts into molded parts can be severely corroded by molten Al. In this study, a 6-μm CrN coating or a 4-μm CrN/TiN multilayer coating for providing physical and chemical barriers between molten reactive Al and the steel substrate were deposited by cathodic arc evaporation onto 10-mm-thick heat-resistant steel plates. Dipping tests were conducted in a 700°C A356 melt for 1–21 h at intervals of 3 h. The damage of the coated steel was evaluated by plane-view and cross-sectional metallography. Experimental results indicate that after a certain incubation period, the coated steel was locally attacked; forming hemispherical pits on both CrN and CrN/TiN coated steel samples. The incubation time for obvious pitting on the CrN coated steel was shorter than that for the CrN/TiN coated steel. Once the obvious pits were formed, the pitted areas increased with dipping time, regardless of the type of coating. Macroparticles were observed on the surface of both the CrN and CrN/TiN coatings. Both the CrN and CrN/TiN coatings exhibited a columnar structure that hardly changed after 21 h of dipping. The different incubation times of the distinct coatings were discussed in terms of the grain structure, macroparticles and thermo-mechanical stress associated with the physical vapor deposition-coated steels.