Thermal spray coatings for corrosion and wear protection of naval Diesel engines components

Intake and exhaust valves of naval Diesel engines suffer from several problems due to the combined and synergistic effects of mechanical stresses and a chemically aggressive environment. Hard chrome plating has been widely used as the standard solution for valves stem, but its limited wear-corrosion resistance and the high toxicity of its galvanic baths suggest to look for suitable alternatives, such as thermal sprayed ceramic-metallic (cermet) and self-fluxing alloys. The present study aims to compare different solutions in terms of corrosion resistance under a selected acid environment and of wear resistance both for as-sprayed and post-corrosion samples. Coatings have been deposited by HVOF both on martensitic steel and Ni-based superalloy. A Design of Experiment procedure has been used for spray parameters optimization, based on porosity and deposition efficiency of the coatings. Corrosion tests have shown the effect of porosity of the coatings, while wear tests confirmed the fundamental role of the dispersed hard phases. All the solutions here proposed have shown a significant improvement in terms of corrosion and wear resistance compared to hard chrome plating.