Ceramic-metal thermal spray coatings for the protection of marine diesel engines components

The present study is focused on the development and testing of thermally sprayed coatings for the protection of exhaust and intake valves of marine diesel engines. Different solutions were tested and compared for both corrosion resistance to the specific acidic environment and abrasive wear resistance, before and after the corrosion attack. Coatings were deposited by high velocity oxy-fuel (HVOF) thermal spraying on both martensitic steel and Nimonic 80A substrates. Optimization of thermal spray parameters were carried out by design of experiment procedures, on the basis of coatings cohesion, adhesion, hardness and thickness. Corrosion experiments were carried out in a boiling solution of 5% sulfuric acid, while the abrasive wear tests were conducted adopting the block on ring configuration on both as sprayed and corroded coatings. The most promising coating was selected for an “on field” test: exhaust and intake valves were coated and put in service on a ship engine in order to evaluate the lifetime of the coatings working in operating conditions.