HOT CORROSION RESISTANCE OF CERAMIC-METALLIC THERMAL SPRAYED COATINGS

Concept: In this work, five different ceramic-metallic composite coatings were produced and characterized: four commercial compositions (Cr3C2 - NiCr, Cr3C2-self fusing alloy, Cr3C2 -NiCrAlY, Cr3C2-CoNiCrAlY) were deposited by thermal spray techniques (HVOF and APS), optimizing the deposition parameters by DOE technique. A new composite powder, composed by ceramic fillers, silica nanoparticles and a NiCr matrix, was developed and the related coating deposited. Characterization of coatings was carried out in terms of microstructural features (porosity, composition, phases distribution, microhardness) and resistance in hot corrosion tests: samples were exposed to a mixture of Na2SO4 and V2O5 salts at 750°C in air. Motivations and objectives: Fuel used in diesel engines for naval propulsion has a high content of impurities, like vanadium and sulfates. Such impurities form aggressive compounds, responsible for hot corrosion phenomena that can lead to severe degradation of the engine’s components, especially of exhaust valves. Thus, a protective coating is needed in order to prevent the valves from degrading because of the corrosive environment. Results and discussion: XRD, SEM and EDS analyses show the formation of nickel vanadate, chromium oxide and nickel oxide on the surface exposed to the salt mixture. The corrosion/oxidation kinetics seems to be strongly dependent to the amount of metallic phases in the coatings and to the porosity of deposited layer. The innovative nanostructured coating exhibits a good hot corrosion resistance even if characterized by a high metallic phases content.