Corrosion and Erosion-Corrosion of Ceramics and Functionally Gradient Material-Coated Steels in Erosion Environments

As a result of their great chemical inertness, ceramics and functionally gradient materials (FGM) are being used increasingly as coatings on machine parts. However, studies have shown that ceramic-coated materials still exhibit corrosion behavior. The objective of this work was to study the corrosion and erosion-corrosion behavior of ceramics and FGM-coated materials in erosion environments. Test results showed that plasma-sprayed ceramic coatings, with or without NiCr (80% Ni-20% Cr) undercoating, did not inhibit corrosion completely but did reduce the corrosion rate. It was found that NiCr undercoating does not improve corrosion resistance; however, it can improve the mechanical cohesion between ceramics and substrate. Although ceramics and FGM coatings were found to decrease the additive and synergistic effects of erosion and corrosion, they did not show erosion-corrosion resistance as expected, particularly FGM coatings. The erosion-corrosion wastage of these coatings was generally greater than that calculated by combining pure erosion and corrosion. To improve the corrosion and erosion-corrosion resistance, it was necessary to modify the composition of the NiCr compound by varying the ratio of nickel and chromium or by decreasing the porosity in the ceramic-coated layers and combining these with a sealing treatment.