Corrosion Properties of Thermally Sprayed Bond Coatings
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Plasma sprayed chromia coatings are known to have excellent wear and corrosion properties in acidic conditions at ambient and elevated temperatures. Thermally sprayed metallic bond coatings are often between the ceramic top coating and the metallic base material in order to guarantee good adhesion of the ceramic coating to the substrate, however corrosion environments can be extremely damaging to such bond coatings due to absence of dissolved oxygen, high concentration of the corrosive electrolyte under the top coating, crevice corrosion mechanisms inside the coating and galvanic coupling between the coatings and even with the corrosion-resistant substrate material. When bond coatings are used, it is therefore of high importance to select the bond layer chemistry and method of manufacturing so that the bond coating can survive in such harsh conditions. In the present study, HVOF sprayed Ni-20Cr, Hastelloy C-276 and Ultimet alloy coatings, and plasma sprayed tantalum coating were studied. The substrate material was solid Hastelloy C-276 while the top coating was plasma sprayed Cr2O3. Corrosion properties were studied in sulphuric acid solutions of various concentrations (0.1M, 0.5M, 1M) at room temperature and at the temperature of 60°C. The corrosion measurements used in this study were electrochemical polarization, Electrochemical Impedance Spectroscopy measurements, and immersion tests. The coating microstructures were studied before and after the immersion test. At room temperature, the results showed that between all the bond coatings the plasma sprayed tantalum performed significantly better, in fact it had very good respond either in the electrochemical measurements and in the immersion test. The Ultimet alloy had the lowest corrosion resistance according to the tests performed. The HVOF sprayed Ni-20Cr and HVOF sprayed Hastelloy C-276 showed an intermediate corrosion resistance between the tantalum bond coating and the Ultimet alloy bond coating. At the temperature of 60°C the corrosion resistance of the different bond coatings changed especially for the Ni-20Cr; in fact the immersion test caused the completely dissolution of HVOF sprayed Ni-20Cr and the considerable attack of Ultimet alloy while the plasma sprayed tantalum and HVOF sprayed Hastelloy C-276Ni-20 Cr resisted to such corrosion conditions fairly well.