Corrosion Properties of Thermally Sprayed Bond Coatings under Plasma Sprayed Chromia Coating in Sulphuric Acid solutions

Plasma sprayed chromia coatings are known to have excellent wear and corrosion properties in acidic conditions at ambient and elevated temperatures. Corrosion environments can be extremely damaging to such bond coatings due to absence of dissolved oxygen, concentration of the corrosive electrolyte under the top coating, crevice corrosion mechanisms inside the coating, galvanic coupling with the corrosion-resistant substrate, etc. 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 Ta coating were studied. The substrate material was solid Hastelloy C-276. The top coating was plasma sprayed Cr₂O₃. Corrosion properties were studied in sulphuric acid solutions of various concentrations at RT and at the temperature of 60°C. The corrosion measurements were electrochemical polarisation and EIS measurements, and immersion tests. The coating microstructures were studied before and after the corrosion tests. The results showed that both HVOF sprayed Ni-20Cr and Ultimet alloy coatings were attacked by the sulphuric acid electrolyte whereas the HVOF sprayed Hastelloy C-276 and plasma sprayed Ta coatings performed significantly better.