Microstructural characterisation studies on hot corrosion behaviour of YSZ/Gd2Zr2O7 based plasma spray coatings on SS316

The present study uses microstructural analysis to examine the impact of integrating the rare earth oxide Gadolinium Zirconate (Gd2Zr2O7) into the primary YSZ powder during plasma spray coating on an SS316 substrate. The ceramic coatings are formulated with two distinct concentrations: 5 wt.% Gd2Zr2O7, designated as 5GDZ, and 15 wt.% Gd2Zr2O7, designated as 15GDZ. The coating thickness was consistently maintained at a bond coating of 50µm and a top coating of 200µm across all coated samples by controlling relevant process parameters, including current, powder feed rate, and standoff distance. Hot corrosion tests were conducted on the samples using 50 mg/cm² of molten salt comprising 60 wt.% V2O5 and Na2SO4 at a temperature of 700°C for 12 hours. The results indicated that YSZ and 15GDZ effectively prevented corrosion in the hot molten salt environment. The corrosive products containing YVO4 and m-ZrO2 in the YSZ and 15GDZ coating act as a passivation layer to inhibit corrosion to a certain extent. Compared to YSZ, 5GDZ shows a weight gain of 162.5 mg/cm², 58.54% higher. However, there is no noticeable improvement in hot corrosion resistance. The 5GDZ coating exhibited the formation of thin, corrosive products. More spallation, cracks, and fractures are evident in the 5GDZ coating. The weight gain of 15GDZ is quantified at 115.32 mg/cm², representing a 40.9% reduction compared to the 5GDZ coating. Hence, further increases in the weight of Gd2Zr2O7 were added with YSZ beyond 5 wt.% demonstrate an enhancement in hot corrosion resistance. The penetration of molten salt into the bond coating interface and substrate is completely inhibited in all three coatings, as evidenced by the SEM and EDAX analysis.