Studies on corrosion resistance (in 3.56 wt.% NaCl solution) of thermal-sprayed CoNiCrAlY (mechanically milled) on Inconel 718 substrate

In the present study, nanocrystalline CoNiCrAlY powder has been synthesized by mechanical milling and has been subjected to coating on INCONEL 718 substrate by high velocity oxyfuel (HVOF) and air plasma spraying (APS) techniques. Followed by coating, a detailed evaluation of the corrosion behavior of the coated surface has been carried out in a 3.56 wt.% NaCl solution by potentiodynamic polarization study and electrochemical impedance spectroscopy. Commercially available CoNiCrAlY powder was milled for 10 and 25 h using stearic acid as a process control agent (PCA) in a tungsten carbide (WC) ball and vial at a speed of 300 RPM. A 150 μm thick coating of both as-received and milled powders was sprayed onto Inconel 718 substrate pre-treated by sand blasting. The microstructure of the coating consists of γ (Co, Ni, Cr), γ′(Co, Ni) 3 Al, and β (Co, Ni)Al phases. A significant improvement in corrosion resistance was observed in the coating with 25 h milled CoNiCrAlY as compared to the as-received one due to the uniform distribution of β phase along the matrix. The study established a relationship between corrosion resistance, crystallite size, and microstructure, highlighting how these factors influence the overall performance of coatings. A comparison of the coating shows that the HVOF coating offered a superior corrosion resistance than plasma spraying. It also compared the benefits of mechanical milling in improving the properties of coatings applied using High-Velocity Oxy-Fuel (HVOF) and Air Plasma Spray (APS) techniques.