Microstructural characterization and electrochemical corrosion behaviour of HVOF sprayed Alloy718-nanoAl2O3 composite coatings

In the present research work, microstructural evolution and corrosion behaviour of the high-velocity oxy-fuel (HVOF) sprayed Alloy718-nano Al2O3 composite coatings were examined. These coatings were deposited on the grey cast iron (C.I) substrate. The agglomerated nano-Al2O3 reinforcement particles in three different proportions (10, 20 & 30 wt.%) were varied in the Alloy718 based matrix to develop the composite coatings. Corrosion testing of bare and coated samples was carried out at an ambient temperature in the NaCl solution (3.5 wt.%) using the potentiodynamic polarization technique. The polarization resistance (Rp), the corrosion efficiency and the corrosion rate (CR) were measured for the developed coatings and substrate. The surface morphology of as-sprayed and corroded coatings was studied using scanning electron microscopy coupled with energy-dispersive spectroscopy. X-ray diffraction analysis was conducted to determine the different phases present in the feedstock powders and as-sprayed coatings. The as-sprayed coatings were seen with nano-Al2O3 particles uniformly embedded in the melted Alloy718 matrix. The coating with 30 wt.% nano-Al2O3 particles revealed the maximum microhardness of about 1296 ± 21 HV0.2 compared to other coatings. The results showed that the corrosion resistance of the composite coatings increases with the increase in alumina particles in the Alloy718 matrix. The 30 wt.% alumina coating showed maximum resistance to corrosion due to the maximum decrease in the electrochemically active area associated with this coating.