Electrochemical Behavior of HVOF-Sprayed Amorphous and Nanocrystalline Fe-Based Fe73.13Si11.12B10.79Cr2.24C2.72 Composite Coatings

The present work describes a series of new amorphous and nanocrystalline composite coatings (composition: 87.6% Fe, 6.7% Si, 2.5% B, 2.5% Cr, 0.7% C in wt.%) on a mild steel substrate made by high-velocity oxy-fuel thermal spray at different feed rates of 20, 30 and 40 g/min. The microstructure characterization using scanning electron microscopy coupled with energy-dispersive spectroscopy shows uniform and adherent coatings of different thickness depending on the feed rate. The structure of the coating is composite in nature (mixture of amorphous and nanocrystalline phases) as confirmed by the x-ray diffraction and transmission electron microscopy. The hardness of the coating is almost 6-7 times than that of the substrate. Though polarization test of the coating demonstrates corrosion resistance in 3.5% NaCl solution similar to the substrate, the difference of corrosion potentials of the coatings and the substrate is largely negative (similar to more than 200 mV against saturated calomel electrode) suggesting anodic nature of the coating as compared to the substrate resulting in sacrificial effect.