Effect of Si3N4 particles and bath temperature on characteristics and corrosion protection abilities of Co-W coatings on carbon steel

In this study, Co-W and Co-W/Si3N4 coatings were electrodeposited at different temperatures and Si3N4 concentrations, respectively, to investigate the effects of these parameters on the chemical composition, thickness, morphology, structure, and corrosion behavior of the resulting coatings. Increasing the bath temperature from 25 to 65 degrees C resulted in less defective Co-W coatings with fine nodular morphologies, higher tungsten content (43.1 wt% at 65 degrees C), and smaller corrosion current density (2 vs. 7.8 mu A cm-2). With the addition of 2-25 g L-1 Si3N4 particles to the electrodeposition bath and the co-deposition of up to 8.1 wt% of these particles within the Co-W matrix, the nodular morphology gradually changed to cauliflower and then powdery form, and the tungsten content of the coatings decreased. The decrease in tungsten content resulted in structural changes from amorphous to nanocrystalline. According to the potentiodynamic polarization and EIS results, the Co-W/Si3N4 coatings deposited from baths containing 5 and 10 g L-1 of Si3N4 particles had the best corrosion resistance among the samples. The corrosion current density of Co-W/10 g L-1 Si3N4 coating was 0.7 mu A cm-2, which is significantly smaller than that of bare steel substrate (27 mu A cm-2).