Effect of nano-Al2O3 particles concentration on microstructure and corrosion behavior of Co-P-nano-Al2O3 composite coating

Composite coatings containing P and ceramic particles are considered in terms of properties such as corrosion resistance, oxidation resistance and high hardness. In this study, Co-P-nanoAl2O3 composite coating was fabricated on the AISI 430 stainless steel substrate using a direct current electric deposition technique. The quality of the coating and its corrosion resistance were studied by changing the amount of alumina (3, 4, 5 and 6 g/L) in the electrolyte bath. In order to investigate the morphology of the composite coating and the analysis of the Co-P-nanoAl2O3 composite coated samples, a scanning electron microscope (SEM) and EDS analyzer were used, respectively. The results showed that by increasing the concentration of nano-alumina particles in the electrolytic solution from 3 g/L with a 1 unit increase to 5 g/L, the weight of nano-alumina particles in the coating increased. Then by increasing the concentration of nano-alumina particles in the electroplating solution the weight percentage of nano-alumina particles in the coating reduced. Also the results showed that, the composite coating formed in the bath containing 5 g/L alumina is perfectly uniform and continuous, while with increasing amount of alumina, it is not possible to form a suitable coating on the entire surface of the substrate. In order to investigate the corrosion resistance, potentiodynamic polarization experiments were applied in aqueous solution of 3.5% NaCl on coated and non-coated samples. The results of Tafel and electrochemical polarization tests were also correlated with microscopic images and showed that the coated in the bath containing 5 g/L alumina had the highest corrosion resistance.