Synthesis and Characterization of bi-cationic Zinc-Calcium Phosphate Coating on Plain Carbon Steel Using an ECO-friendly Accelerator

The purpose of this study is to use of an Eco-friendly accelerator in zinc-calcium phosphating process. For this, the conventional phosphating parameters (containing nitrate and nitrite) were optimized using literature; Then, the accelerators were substituted with H2O2. The morphology of the phosphate coatings was observed by a scanning electron microscope (SEM), and the chemical compositions were investigated by energy dispersive X-ray spectroscopy (EDS). The crystalline structures of the phosphate coatings were analyzed by X-Ray Diffraction (XRD). The corrosion resistance of samples was assessed through polarization tests in 3.5 wt.% NaCl solution. According to the results, about 0.6 mL/L was obtained as optimum concentration of H2O2. Also, the optimum temperature and pH of phosphating were about 47 °C and 3.2 respectively. The XRD results indicated that the coating structure include phosphophyllite, hopeite and some scholzite phases. SEM images showed that, in the presence of H2O2, the geometrical shape of coating crystals is cubic, which are grown uniformly on the entire area the substrate. Also, according to the electrochemical parameters, the coating obtained from the bath containing 0.6 mL/L of H2O2, has the highest corrosion resistance. Therefore, it can be deduced that, H2O2 could be used to replace the traditional accelerators like nitrate and nitrite, since it is less pollutant and provides better quality of the coating.