Investigation of the Anticorrosion Behavior of Zinc Phosphate Coating on Carbon Steel Substrate with Nickel and Cobalt Dopants in the Coating Structure in 3.5 wt.% NaCl Solution

In this research, zinc phosphate coating was applied to carbon steel substrate by an immersion method to decrease the substrate's corrosion rate. Surface morphology, elemental analysis, and microstructure were studied by FESEM, EDS, and XRD, respectively. Electrochemical evaluations were conducted to evaluate the corrosion resistance behavior of the coating, including potentiometry, potentiodynamic polarization (PDP) measurements, immersion corrosion testing, and electrochemical impedance spectroscopy (EIS) tests. The addition of nickel and cobalt to the coating structure improved the surface morphology to a more uniform structure. Nickel had a more intense effect than that of cobalt on reducing the crystallite size. According to the results of PDP and immersion corrosion tests, the application of the phosphate coating reduced the corrosion rate compared to the uncoated carbon steel. Moreover, with the addition of cobalt and nickel additives, the corrosion rate decreased by approximately three times compared to carbon steel. After one week of immersion in 3.5 wt.% NaCl solution, the coated samples exhibited good stability with minimal changes in the corrosion rate. According to the EIS, the epoxy coating applied on the phosphate-coated sample surface had good adhesion, as the coating resistance did not decrease after one week of immersion in a 3.5 wt.% NaCl solution .