Investigation of Corrosion Resistance and Surface Properties of Carbon Steel Treated with Phosphate Coating in Aqueous Media

Carbon steel is widely used in industrial applications due to its mechanical properties and high availability. Zinc phosphate conversion coating is commonly used in industry for surface pretreatment. It enhances corrosion resistance and improves adhesion between metal surfaces and subsequent treatments. The practical aspect of the study involved treating carbon steel samples with zinc phosphate at 90 °C using nitric acid, zinc oxide, phosphoric acid, and sodium carbonate. Then, nickel carbonate was added to the same metal samples for phosphating. The results revealed that adding nickel carbonate effectively improved the corrosion resistance of the samples, evaluated using weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy techniques. Furthermore, surface properties, particularly surface roughness, were assessed using scanning electron microscopy (SEM). The samples were tested before and after exposure to three corrosive media, including hydrochloric acid with pH 3, seawater with 3.5% sodium chloride, and sodium hydroxide with pH 11. The SEM showed the effect of the corrosive solutions on the zinc phosphate coating layer, in the HCl solution dissolving it and exposing carbon steel. Also, there was localized corrosion with pits and erosion. 3.5% NaCl exposure made the surface rough and deposited pitting corrosion products. NaOH immersion for 15 mins formed a uniform, dense, crystalline corrosion-resistant layer. The coating transformed to light gray, indicating improved properties. Also, incorporating nickel carbonate in treating carbon steel can substantially enhance its corrosion resistance.