Characterization of Zn–Mn Phosphate Coating Deposited by Cathodic Electrochemical Method

Abstract

Electrophosphating is a novel method that is used for accelerating the low-temperature phosphating bath. In this study, Zn–Mn phosphate coating was fabricated on mild steel substrate using the cathodic electrochemical method. Afterward, the effect of electrolyte pH and applied current density was investigated on coating properties. Microstructure, phase analysis and corrosion resistance of coating were evaluated using SEM, X-ray diffraction, potentiodynamic, immersion test and electrochemical impedance spectroscopy tests, respectively. Results indicated that the metallic and nonmetallic phases are present in the coating fabricated by the cathodic electrochemical method. The results of the corrosion evaluation tests indicated that the best coating resistance is obtained at pH = 2.5 and 20 mA/cm2 applied current density, due to compact phosphate layer of the coating. By further increase in pH and applied current density, the coating with poor corrosion resistance is formed due to high porosity of coating obtained at these conditions.