Influence of thermal treatment on the corrosion resistance of electrolytic Zn-Ni coatings

This study was undertaken in order to obtain and characterize the corrosion resistance of Zn-Ni coating. The process was carried out under galvanostatic conditions (j = 50 mA·cm−2) chosen on the ground of an analysis of the deposition process in the Hull’s cell. The Zn-Ni coatings were deposited on austenitic (OH18N9) steel substrate from the ammonia bath. Thermal treatment of Zn-Ni coating was carried out in argon atmosphere. Structural investigations were conducted by X-ray diffraction method. Surface morphology of the obtained coatings was determined using a scanning electron microscope (JEOL JSM-6480) with EDS attachment. The electrochemical corrosion resistance of the prepared Zn-Ni coatings, austenitic (OH18N9) and (St3S) steels, was defined. The studies of electrochemical corrosion resistance were carried out in 5 % NaCl, using potentiodynamic and electrochemical impedance spectroscopy (EIS) methods. Examinations of localized corrosion resistance were conducted using scanning vibrating electrode technique (SVET). On the grounds of these investigations it was found that Zn-Ni coating after thermal treatment was more corrosion resistant than the Zn-Ni coating before thermal treatment. The relatively good corrosion resistance of Zn-Ni coatings is not as high as the resistance of (OH18N9) steel substrate, but higher compared to (St3S) steel. Therefore, the Zn-Ni coatings may be regarded as a protective coating for St3S steel.