ON PULSED CURRENT ELECTRODEPOSITED ZINC COATING OVER MAGNESIUM ALLOY AZ31 FOR ATMOSPHERIC CORROSION RESISTANCE

Abstract

Magnesium and its alloys have excellent physical and mechanical properties for aerospace applications. Its, low density, high strength to weight ratio, recyclability makes it an ideal metal for automotive and aerospace applications. It is some-times limited by its inadequate resistance to aqueous corrosion. In order to improve corrosion resistance, several surface treatments such as electrodeposition, conversion coatings, anodization, micro-arc oxidation treatment and organic coatings have been tried in literature. Among such techniques, electrodeposition provides advantages lower energy consumption, lower cost and easier to control. Electrodeposition using pulsed currents is known as pulse plating. The pulsed currents can be unipolar (onoff) or bipolar (current reversal). By using the bipolar pulse, metal deposition occurs in the cathodic pulse period, with a limited amount of metal being redissolved in the anodic period. This repeated deposition and partial redissolution could im-prove the morphology and the physical properties of the deposit. In the present study, only peak current density and temperature were varied keeping all other pulsed current deposition parameters constant to electrodeposited nanocrystalline coating on AZ31 magnesium alloy. The Zinc coating were electrode-posited over AZ31 Mg alloy using pyrophosphate bath. However, in order to prevent AZ31 Mg alloy from corrosion during electrodeposition and to ensure excellent ad-hesion of coatings to the substrate, AZ31 Mg alloy was pretreated using alkaline bath and activation bath. The pulsed current density was varied between 14 and 52 mA/cm2. The electrolyte temperature was either ambient or 50 °C. Effect of pulsed current density over morphology, grain size and corrosion resistance of the v coating was studied using scanning electron microscope, x-ray diffraction, electro-chemical tests respectively. Electrochemical tests, such as free corrosion potential, linear polarization, Tafel polarization and electrochemical impedance spectroscopy were conducted using 3.5% NaCl electrolyte. The electrochemical tests showed that the corrosion rate decreased four times due to coating zinc using pulsed current density 22 and 45 mA/cm2 over the alloy.