CORROSION MECHANISM OF A NON-CHROMATE CONVERSION COATING FOR MAGNESIUM ALLOYS AND MAGNESIUM-BASED METAL MATRIX COMPOSITES IN NaCl AT DIFFERENT pH USING EIS

摘要

The use of magnesium alloys in applications requiring a combination of high strength, stiffness and lightweight has been applied in the automotive and transport industry nowadays. Despite of all of these achievements, one of the main problem that limits the use of magnesium and its alloys in many structural applications, is the poor corrosion resistance of these materials, especially in comparison with aluminium and its alloys. In order to overcome this problem, surface treatments, such as chemical and electrochemical anodising, surface sealing, painting and electroplating have been routinely used. In the present work, electrochemical Impedance Spectroscopy (EIS) was used to evaluate and characterize the corrosion mechanism of a non-chromate conversion coating used to protect Mg comm, ZC71 Mg alloy, WE43 Mg alloy and a 12%SiCp/ZC71 Mg alloy MMC. Evaluation of the coating was carried out by immersion of coated specimens in a 3.5 wt. % NaCl solution at room temperature and three pH values, namely 3.5, 7 and 12.