Laser surface melting and application of silane coating to improve the corrosion resistance of ZE41 magnesium alloy

Magnesium (Mg) alloys are the lightest metallic material with excellent physical and mechanical properties. High strength to weight ratio makes them very attractive to the automobile and aerospace industries, but the use of Mg alloys is restricted due to their poor corrosion resistance. For wider use of Mg and its alloys for structural applications, it is necessary to find a durable solution for their poor corrosion resistance. Corrosion is essentially a surface degradation process, which can be minimized by suitable surface modification and/or coatings. In this study, the influence of laser surface melting and application of silane coating on the corrosion resistance of ZE41 alloy has been investigated. Influence of laser surface melting and laser processing parameters on the resultant microstructure, and as a result, on the corrosion kinetics has been investigated. Although laser surface melting has profound influence on the resultant surface microstructure, contrary to what has been reported for other magnesium alloys, this treatment has been found to have little effect on the corrosion resistance of ZE41. This has been explained on the basis of the results of the scanning electron microscopy (SEM), X-ray mapping, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). SEM investigations of the corroded specimens revealed that the corrosion morphologies of the laser melted and the untreated alloy were significantly different. Development of a uniform silane coating necessitates the presence of a reasonably uniform hydroxide layer on the metal surface. Hence, in the present study, the influence of alkali surface pre-treatment on the corrosion resistance of the silane coated ZE41 has been investigated using potentiodynamic polarization and EIS. This study concludes that alkaline pre-treatment prior to silane coating considerably improves corrosion resistance. A thorough morphological and electrochemical characterization suggests that a particular alkaline pre-treatment, that develops a thick and uniform hydroxide film on the surface, also facilitates formation of a protective silane film on the substrate. Octadecyltrimethoxy silane (ODTMS) has been applied on alkaline pre-treated ZE41 alloy, and, a thorough and systematic electrochemical and analytical investigation of the influence of different coating parameters (such as, silane concentration, pH of hydrolysis and dipping time) on the resultant ODTMS coating, and hence on the corrosion resistance of ZE41 alloy in 0.1 M NaCl has been undertaken. This study suggests that the concentration of ODTMS, pH of hydrolysis and dipping time play crucial roles in the determination of the concentration of the silanols in the coating bath available for bonding with the metal substrate. ZE41 alloy, pre-treated in 3 M sodium hydroxide solution for 48 h, and then immersed for 60 min in the hydroalcoholic solution of 5% ODTMS hydrolysed at pH 7, provided about two and half orders of magnitude improvement in corrosion resistance at 170 h of immersion in 0.1 M sodium chloride solution.