Micro-arc Oxidation Surface Treatment for Better Salt Spray Durability in CMT-Welded AA6061-T6 Aluminum Alloy Joints

ABSTRACT

Aluminum alloys, in particular 6xxx series alloys, find vast applications in the automobile sector, because of the intense need for light-weight materials to increase fuel efficiency. Gas metal arc welding is an extensively accepted joining technique for fabricating these aluminum alloys owing to the high productivity. But aluminum alloy joints are extremely vulnerable to salt spray corrosion because welding is associated with the breakdown and dissolution of intermetallics, and it is severe in case of high energy consumption processes. Therefore, low energy consumption welding processes like cold metal transfer (CMT) and pulsed CMT (PCMT) were employed. A salt spray corrosion test was employed to evaluate the corrosion properties of these welded joints. Salt spray testing revealed that corrosion attack was most severe in the fusion zone in contrast to the heat affected zone and base metal. Further, to enhance the corrosion resistance of the welded joints, micro-arc oxidation (MAO) coating technique was employed. From the electrochemical impedance spectroscopy (EIS) test data, it is confirmed that the absence of inductive characteristics in the coated specimens indicates the absence of localized corrosion, in contrast to the uncoated sample. The results show that MAO coatings exhibited higher corrosion resistance (i.e., ∼99 % improvement for CMT and PCMT joints) than the uncoated samples because of the formation of a thick oxide layer by a series of electrical discharges that can greatly resist corrosion.