Inhomogeneity And Application In Corrosion Resistance Of Microarc Oxidation Coatings On Al Alloy Welded Joints And Structure

Aluminum alloy is widely used in various fields for its excellent properties. As common joining methods, welding is usually utilized to connect aluminum alloy structure. However, welded joint of aluminum alloy can be easily corroded and weaken, leading to the failure of welding structure. Micro-arc oxidation (MAO), a newly developed technique, could improve surface properties, especially for the metals such as magnesium, aluminum, titanium and their alloys. Welded joints of aluminum were oxidized and ceramic coatings formed on the surface of samples by MAO process. The characteristics of the ceramic coatings and the properties of welded joints were tested and analyzed systematically.Firstly, inhomogeneities of MAO layers on the surface of aluminum samples welded by TIG were investigated, through analyzing the distribution of current density, morphology and thickness of layers. Also effect of micro-arc oxidation on surface morphology, phase composition and corrosion resistance of aluminum alloy were investigated. SEM, XRD, Electrochemical analyzer and stress-corrosion analyzer were used to investigate surface morphology, phase composition and corrosion resistance of MAO coatings.The results show that the phase composition of the coatings on the surface of inner surface of V-shaped specimen does not demonstrate obvious difference for the inner surface of V-shaped specimen. In contrast, the thickness of the coatings varies substantially at different sites. With increasing of the V-shaped angle of specimens, the thickness difference becomes slighter. And corrosion resistance of the coatings near the edge of the samples was better. The effect of pulse duration on inhomogeneity has not been observed, but with the increase of oxidation time, inhomogeneity degree will enlarge. As to blind pipe, thickness of the inner surface coatings is not uniform and decreases gradually from the tube top to bottom. Surface morphology varied substantially, though phase composition only has slight difference in diffraction peak intensity. The enhancement of electrolyte circulation and utilization of auxiliary electrode could largely decrease the inhomogeneity of MAO. In earlier stage of micro-arc oxidation, discharge-difference exists between welded joints and base metal, but with oxidation time increasing, the difference becomes smaller and smaller, and finally their thickness, morphology and corrosion resistance will all be the same.The biggest conversation of MAO coatings to the welded joints was the enhancement of corrosion resistance. The difference of corrosion resistance of welded joints is small with various processing parameters including current density, frequency. The weight-loss of original welded joints rate after saline soaking is more than those with MAO coatings, and with increasing current density and frequency of MAO discharge, weight-loss ratio will decrease. When the welded joints with MAO coatings are immersed in 3.5 wt% NaCl solution with the load of 80% fracture strength, corrosion traces on the surface of specimens could be clearly observed, but MAO coatings can be hardly corroded in corrosion liquid.