Research On Rare Earth Conversion Coating Technology On Aluminum Alloy Surface

Aluminum alloy is widely used in daily life because of its low price, low density, good machinability and so on. A lot of research has been done and a variety of surface treatment technologies have been developed to improve the appearance and corrosion resistance of aluminum alloy. Chromate conversion coating technology will be replaced by rare earth conversion coating which is eco-friendly for the protection of aluminum alloys in all probability. In this paper, the chemical polishing and rare earth conversion coating technology of 6061 aluminum alloy were studied.Polishing is an important pretreatment for the preparation of rare earth conversion coating on aluminum alloy surface. The basic composition of polishing liquid was obtained by reasonable design, the orthogonal experiment was used to study the main components of the polishing liquid, and the optimum content of the above components in polishing liquid was determined. Also, the optimum technological conditions, polishing time and temperature, were achieved by single factor experiment. The polished 6061 aluminum alloy showed smooth, bright and uniform surface. The roughness of sample was reduced significantly. Meanwhile, the polishing liquid showed low damage to aluminum alloy. The results indicated that the polishing technology had good polishing effect to the 6061 aluminum alloy.The preparation of the rare earth chemical conversion coating on aluminum alloy surface was designed. The optimum composition and process parameters were determined by orthogonal experiments. Further more, the properties of the prepared coating were studied. The prepared conversion coating is complete, golden in colour, and with a uniform morphology. The corrosion rate was calculated by immersion test, and the corrosion rate of the conversion coating was lower than that of the untreated sample. The corrosion potential of the conversion coating is higher than that of the untreated sample. The corrosion current density of the conversion coating is much lower than that of the untreated sample from the potentiodynamic polarization curves. Therefore, conversion coating sample has a higher corrosion resistance.The growth kinetics of the conversion coating was studied by measuring the change in thickness and weight of the coating. It was found that the coating increased rapidly in the first 30 minutes of the coating formation, and then increased slowly. The formation of the conversion coating was analyzed by scanning electron microscope(SEM), energy dispersive spectrometer(EDS), x-ray photoelectron spectroscopy(XPS) and electrochemical technologies etc. The microstructure of the conversion coating with different growth time indicate the growth process intends the generation of three-dimensional skeleton and the filling of the skeleton. Also, above steps were repeated to form the final coating. Moreover, it is noted that the rare earth element Ce plays a role of skeleton during the formation of the conversion coating. The EDS result revealed that the coating mainly include C, O, Al, Si, Mn, Fe, Ce, Mg. The main substance of the coating mainly include the oxide or hydroxide of Al, Mn, Ce. The chemical reaction equation of the coating formation was also inferred.