Plasma electrolytic oxidation（PEO） coatings on AZ31 magnesium（Mg） alloy were developed using the aqueous solution with alkaline silicate and sodium hydroxide as a base electrolyte system.The effects of cerium（Ce） nitrate and lanthanum（La） nitrate additives on the voltage response,microstructure,compositions and corrosion resistance of PEO coatings were investigated by scanning electron microscopy（SEM）,energy-dispersive spectrum（EDS）,X-ray diffraction（XRD） and potentiodynamic polarization tests,etc.The results showed that Ce and La additives increased the stable voltage and compactness of the PEO coatings,while,those did not change the compositions of the PEO coatings.The corrosion resistance of the PEO coating obtained in solutions with La nitrate of 0.1 g/L was the best,followed by that with Ce nitrate of 0.1 g/L and that without additives.
Most researches on micro-arc oxidation mainly focus on the application rather than discovering the evolution of residual stresses. However, residual stresses in the surface coatings of structural components have adverse effects on their properties, such as fatigue life, dimensional stability and corrosion resistance, etc. The micro-arc oxidation ceramic coatings are produced on the surfaces of 6061 aluminum alloy by a homemade asymmetric AC type of micro-arc oxidation equipment of 20 kW. A constant current density of 4.4___0.1 A/dm2 and a self-regulated composite electrolyte are used. The micro-arc oxidation treatment period ranges from 10 min to 40 min, and the thickness of the ceramic coatings is more than 20 Bin. Residual stresses attributed to 7-A1203 constituent in the coatings at different micro-arc oxidation periods are analyzed by an X-ray diffractometer using the sin2~u method. The analysis results show that the residual stress in the ceramic coatings is compressive in nature, and it increases first and then decreases with micro-arc oxidation time increase. The maximum stress value is 1 667_＋20 MPa for period of 20 min. Through analyzing the coating thickness, surface morphology and phase composition, it is found that the residual stress in the ceramic coatings is linked closely with the coating growth, the phase composition and the micro cracks formed. It is also found that both the heat treatment and the ultrasonic action release remarkably the residual compressive stress. The heat treatment makes the residual compressive stress value decrease 1 378 MPa. The ultrasonic action even alters the nature of the residual stress, making the residual compressive stress change into a residual tensile stress.
