Preparation And Corrosion Resistance Of Bio-Ceramic Coating By Micro-Arc Oxidation On Magnesium Alloy

In this work, the ceramic coatings containing calcium, phosphorus were formed on magnesium alloy AZ91D substrate by micro-arc oxidation (MAO). In the NaOH system and NaOH-calcium salt system, the effects of the composition of the electrolyte and technical parameters on the ceramic were studied. The coating thickness and roughness, phase composition, morphology and elemental distribution were charaterized by thickness gauge, roughness gauge, XRD, SEM and EDS, respectively. The corrosion resistance of the ceramic coating in simulation body fluid (SBF) solution was evaluated by the electrochemical methods. Besides, the growing process of the ceramic coating was discussed.In the NaOH electrolyte system, the prepared ceramic coatings are porous, and the diameters of the micro-pores are within a few microns in general. The coatings prepared with longer reaction time or higher current densities are more thick and rough, presenting the flake-staked structure. Calcium from the electrolyte is successfully came into the ceramic coating prepared in the NaOH-calcium salt electrolyte system. With the increase of the concentration of Ca2+, the thickness and roughness of the ceramic coatings are increased. The coating prepared in the two systems are mainly composed of MgO, the amount of which changs with different compositions of the electrolyte or technical parameters.The corrosion resistance of the coatings in NaOH-calcium salt system is decreased a little, compared with NaOH system. Generally, the corrosion resistance of the coatings is improved by increasing the reaction time and the frequency, or decreasing the peak current density. When the ceramic coatings were immersed into the SBF solution, the SBF permeated the micro-pores of the coatings gradually, and the equilibrium of the system was achieved in about 4h. During this period, the corrosion resistance of the coatings changed less. Then the corrosion resistance of the coatings was decreased with extending the immersing time. The calcium salt in the electrolyt system brought forward the equilibrium time.Under the same treatment voltage, the coatings prepared with constant voltage (CV) mode are fairly thick and rough, the amount of micro pores was few, the outlayers are rough and coarse whereas the inner layer are dense and therefore the coating samples show the best corrosion resistance; The coatings prepared with constant power (CP) mode are of the least roughness and a better corrosion resistance; and the coatings prepared with constant current (CC) mode present the worst corrosion resistance. Under the same electrical source mode, the coatings prepared at around 400-450V for the CV mode have better corrosion resistance, the ones prepared at aroud 450-480V for the CC mode have better corrosion resistance, and the ones prepared at 450V for the CP mode have better corrosion resistance.During the MAO process conducted under the CP mode in NaOH system and NaOH-calcium salt system, the porosity and roughness of the coating are increased gradually in both eletrolyte systems. In the spark region, the thickness of the inner compact layer is increased with the treatment voltage. In the micro-arc region, some damage to the compactness of the coating was caused by the larger spark discharging. Adding of the calcium salt into the electrolyte, the porosity of the coatings is decreased while the depth of the micro pores is increased, and the density of the coating is decreased to a certain extent.