Investigation Of Growth And Corrosion Behavior Of The Micro-arc Oxidation Coating Based On The Phosphate And Silicate Electrolytes

In this work,the micro-arc oxidation coating（MAO）was fabricated on the AA1060pure aluminum by a self-made WH-1A power supply device in the alkaline sodium hexametaphosphate and sodium silicate electrolytes.The evolution of the coating microstructure,the phase and elemental compositions and the corrosion behavior of the coating in 3.5 wt.%NaCl solution were investigated;the coating growth behavior and coating corrosion resistance were studied by means of“two-step”MAO treatment.The result indicated that the growth rate of the coating obtained in the alkaline sodium hexametaphosphate electrolyte under the constant current model decreased with the prolonging of the MAO treatment time,the coating surface microdefects increased and the corrosion resistance decreased.The investigation based on the electrochemical detachment technique showed that there is a compact layer of¹μm at the coating/substrate interface,and the thickness of the compact layer rarely changed during the treatment,and the density decreased slightly.The electrochemical test of the layer showed that the contribution of compact layer to the corrosion resistance of the coating was much greater than that of the outer porous layer.Surface elemental analysis and open-circuit potential test indicated that the ratio of Al/O on the surface of the coating gradually increased with the prolonging of treatment time,and the open-circuit potential of the coating gradually decreased.Immersion experiments showed that the corrosion process of MAO coating included the formation,migration,dissolution and regeneration of corrosion products.The dissolution of the coating is related to the reaction of the electrolyte and the amorphous phase in the coating.The study based on the“two-step”method indicated that silicate ions could be rapidly incorporated into the MAO coating by reacting with theγ-Al₂O₃ phase in the coating.Sodium hexametaphosphate was conducive to the homogenization of the inward-growth of the coating and the refining of the discharge channels.The electrochemical tests showed that sodium silicate electrolyte was not conducive to the improvement of corrosion resistance of the inner barrier layer of the coating,and the“two-step”method could improve the corrosion resistance of the prepared coating in the sodium silicate electrolyte by changing the structure of the coating.