Study On Influences Of Processing Parameters On Corrosion Resistance And Hardness Of Anodic Coating Developed On Magnesium Alloy In Neutral Solutions

Magnesium alloy has good application prospects in aerospace and automobile industry due to its low density,high specific strength and damping capacities,and other properties.However,the poor corrosion resistance and wear resistance of magnesium alloy seriously limit its wide applications.Micro-arc oxidation（MAO）is a comparatively new surface treatment technology,which can prepare ceramic coatings with controllable chemical composition on the surface of magnesium alloy by adjusting the electrolyte compositions and electrical parameters.In this thesis,four additives,including potassium fluotitanate（K₂Ti F₆）,potassium fluozirconate（K₂Zr F₆）,sodium phosphate（Na₃PO₄）and sodium tetraborate（Na₂B₄O₇）were selected to prepare ceramic coatings on the surface of magnesium alloy in neutral solutions by single-factor and an orthogonal experiment to clarify the affecting regularity of electrolyte compositions on the properties of oxide film.In addition,the inhibition mechanism of electrolytes on magnesium alloy was also revealed and the process with the best corrosion resistance and hardness was successfully screened.The following results are drawn in this thesis.1.In a neutral solution,the effects of K₂Ti F₆ concentration on the surface morphology,chemical composition,corrosion resistance and hardness of oxide films were studied.The results show that the corrosion resistance of the Ti-containing composite coating is synergistically determined by the Ti element in the loose layer and the F element in the compact layer.Among them,the microporous diameter and porosity of anodic coating are closely related to the corrosion resistance.With the increased K₂Ti F₆ concentration,the Ti content and the hardness of MAO coating also increase.The best corrosion resistance of the coating is achieved when K₂Ti F₆concentration is 1 g/L.2.K₂Ti F₆,K₂Zr F₆,Na₃PO₄ and Na₂B₄O₇ were selected as electrolyte components,and their effects on the coating performance was investigated by an orthogonal experiment.The results show that coating chemical composition and thickness play important roles on its corrosion resistance and hardness.The influencing order of four electrolytes on the corrosion resistance of oxide film is Na₃PO₄>K₂Zr F₆>K₂Ti F₆>Na₂B₄O₇.The best corrosion resistance is achieved in the solution composed of 2 g/L K₂Ti F₆,2 g/L K₂Zr F₆,5 g/L Na₃PO₄ and 4 g/L Na₂B₄O₇,and the coating corrosion current density is 5 orders of magnitude lower than that of the substrate.The sequence on hardness is K₂Zr F₆>Na₃PO₄>K₂Ti F₆>Na₂B₄O₇.Zirconium oxide can gradually increase the coating hardness;The selected four electrolytes are all the corrosion inhibitors of magnesium alloy,and Na₃PO₄ is the most important factor affecting the corrosion resistance.5 g/L Na₃PO₄ can play the best inhibition role on magnesium alloy substrate.3.Based on the previous experimental results,the effects of Na₃PO₄ concentration under constant current mode and three constant voltages on the corrosion resistance of oxide films were investigated to further screen the process with the best corrosion resistance.The results show that the ceramic coating prepared with 5 g/L Na₃PO₄ under constant current mode achieve the best corrosion resistance,but the coating micropores are uneven in size with thin thickness.When the constant voltage is increased from 300V to 500 V,the micropore number on the surface of the coating is greatly reduced and the coating thickness gradually increased,When the voltage is 500 V,the fabricated coating is the thickest with 46.06μm and achieves the best corrosion resistance,and its corrosion current density is 5 orders of magnitude lower than that of the substrate.