Study Of Phosphate Conversion Coating On Magnesium Alloys

Magnesium alloys as new structural materials have low density and high strength/weight ratio, and excellent dampingcapacity. These properties make them have huge potential applications for lightweight engineering applications, especially in the automotive, aerospace and household appliances, computers, communications and other electronic devices. However, the application of magnesium alloys has been limited due to the undesirable properties, including high chemical activity and poor corrosion resistance.Chemical conversion coating is an effective method to improve the corrosion resistance of magnesium alloys, as it is simple and easy to be manipalated, economic, corrosion resistant, etc. A novel zinc-calcium phosphate conversion coating on wrought magnesium alloy AZ31 was prepared. And their structure, corrosion properties and process parameters of phosphate conversion coating as well as mechanism of formation were investigated.The orthogonal tests were used to optimize the parameters of the bath component. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), immersion corrosion tests, electrochemical techniques and other surface analysis techniques were applied to investigate the surface morphology, structure and formation mechanism of the zinc-calcium phosphate conversion coating.The results showed that the optimum process variables of the zinc-calcium phosphate conversion coating was bath components, which is composed of 10g/L Na₂HPO₄, 4g/L NaNO₂, 6g/L Zn(NO₃)₂, 2g/L NaF, 2g/L Ca(NO₃)₂ at a temperature of 50℃and a pH value of 2.5. The coatings exhibited crystal features. Compared with the zinc phosphate conversion coating on extruded AZ31 magnesium alloy, the zinc-calcium phosphate conversion coating was interlaced and dense and defect-free. As a result, it significantly improved the corrosion resistance of magnesium alloys in comparison with the zinc phosphate coating. Moreover, the chemical composition and forming process of magnesium alloys have an important influence on the structure of conversion coating. The corrosion resistance of the phosphate coating is closely related to the matrix. In other words, the higher the corrosion resistance of the substrate, the better the corrosion resistance of phosphate coating obtained on the surfaces.The formation of zinc-calcium phosphate conversion coating on extrude AZ31 magnesium alloys experienced three stages: (1) the magnesium matrix as anode dissolved; (2) the formation of phosphate nuclei and (3) growth of the phosphate nuclei. At the initial stageα-Mg matrix as the anode dissolved, and the hydrogen evolution occoured on cathode of the secondary phases. And then, the phosphate nuclei were formed around corrosion areas neighboring the secondary phases (Mg₁₇Al₁₂ and AlMn phases). Consequently, a part of insoluble phosphate salt generated new phosphate nuclei on the surface of magnesium alloy, and the other part have been absorbed and grew up on the formed phosphate nuclei. Finally, all the grain boundaries of the coating combined each other and the conversion coating was formed on magnesium alloys completely.