Plasma Electrolytic Oxidation And Electroless Plating On The PEO Coating Of Magnesium Alloys AZ91D

Mgnesium alloys, which is the lightest metal in the engineering metal, named as the "environmental engineering material in the 21^st century". Its potential applied field including automobile, aerospace and cosuming electronics, etc. In order to solving the corrosion problem of magnesium alloy, the plasma electrolytic oxidation which was developed from the traditional anodic oxidation was introduced into the research of magneisum protection. An in situ ceramic protective coating can formed on light metal surface by this technology.A plasma electrolytic oxidation coating prepared on AZ91D magnesium alloy by the means of step controlling the electric parameters. SEM morphologies show that the PEO coating can be divided into compact layer and loose layer. Lots of discarged pores existed in the coating. The dynamic potential polarization research show that j_corr has been reduced about four orders of magnitude and a 0.3V opassivation potential presented in the dynamic potential polarization curve. The electrochemical impedance magnitude improved by four orders. The electrochemical research results show that the corrosion resistance has been improved apparently. In the corrosion process, the PEO coating has blocked the transportation of corrosion medium and corrosion products.The research results shows that in order to ensure the processing of electroless plating on the PEO coating, the PEO coating have to be acted by ionic pallarium solution, the main salt of electroless bath have to be NiCO₃·2Ni(OH)₂·4H₂O.The nickel coating was prepared on the PEO coating according to the process which was set down in this paper. The content of P was 10% according to EDAX result, which was a high P nickel coating. XRD result shows that the structure of nickel plating coating was amophous. The coating is in an classical cellular mopology. The Ni coating grew from the loose layer of PEO coating which was recognized by the cross sectional structure analysis. The Ni coating can be physical bonded to PEO coating because of "physical locking". The cohesion of Ni coating was excellent according to hot shock and file test. Electrochemical results show that the protective coating is corrosion resistant.