PREPARATION AND CORROSION ELECTROCHEMISTRY BEHAVIOR OF CERIUM-BASED CHEMICAL CONVER- SION COATING ON AZ91 MAGNESIUM ALLOY

In order to develop an environmental friendly surface coating technology for mag- nesium,a chrome-free Ce-based process to form a conversion coating on AZ91 magnesium alloy was investigated.The open circuit potential and polarization curves were used to research AZ91 electro- chemical behavior in Ce(NO_3)_3+H_2O_2 solution.Electrochemical impedance spectroscopy(EIS)was chosen to evaluate corrosion resistance due to its low effect on electrochemical test system for getting high reproduction result.The effects of treatment time,temperature of electrolyte,concentration of Ce(NO_3)_3 and accelerant concentration on corrosion resistance were summarized,while an optimized technology was achieved,0.02 mol/L Ce(NO_3)_3,4 mL/L accelerant at 35℃for 35 min.The results indicated that the chemical conversion coating formed in optimized technology presented yellow and impact surface by naked eye,while took on micro cracks on the outer layer.The content of Ce in the surface layer is high.Tafel plots indicated that conversion coating restrained the anodic and cathodic reaction on the interface,while the corrosion potential raised anodic 250 mV and corrosion current density decreased about 100 times.EIS results of optimal coating in NaCl solution for 96 h also indicated that the conversion coating protect the AZ91 magnesium alloy effectively in the first 60 h.