The Preparation And Study Of Magnesium Alloys Chemical Conversion Coating Performance

Magnesium alloys have low density, rich resources, superior heat and electricity conduction properties, mechanical properties and electromagnetic shielding effectiveness etc. It was widely used in vehicles, aerospace industry, electronics industry and other fields. The main components of magnesium alloys is metallic magnesium. The metallic magnesium has low electrode potential, so the chemical activities of magnesium alloys is high. It is easily corroded in the air and the oxide layer formed on the surface of magnesium alloys is loose and porous, which leads to the poorer corrosion resistance. That sharply limits the application of magnesium alloys.In order to overcome the deficiency that is the poor corrosion resistance. In the paper, AZ91 D magnesium alloys is as raw material, a film which is nonmetallic and non conducting is formed on the surface of magnesium alloys by chemical conversion and electrochemical conversion. Using permanganate drip, electrochemical test study the corrosion resistance of conversion coating and determine the optimal formula conversion solution and process conditions. The microstructure and element composition are studied by metallographic microscope, scanning electron microscopy(SEM)and EDS. The phase composition of conversion coating is analyzed by X-ray diffraction(XRD).A comparative study on microstructure, corrosion resistance, phase composition of magnesium alloys chemical conversion coating with electrochemical conversion coating and the tradition chromate conversion.Experimental results show that, the optimal formula conversion solution and process conditions : Zn O 1.5~2.5 g·L-1, Na F 0.3~0.6 g·L-1,tartaric acid 0.1~ 0.3 g·L-1, additive: SDS 0.05~0.15 g·L-1, Mn CO3 0.1~ 0.2 g·L-1, the time of surface conditioning is 60s~90s, the temperature of surface conditioning is 55~75℃, the acid ratio is 5:1~7:1,the conversion temperature is 60~80℃, the conversion time is 8~10min. The conversion coatings crystallize uniform and fine and the major components are Zn3(PO4)2·4H2O,Mg Zn2(PO4)2 and Mg3(PO4)2.Compared with the traditional chromate conversion coating, the chemical conversion coating and the electrochemical coating is denser and less crack. The corrosion resistance of chemical conversion coating is more prominent, followed by electrochemical coating. So the chemical conversion coating and electrochemical coating can substitute for chromate conversion coating and have more extensive application.