Preparation And Performance Of Phosphating Coatings By Electrochemical Treatment On Magnesium Alloy

As a lightest structure material among all the metal materials, magnesium alloypossesses some excellent characteristics, such as low density, high specific strength,good conductive ability, specific elelctromagnetic shielding property and favorablevibration and damping properties. So it is widely applied in industry fields such asaerospace, automotive industry, communication, optical instrument and computer. Inthe process of magnesium alloy usage, the biggest problem is the environmentaladaptability. In the different kinds of environment, which consist of humid atmosphere,acidic solution, neutral solution and alkaline solution, the magnesium alloy shows apoor corrosion resistance. In this study, the phosphating coatings were prepared byelectrochemical method on magnesium alloy. The effects on the different processparameters and additives on the corrosion resistance and morphology of phosphatingcoating were discussed. The performances of the coating were also investigated.The single factor tests were performed to character the current density ofelectrochemical phosphating, treatment time and treatment temperature. Themeasurement methods, which consisted of scanning electron microscope (SEM), energyspectrum (EDS), electrochemical test and copper sulfate intravenous dropping, wereused to character the performances of the coatings. From the results, it was shown thatthe corrosion resistance of phosphating coating first increased and then decreased withthe increasing of treatment current density. The best corrosion resistance and the mostdense and regular coatings can be obtained when the treatment current density and thetreatment time were about0.5A/dm2and3m min. With the increasing of treatmenttemperature, the corrosion resistance increased remarkably. When the temperature risedto55℃, phosphating coating corrosion resistance increased slow. The appropriatetemperature of the solution was about55℃.The chemical phosphating and electrochemical phosphating were compared in this study. The results showed that the size of electrochemical phosphating coating wasmuch bigger than the chemical phosphating film. The main compositions of thechemical phosphating coating and electrochemical phosphating coating were all Zn,ZnO and Zn3(PO4)2(H2O)4. The crystallization of electrochemical phosphatingcoating was better than chemical phosphating. The corrosion resistance ofelectrochemical phosphating coating was better than chemical phosphating coating.The experimental results showed that the pretreatment of magnesium alloy had greaterinfluence on the corrosion resistant performance and morphology of phosphate coating.After hydrofluoric acid pickling, the surface of magnesium alloy could produce MgF2precipitation to prevent external corrosion matrix. When treated by hydrofluoric acidpickling, morphology and corrosion resistance of phosphating coating were changedobviously to increased corrosion resistance significantly.The adding of sodium3-nitrobenzene sulfonate can improve the performance ofphosphating coating on magnesium alloy. The corrosion resistance of phosphaingcoating on magnesium alloys was enhanced obviously when adding3-nitrobenzenesulfonate, and the phosphating solution was more stable and less sediment. Along withthe increase of sodium3-nitrobenzene sulfonate content, the corrosion resistance ofphosphating coating was first increased and then decreased. The best corrosionresistance and morphology could be obtained when the conent of sodium3-nitrobenzene sulfonate was about0.5g/L. the rare earth cerium adding to thetreatment solution could improve the corrosion resistance effectively. The experimentsshowed that the concentration of cerium nitrate should not be too much. Otherwise theexcess amount of cerium could due to the rapid reaction to generate a large number ofbubbles to make phosphating coating roughness and make the corrosion resistancedecreased. When the concentration of cerium nitrate was40mg/L, the best corrosionresistant performance and morphology of phosphate coating could be obtained.After phophating, the post-processing using sodium silicate sealing could improve thecorrosion resistance of the phosphating coating. The contents of the sealing solution was8g/L sodium silicate, thiourea3g/L and confining temperature is80℃. According to the results of electrochemical test, with the increase of phosphating time, the corrosionresistance of phosphating coating was increased first and then decreased. becomesstrong after reducing, closed time is6min, the corrosion resistance of the phosphatingfilm is the best. When sealing time was6min, the best corrosion resistance ofphodphating coating could be obtained.