Preparation Of Corrosion Resistant And Heat-resistant Coatings By Plasma Electrolysis Discharge

Magnesium alloy has low density,light weight,high specific strength and excellent electromagnetic shielding performance.It is widely used in the fields of aviation,aerospace,automobile,electronic communication and so on.However,because of its loose and porous oxide film produced naturally,its corrosion resistance and high temperature resistance are poor,which greatly hinders its further engineering applications.In this paper,plasma electrolytic oxidation?PEO?technology was used to grow ceramic coatings on the surface of AZ31 magnesium alloy by using the common solution?Na2SiO3and NaOH?with/without additional auxiliary additives?KH₂PO₄?Na₂B₄O₇?10H₂O?.The properties of the different ceramic coatings were tested and the growth mechanism of the coating was discussed.The following work has been done.?1?Orthogonal experiments were conducted to optimize process parameters,and voltage,frequency?duty cycle?,processing time and EDTA-2Na concentration were chosen as the research factors.The effect of the factors on corrosion resistance and heat resistance was systematically studied,respectively.?2?Under the optimized process parameters,the S coating was prepared in common solution without auxiliary additives.The P coating was prepared by using the electrolyte containing KH₂PO₄ additives,and the B coating was prepared by using electrolyte containing Na₂B₄O₇?10H₂O additives.?3?By using VK-X series shape measurement laser micro system and XRD,the micro surface morphology,surface three-dimensional image and phase composition of the S,P and B coatings were systematically studied.?4?The corrosion resistance of the S,P and B coatings were studied by dynamic potential polarization curve test and full immersion corrosion test.?5?The heat resistance of S,P and B coatings was evaluated with the high temperature oxidation experiment and the cold and heat cycle impact test.?6?The ceramic coatings were sealed with boiling water,sodium aluminate solution and water glass solution.Taking S coating as an example,the surface morphologies of the S coatings were studied before and after the sealing treatment.The corrosion resistance and high temperature oxidation resistance of the unsealed and sealed samples were compared and studied.The test results show that,according to the corrosion and heat resistance,the best process parameters are as follows:the frequency is 12KHz,the working voltage is 390V,the oxidation time is 40min,and the EDTA-2Na concentration is 1g?L^-1.Compared with S coating surface morphology,the surface of P coating and B coating were composed of a larger area of sintered oxide.The corrosion resistance of the samples treated by plasma electrolytic oxidation were improved and the corrosion rate was greatly reduced.The high temperature oxidation resistance and thermal shock performance was significantly better than that of the base materials,and the properties of P and B coating prepared in the auxiliary additives solution were better than that of S coating.After sealing,the surface of the samples became more dense,the micro pores greatly reduced,and the surface defect of the porous coating caused by plasma discharge was effectively compensated.