Effects Of Narrow Slit And Treat Time On The Coating Of AZ31B Magnesium Alloy By Plasma Electrolytic Oxidation

Plasma electrolytic oxidation?PEO?process was performed on AZ31 B magnesium alloy using a home-made WH1 A power supply and with sodium silicate as the base electrolyte.Two aspects were mainly investigated:?1?The effects of the different factors on the coating characteristics and corrosion resistance in narrow slit of magnesium alloys by PEO,such as the current density,the width of the narrow slit,the position of the sample,the ultrasonic and so on.?2?The influence of sodium hexametaphosphate in the base electrolyte on the structure and corrosion resistance of PEO coating with different processing time.The coating forming results show that: With the increasing of the current density,the coatings coated in the narrow slit are deeper,and with the width of the cracks increasing,the depth of the coatings in the narrow slit adds either.The coatings with different depth are gained by changing the positions of the sample,when the sample parallels with the bottom of the bath,the coatings in the narrow slit are deeper compared with that vertically places in the electrolyte.The coatings processed by ultrasonic are deeper in the narrow slit than coatings without ultrasonic.That is it is much easier to form coating with ultrasonic in the narrow slit.The electrochemistry tests show that the corrosion resistance of the coatings coated in narrower slit is worse than that in the wider slit,but they are both better than the substrate and the coatings coated outside has the best corrosion resistance.When the sodium hexametaphosphate was added in the base electrolyte,the roughness and thickness of the coating both increased with the processing time increased.According to the EDS,it revealed that the relative mole percent of silicon element increased at first and then decreased,while that of phosphorus was increased gradually.The electrochemical results indicate that the corrosion resistance of the PEO coatings has a rising tendency firstly and declining tendency lately,the coating with 5min treat time has the best corrosion resistance.Therefore,it suggests the phosphate maybe can promote the formation of Mg2SiO4,but for the PEO coating with longer treatment time,its corrosion resistance is decreased due to the formation of Mg2SiO4 is influenced by the increase of Mg₃?PO₄?₂.