Decay Process And Aging Mechanism Of Silicate-containing Electrolyte For Micro-arc Oxidation On Magnesium Alloy

Wearproof and corrosion resistance of magnesium alloy is improved greatly by processing ceramic coating on the surface of magnesium alloy with micro arc oxidation (MAO) technology, however, electrolyte needed by MAO technology is constantly decay with increase of workpiece numbers processed in electrolyte, which results in instability of electrolyte、increase of defects in ceramic coating processed、 decrease of ceramic coating performance, Therefore study of decay process and aged mechanism of AZ91D magnesium MAO electrolyte is a key problem in industrical application of MAO technology.Firstly, variation law of coating in decay process of electrolyte is investigated, it is discovered that before of coating with macroscopical defects, the microstructure and properties of coating did not changed significantly, for example, with decay of electrolyte, increasing of coating thickness slowly, decreasing of density and corrosion resistance slowly, coating phases mainly including MgO, MgF2, Mg2SiO4and Mg2A104; after of coating with macroscopical defects, thickness、density and corrosion resistance of coating decreased suddenly, macroscopical defects including not only original phases of MgO, MgF2, Mg2SiO,4and Mg2A104but also new phases of Al2O3, KAlSiO4, K1.5Al11O17.5.Secondly, decay process of MAO electrolyte was studied. It was found that with increasing of workpiece numbers processed in electrolyte, breakdown voltage of electrolyte constantly rised, in original ions of electrolyte, significantly decrease of silicate and increase of hydroxide, but not obvious change of fluoride. Most of magnesium and aluminum from magnesium alloy to electrolyte and silicate、 hydroxide and fluoride in electrolyte reacted into mostly precipitation and colloid, only a small portion of magnesium and aluminum existed in form of ion, precipitation mainly contained silicon acid magnesium and colloid mainly included colloid of aluminum hydroxide、colloid of silica and alumina、micelles of large molecular size formed by adsorption of ions in electrolyte. In addition, conductivity、salinity and TDS of electrolyte rised, which proved decomposition of electrolyte and new ions of produce constantly, however, ionic strength of the electrolyte reduced, which proved consumption level of ions needed micro-arc oxidation. Finally, aged mechanism of micro-arc oxidation electrolyte was discussed. micelles with large molecular size were adsorbed on the surface of the workpiece and blocked the channel of moving of ions in the electrolyte, at the same time, electrolyte cannot provide the material needed by micro-arc oxidation because of consumption of precipitation formation and decomposition of electrolyte, it is most serious that enough energy produced by MAO voltage must been used, under of the condition, electic arc gathered on the surface of region with higher electric field, which resulted in forming macroscopical defects on the surface of coating.In addition, after coating with macroscopical defects, increase of temperature in electrolyte would stimulate electrolyte each ion reaction activity and exacerbated side reactions the electrolyte, eventually electrolyte aged...