Study On Technology Of Barium Phosphate Surface Treatment For Magnesium Alloy

Magnesium alloys are considered as green engineering materials of 21 century because of their attractive properties, including low density, high strength-to-weight ratio, high thermal and electrical conductivities, and easy recyclability. However, magnesium-based alloys are susceptible to galvanic corrosion and thus the use of magnesium and its alloys is severely limited. Chemical conversion treatment is a widely used protective method because of its low cost and simple operation. Conversion coatings for magnesium alloys have traditionally been based on poisonous hexavalent chromium (Cr⁶⁺) compounds.Phosphate is a development trend of surface chemical treatment for magnesium alloys because of the good corrosion resistance and no poisonous.The main work and results are as follows:1 The technics of pretreatmentThe optimal pretreatment, which is NaOH degreased, and then HF, C₂H₆O₂ acid pickled, and finally NaOH activation processed, was determinde after lots of screening experiments and orthogonal tests. By Effect of the optimal pretreatment on the AZ91D matrix was been studied systematically. The results show that the optimal pretreatment can make the magnesium matrix clearly, and do not erode the matrix much, and offer a good preparetion for the phosphate conversion treatment.2 The properties of conventional phosphate conversion coatingThe surface of convertional phosphate conversion coating is gray, uniform, smooth, integrity and micro network, and there is some micro-crack on the surface. The phosphate coating is mainly composed of some amorphous phase and mainly comprises P, O, Al, Ba, F, Mg, Na. The conventional phosphate coating can provide a good protection for the AZ91D alloy matrix, and the adhesion power of the conventional phosphate coating is very good.3 The effect of seal on the conventional phosphate coatingThe effect of seal on the conventional phosphate coating was researched systematically. There are much less cracks and cracks narrow significantly on the surface of the coating after sealing. The surface morphology shapes leaves and accumulates along the same direction, which makes the surface layer more flat and smooth, dense. The sealing coating layer has crystal materials which contain SiO₂, BaSi₄O₉, Na₂SiO₃, MgF₂ and a litter Na₂CO₃. Based on full immersion corrosion test and electrochemical test it is found that the phosphate conversion coating after sealing can provide better corrosion resistance than the conventional phosphate conversion coating.4 The phosphate convesion coating of preimmerse with magnesium alloyThe method of preimmerse with magnesium alloy was used to phosphate conversion treatment supply for the first time. The results demonstrated that the surface of the phosphate convesion coating of preimmerse with magnesium alloy is flater, more compact and smooth, and there are much less cracks on the surface than the conventional phosphate conversion coating. The method can reduce corrosion of the AZ91D matrix, and make the active centre increases, thus the number of crystal nucleus increases, so during the phosphate conversion treatment, the conversion time shortens and the conversion rate increasees. Phosphate convesion coating of preimmerse with magnesium alloy can privode much better protect than the conventional phosphate conversion coating. The method is simple, convenience, but it can improve the corrosion resistance of the coating remarkably and perfect the phosphate technics.5 The phosphate convesion coating under magnetic fieldMagnetic field was used to phosphate conversion treatment supply for the first time. The results demonstrated that the surface of the phosphate convesion coating under magnetic field is flater, more compact and smooth, and there are much less cracks on the surface than the conventional phosphate conversion coating because the growth of coating increases. The phosphate convesion coating under magnetic field can privode better protect than the conventional phosphate conversion coating. Moreover the time of forming compact layer shortens and the work efficiency raise.6 The growth mechanism of the conventional phosphate coatingThe growth mechanism of the phosphate conversion coating was studied systematically. In order to find the optimal time and the conversion rule, the process of coating formation is mainly divided into initial stage (<10s), metaphase stage (10s～10min) and late stage (10～20min). During the initial stage, the increased weight of the coating is negative because the erode dissolve of AZ91D matrix, which is importantest at the stage. The conversion coating grows continually at the metaphase stage of compact coating formation and the increased weight of the compact coating is positive. During the late stage, the growth of coating slowers rapidly and the coating weight almost keeps changless.The surface of coating layer becomes loose, the cracks increase and broaden because the inner coating dissolves and H₂ runs over.7 The technics of phosphate conversion coating on AZ91D die-casting magnesium alloy is been applied to the AZ31 wrought magnesium alloyThe technics of phosphate conversion coating on AZ91D die-casting magnesium alloy can be used to dispose the AZ31 wrought magnesium alloy. The phosphate coating can improve the corrosion resistance of the AZ31 matrix effectively. The growth mechanism of the phosphate conversion coating on the AZ31 matrix was studied. The growth and the corrosion resistances of the phosphate conversion coating on AZ31 magnesium alloy are related to the forming process, microstructure, and component of the AZ31 matrix.8 Application of the phosphate conversion coating on AZ91D magnesium alloyAccording to the performance of the AZ91D magnesium alloy surface treatmentof phosphate on small and medium-sized diecasting parts such as electromotion screwdriver part, the production quality met the requirement of surface treatment process of magnesium alloy, which laid the groundwork for the application of chromium-free treatment technology.