| Magnesium alloy complex organic coatings inherit advantages of many protection methods such as chemical conversion,micro-arc oxidation and organic coating. These coatings exhibit good comprehensive properties, become an important direction for protection of magnesium alloys in future.In this paper, pretreatment processes of magnesium alloy are optimized by orthogonal experiments, and films under the optimization process are analyzed by SEM, EDS and XRD. 17 different kinds of organic coating systems with different pretreatments and paints prepared on this basis,and the effects of process on coating properties are analyzed. At the some time, the technical & economic performance of complex organic coatings under the two service conditions is evaluated combined with value engineering theory. The main conclusions are as follows:①Process of micro-arc oxidation is optimized by orthogonal experiments. The oxidation film with best corrosion resistance have obtained when the parameters are as follows:15g/l Na2SiO3.7H2O, 6g/lKF.H2O, 9g/l KOH, 10ml/l C3H2(OH)3, 500Hz frequency; 18 % duty cycle; Under the optimization oxidation process, the surface of oxide film feel smoothly and uniformly, and color present milky white. The films compose of bottom layer combined with substrate and surface layer with a large number of uniform crateriform holes which diameter between 1 and 2μm. The chemical composition of film surface is consists of a large amount of Mg, O and Si, and a small amount of Al, F, K and other elements. Furthermore, the phase composed of massive Mg, some MgO, Mg2SiO4, MgAl2O4 and some non-qualitative phase.②Optimal process of chromium-free conversion is obtained by orthogonal experiments. The best corrosion resistance of subsequent organic coatings on conversion film when the concentration of solution is 80g/l J-A, 1g/l J-B,J-C 1g/l,and improved as the treatment time. Under the optimum process conditions, conversion films compose of bottom layer, dense middle layer, and surface layer which divided into island by a large number of crossbar staggered "cracks" with a 1-3μm width. Furthermore, the chemical composition of film surface consists of Mg, Al, O and P, as well as a small amount of Ca and V, while the phases compose of Mg, Mg17Al12 and amorphous phase③Based on optimized pretreatment process,17 different complex organic coatings are prepared with 4 different kinds of pretreatments such as sand-blasting and 5 different kinds of organic paints such as acrylic acid for final disposal. The performance differences and the effects of process on coating performance are comparative studied:1) Effects of pretreatment on complex organic coating properties such as adhesion, corrosion resistance, abrasion resistance are significant, while less significant effects on remaining properties of coatings. The properties in terms of corrosion resistance and wear resistance of coatings based on oxidation film are better than the other pretreatment.2) Paint types affect all the coating properties such as corrosion significantly. Some type coatings have significant advantages in some properties. For example, epoxy coating in corrosion resistance, hardness of ceramic coating of silicone, the organic fluorine coating in gloss, the silicone coating in the wear resistance.④Costs of complex organic coating are calculated from the source of costs. And then, select the corrosion resistance, hardness, gloss of coatings as objects of value evaluation, and establish a assessment mechanism of coating properties and weight parameters in two typical service environments. On this basis, technical & economic performance of the coatings is comparative researched thought the basic principles of value engineering.1) Cost of micro-arc oxidation process is the highest of the all pretreatments because of its high energy consumption and low productivity. While cost of ceramic silicone coating is higher than other coatings,that due to its expensive paint, high baking temperatures and so long baking time.2) Epoxy resin, silicone resin coatings exhibit an excellent technical& economic performance under two typical service conditions due to prominent in terms of corrosion resistance and their low costs. Although acrylic resin coatings costs are relatively low, their bad corrosion resistance, weak technical lead to technical &economic performance. Ceramic silicone and organic fluorine coatings possess excellent gloss, hardness, moderate corrosion resistance, but their costs are so high, their technical & economic performance is unfavorable.
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