Investigation On Pretreatment Process Before Painting On Magnesium Alloy For Automobile

Magnesium alloys have been extensively used in automobile, aerospace and other fields owing to their low density, high strength/weight ratio and excellent machinability. However, the relatively poor corrosion resistance results in a decrease mechanical stability and unattractive appearance. All these greatly limit the application of magnesium alloys. Chemical conversion coating is one of the most effective methods to increase the corrosion resistance of magnesium alloys and it also can enhance the adhesion between the outer paint and the base. It is well known that conversion coating for magnesium alloys have been traditionally based on chromate ions. However, the use of chromate solution is being progressively restricted due to the high toxicity of the hexavalent chromate compounds. Therefore, the development of environment-friendly treatment methods is the most important question to investigate.In this work, an innovative conversion coating on AZ91D magnesium alloy in phytic acid solution was developed. The effect of different acids pickling pretreatment on the corrosion resistance of phytic acid conversion coating and the adhesive ability between the conversion coating and the outer paint was analyzed. The influences of technological parameters such as pH value, immersion time, and the solution concentration on the properties of the phytic acid conversion coating were studied through single factor experiment. And the optimal process was obtained through orthogonal experiments. The morphology, composition, the corrosion resistance and the adhesive ability of the coating were analyzed. On the basis of the above discussion, the formation mechanism of the phytic conversion coating was speculated.The effect of different acids pickling pretreatment on the corrosion resistance of the conversion coating and the adhesive ability with the outer paint was investigated. The results indicated that the pretreatment process which adopts HCl (10-20ml/L) pickling20s under room temperature is best. The influence of technological parameters on the properties of the phytic acid conversion coating is studied. With the increase of pH values, the cracks in the coating decreased, and the corrosion resistance of the conversion coating was firstly reduced then enhanced. As the immersion time prolonging, the coating becomes thicker, which increased the cracks and the corrosion resistance of the conversion coating decreased. As the conversion temperature going up from20℃to80℃, the conversion coating became rougher and the cracks gradually increased. With the concentration of phytic acid solution enhancing, the cracks on the conversion coating increased, while the corrosion resistance of the coating decreased.The optimal process conditions contain5g/L phytic acid at40℃under pH=7.5for10min. A uniform and compact conversion coating with ramiform and reticular morphology formed on AZ91D magnesium alloy. The conversion coating does not only provide effective protection for AZ91D magnesium alloy, but also provides favorable base for the outer paint with increased adhesion.The chelate manners of phytic acid varied under different pH value solutions during the formation of the conversion coating. When pH value is4-5, the chelate compounds are magnesium phosphate, metaphosphate and P-OH (the phosphate carboxyl groups are not reacted with magnesium alloy). When pH value is7～8and10～11, the chelate compounds are magnesium phosphate, pyrophosphate and metaphosphate.