| Magnesium alloys, which are considered as green engineering materials of 21 century, have unique characteristics of low density, high strength-to-weight ratio, high rigidity and elastic modulus, good recyclability, excellent castability and cutting properties. However, their corrosion resistance is poor and this restricts their wide application. 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. It is urgently necessary to find Cr-free substitute processes, and this is the development trend of surface chemical treatment for magnesium alloys.In this paper, a chromium-free multi-elements composited coating (MECC for short) process has been developed, which contains new solution formula and relative techniques. It could improve the surface corrosive protective property of magnesium alloy significantly. The effects of all kinds of factors on the formation of MECC and corrosive property of MECC were investigated sufficiently. In order to make the MECC treatment bath cyclic used, the supplemention method of solution was devised, which perfects the MECC conversion process.On the basis of the electrochemical methods such as the curve of open circuit potential and time, cyclic voltammetry and potentiodynamic polarization, the performances of the conversion coatings treated with different treatment solutions were evaluated and analysed, the effective ingredients of MECC treatment solution and its optimum formula were determined after lots of screening experiments and orthogonal tests. Its constituents were 2.2 g/l Ca(NO3)2; 9 g/l coating former;23.56 g/l phosphorus acid(85%),0.33g/L accelerator. The effects of treatment prior to MECC conversion treatment on the surface of magnesium alloy and the anti-corrosion property of MECC coating have been studied thoroughly. An optimum MECC treatment process that contains steps of alkaline cleaning, acid cleaning, immersion in NaHF2 solution and MECC treatment was confirmed. The results of salt-fog spray test and electrochemical potentiodynamic polarization experiment indicated the corrosion resistance of the MECC coatings was much better than the protection afforded by Dow 1 chromate treatment, which contained conventional Cr6+ compounds.The relation among MECC treat solution parameters such as total acidity (Ta), free acidity (Fa), the ratio between total acidity and free acidity, the value of PH and the treated area of magnesium alloy were researched systematically. The results demonstrated that the values of Ta and PH increased with the increase of the area of magnesium alloy specimen treated in MECC solution. However, the value of Fa was just the reverse. Therefore, it is obvious that the ratio between total acidity and free acidity also increased with the area increase of magnesium alloy treated in MECC solution. It was the most important parameter, namely the ratio of acidity, that contained almost all information of the parameters of MECC treatment solution. Based on the method of impendent sample T test and levene's test for equality of variances, the results of anti-corrosion test of MECC coatings treated in different acid ratio solutions indicated that the range of optimum acid ratio was between 3 and 9.The method of factor analysis and varimax was used to evaluate the character of treatment solution supply for the first time. The results demonstrated that there were strong correlation among coating former, Ca(NO3)2 and accelerator. The performance of treatment solution depended on two factors. One was the factor of coating formation, and another was PH factor. The results proved that the control parameter of treatment solution could be acidity ratio in condition of strong acidity.The methods of scanning electron microscopy (SEM), EDX, XED, Atomic Force Microscopy (AFM) and electrochemical methods such as the curve of open circuit potential and time (OCP-t), cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were utilized to study MECC structure, the growth mechanism of MECC and its deterioration process in 5% NaCl solution. The results showed that wavy profile was detected over the surface of the conversion coating that showed a relatively smooth appearance with shallow valleys, and its color was gray and black. The MECC layer was mainly consisted of amorphous materials which contained Mg, Al, P, K, Ca, Mn, Zn, O and a few crystalline materials of Ca0.965Mg2Al6O27, Mn5.64P3, ZnAl2O4, (Mg0.66Al0.34)(Al0.83Mg0.17)2O4. Network-like tiny cracks obviously distributed all over the surface of the MECC layer and the cracks did not penetrated through the surface. Al, Mn, P and Zn of MECC were definite selectivity, which accumulated in some area in MECC layer.We discover that the process of coating formation is divided into initial stage(<3min), metaphase stage(35min) and late stage(57min). The conversion coating grows continually at the initial stage of coating formation. The compact coating layer is composed of amorphous materials contained O, Mg, Al and P. The thickness of coating layer becomes thin at the metaphase stage of coating formation and the coating layer degenerates. The surface of coating layer becomes smooth and compact leading to steady condition at the late stage of coating formation. The degradation mechanism of MECC in NaCl solution follows three steps. The first step is the period of adsorption of selective chlorine anions. The dissolution of conversion coating is negligible at this period. The second step is the period of dissolution of conversion coating. The third step is the period of penetration of conversion coating at the chlorine anions absorption area.The EIS results of MECC in 5wt% NaCl solution indicated that electrochemical reaction controlled the degradation process in NaCl solution, and there was no diffusion impedance. The immersion process of MECC in NaCl solution indicated there were two surface state variables that could affect the reaction velocity of electrode. Namely, one was electrode potential E, another was coating thicknessδ. We proposed a reasonable mathematical model of two surface state variables to interpret the electrochemical impedance spectroscopy of MECC on AZ91D in 5wt% NaCl solution. Based on the mathematical model and general equivalent circuit method, the inductance of general equivalent circuit was explained successfully. Moreover, three-step degradation mechanism of MECC in NaCl was proved theoretically.A production line and equipments based on MECC technology were designed and set up. According to the MECC's performance on small and medium-sized diecasting parts such as truck part, the production quality met the requirement of chromium-free MECC treatment process of magnesium alloy, which laid the groundwork for the application of MECC technology.
|
PDF Full Text Request