| Magnesium alloy is lightweight metal with advantages like low density, high strength/weight ratio, good heat dissipation and damping performance and others, which has got huge applications in aerospace, machinery and3C fields (Communications, Computers, Consumer Electronics) and become a hot and new direction in the development of material. However, attribute to its lively chemical property and poor corrosion resistance, magnesium alloy is hampered a lot from promotion and applications, therefore, of which the surface treatment is deadly needed.Chemical conversion coating is one of the most commonly surface treatment processes for magnesium alloy and mainly used for the bottom coating, which have a number of excellent features, including simple operation, low cost, good corrosion resistance and other characteristics. However, the film and processing solution of existing conversion coating have heavy metals and other toxic substances and poor corrosion resistance and other shortcomings. This paper presents that using the environmentally friendly calcium phosphate conversion to replace the traditional conversion coating process, achieves non-pollution and abtains the film of preferable corrosion resistance (Ecorr:-1.2958mV, icorr:11.13μA·cm2) and cohesion (0).Through the study of the basic solution, complexing agents and ethanolamine additive, obtaining the optimal formula of solution of conversion coating. Exploring the phosphating process conditions (pH, processing temperature and time), sealing treatment and drying procedure affect film morphology and structure as well as corrosion resistance. During the experiment, using metalloscope and scanning electron microscopy (SEM) to observe surface morphology of the film; Investigating the microstructure of the film by X-ray diffraction (XRD); Utilizing experiment of Saline Immersion to calculate continuous bubble point and combining test data of porosity and electrochemical workstation to analyze corrosion resistance.On the basis of consulting a large number of domestic and international literature and material, and combining four kinds of analytical instruments (metalloscope, SEM, XRD and electrochemical workstation) to test.The main conclusions include:(1) Through the study of the basic solution, complexing agents and ethanolamine additive, the optimum formula of phosphating solution was obtained:H3PO47g/L, Ca(H2PO4)12g/L, KClO34g/L, C3H6O30.92g/L, MEA1g/L.(2) Results shows that conversion coating of high cover degree and excellent corrosion resistance obtained in process conditions(pH2.5-3.0, the processing temperature(T) is35～50℃, phosphating time(t) is2-5min). Among them, the best properties of coating is got in pH=2.5, T=35℃. t is depended(3) The formation of calcium phosphate conversion coating is divided into four stages:1) substrate dissolving-mucleation(0-27s),2)short-term balance between dissolution of substrate and film and growth of film(27～31s),3)phosphate coating rapid growth(31-44s),4)growth and dissolution of film(after44s).(4) By thermodynamic and kinetic of phosphating, we investigated the impact of temperature and it showed that the raise of temperature was in favor of the reaction of phosphate film deposition. In different phosphating systems, temperature should be controlled within a certain range, too low or too high will affect the quality of phosphate film.(5) Sealing treatment by boiling water and drying procedure at80℃can further improve the coverage and corrosion resistance of the film.(6) Combining the results with phenomena of the experiment describe the process and the principles of the phosphate reaction, which show that the main line of phosphating process is the electrochemical reaction, and the Coordination Reaction of Ca2+and ethanolamine physical adsorption regulate the reaction process. At the same time, the main component of film is Ca10(PO4)6(OH)2and may contains CaHPO4·2H20.Through research this paper achieves the target of the preparation of environmentally friendly calcium phosphate conversion coating. Nontoxic phosphate solution and coating acquired and a better corrosion resistance and adhesion force ensured. Systematically study of magnesium alloy calcium phosphate conversion coating to offer a theoretical basis for the exploration of calcium phosphate coating. The process with simple operation and low energy consumption offers the possibility of practical application. |
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