| Magnesium alloys, one of the lightest structural metallic materials in the real applications, have been widely used in automotive, mechanical, aircraft, 3C (computer, communication and consumer electronic) manufacturing industry and other fields due to a series of excellent performances, such as, the low density, high strength-to-weight ratio, and good castability, machinability and weldability and so on. It is considered to be the most potential for the development and application of "green engineering materials in the 21st century". However, the poor corrosion resistance of magnesium alloys plays a major factor to limit its practical applications. Much effort on surface treatment has been devoted to the improvement of its corrosion resistance. The chemical conversion processing, one of surface protection technologies, has been applied widely owing to its advantages of low energy consumption, simple equipment, low investment and so on. As the most popular and traditional method, conversion coatings are prepared by treating magnesium alloys in the solution containing hexavalent chromium (Cr6+) compounds which are toxic and caustic. In view of the serious impact from Cr6+ to the environment and human and the imminence of associated restriction, it is necessary for the development of an environmentally friendly methods.The zinc phosphate coating, cerium conversion coating, phytic acid conversion coating and cerium-phytic acid synergistic conversion coating were prepared on AZ31B magnesium alloy. The chemical principle and technology were studied in this paper. The corrosion resistance and the optimum processing parameters of the four conversion coatings mentioned above were investigated by hydrogen evolution method and the polarization curves. The morphology, compositions and functional group of these coatings were characterized and analyzed by scanning electron microscope (SEM), atomic force microscope (AFM), energy disperse spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The chromium-free environmentally friendly chemical conversion surface treatment technology has been optimized. The environmentally friendly cerium-phytic acid synergistic conversion coating, consisting of the inner phytic acid conversion coating and the outer cerium conversion coating, was formed on AZ31B magnesium alloy, and the synergistic conversion coating has good corrosion resistance. The main results obtained are as follows: 1) The optimum phosphate technology of AZ31B magnesium alloy was obtained by experiment. The phosphate coating formed at the phosphate technology is slablike crystals. The chemical composition of zinc phosphate coating was presented definitely and the element dissolution phenomenon of AZ31B magnesium alloy in a phosphating solution was analyzed. The technology parameters that the phosphate coating formed at the technology parameters has best corrosion resistance performance were obtained.2) A corrosion model of the AZ31B magnesium alloy zinc phosphated sample in 3.5% NaCl solution is proposed. The model shows the corrosion process of the AZ31B magnesium alloy zinc phosphated sample includes three stages. The magnesium and zinc are dissolved at the first stage, because of the shielding effect of the zinc phosphate coating, the corrosion resistance of the AZ31B magnesium alloy treated in the phosphating solution is improved obviously. The insoluble species will be formed at the second stage, because of the shielding effect of Zn(OH)2 and Mg(OH)2, the corrosion rate of the AZ31B magnesium alloy treated in the phosphating solution basically maintains constant. At the third stage, the potential is different between the zinc and magnesium, the Zn-Mg galvanic cell and galvanic corrosion will be formed between zinc and AZ31B substrate. So the corrosion resistance of the phosphate sample is lower than that of the uncoated AZ31B magnesium alloy sample.3) The technology parameters that the cerium conversion coating formed at the technology parameters has best corrosion resistance performance were obtained. The cerium nitrate concentration is the most important influencing factor on the corrosion resistance performance of cerium conversion coating. The hydrogen peroxide concentration, treating temperature and treating time are the second, third and fourth important influencing factor. The hydrogen evolution rate of cerium conversion sample that formed in optimum processing parameters relative to the bare sample is 60.35%. The cerium conversion coating is composed of O, Mg, Al and Ce. It has many cracks and two layers.4) The formation mechanism of cerium conversion coating shows Ce(OH)3 and Ce(OH)4 will be precipitated from the solution at specifically condition when magnesium alloy contacts with the cerium nitrate solution. The insoluble precipitations lay over the magnesium alloy surface and become the cerium conversion coating. The cerium conversion coating will be decomposed at drying process and form CeO2. So the conversion coating has more CeO2 and a small quantity of Ce(OH)3 in the inner layer.5) The pH value of phytic acid is an obvious influencing factor on the formation process of the phytic acid conversion coating. When the pH value is low, the phytic acid conversion coating can be formed rapidly on the surface of magnesium alloy at initial stages. But the rapid release of hydrogen prevented from the formation of conversion coating. The conversion coating did not cover the surface of sample completely, which reduced its corrosion resistance. When the pH value is high, the electrochemical reaction that magnesium ions can be formed is not easy to start. The magnesium ions that can react with the phytic acid radical ions are less and the conversion coating formation rate is slowness. So the conversion coating was thin and could not offer an effective corrosion resistance. The conversion coating formation rate can be controlled when the pH value is adjusted. So the phytic acid conversion coating that has best corrosion resistance performance can be formed.6) The optimum technology parameters of phytic acid conversion coating on AZ31B magnesium alloy are pH value is 2, treating time is 40 min, treating temperature is 40°C and solution concentration is 4g/L. The phytic acid conversion sample formed at optimum processing parameters is compact and has less hydrogen evolution rate, less corrosion current density, biggish corrosion potential, better corrosion resistance performance than that of untreated sample. The main elements of the phytic acid conversion coating are Mg, Al, Zn, O and P element, the functional groups are PO43-, HPO42-and OH-. The thin phytic acid conversion coating is formed on the crack.7) The formation mechanism of phytic acid conversion coating shows the active groups of phytic acid can react with the magnesium and aluminum ions that are dissolved from the magnesium alloy and form the steady chelate compounds. The compact conversion coating can be formed on the surface of magnesium alloy and the corrosion resistance performance can be improved.8) The conversion technology that the synergistic conversion coating on the AZ31B magnesium alloy formed at the conversion technology has best corrosion resistance performance has been optimized. The synergistic conversion coating, consisting of the inner phytic acid conversion coating and the outer cerium conversion coating, has better compact performance and corrosion resistance performance than cerium conversion sample and phytic acid conversion coating sample. The main elements of the synergistic conversion coating are C, O, P, Ce, Mg and Al, the functional groups are PO43-, HPO42-and OH-. The synergistic conversion coating contains CeO2 and a little amount of Ce(OH)3.9) The formation mechanism of synergistic conversion coating shows the active groups of phytic acid radical dissolved from the phytic acid conversion sample can chelated with cerium ions in cerium nitrate solution when the phytic acid conversion sample will be put into the cerium nitrate solution, the steady chelate compounds will be formed. The chelate compounds and cerium oxide and hydroxide deposit on the surface of phytic acid. Because the chelate compounds have the repair effect on some cracks on phytic acid conversion coating. The synergistic conversion coating has better corrosion resistance performance than cerium conversion sample and phytic acid conversion coating sample.10) The corrosion kinetics analysis shows that the corrosion dissolution rate coefficient of untreated sample, cerium conversion sample, phytic acid conversion sample and synergistic conversion sample become smaller in turn. The corrosion resistance performance will become better in turn. Furthermore, the corrosion autocatalytic coefficient of synergistic conversion sample is least in the four samples. That is to say, the synergistic conversion sample has best corrosion resistance performance.
|
PDF Full Text Request