| Advantageous properties of magnesium alloy makes them be required of a sharp increase in the amount, especially in the aerospace industry, automotive industry, telecommunications industry and other industries in the lightweight. Magnesium is seen as an important strategic material in the 21st century. Magnesium is an important metal element in human body, magnesium density and is similar to bone mineral density of the human body. Activity of magnesium in the body plays a beneficial promoting role: magnesium can promote the formation of bone cells and accelerate bone healing; Magnesium did not inhibit for the growth of bone marrow cells. These characteristics make magnesium have potential to be applied to the biomedical area, but the chemical properties of magnesium alloy in all industries alloys is the most active, and poor corrosion resistance limits its further application, thus improving the corrosion resistance of magnesium alloys becomes the hotspot of magnesium reseach in recent years. Chemical conversion technology was widely applied for its advantage: simple process, low cost and coating having adhesion. Chromate conversion coatings with excellent performance have been used to improve corrosion resistance of magnesium alloy, but in recent year it was strictly limited for toxic hexavalent chromium ions. Thus various non-chrome chemical conversion films came into being.Today, with the further development of magnesium alloys, magnesium is used as a biological implant material again. We try to introduce organic conversion coating to on the magnesium alloy surface to improve corrosion resistance of magnesium alloy, which has no harm to human body, and we do early exploration on the magnesium alloy applications in the biomedical area. Based on this consideration, we prepared and studied two different organic conversion coating on different magnesium alloy surface. One is tannic acid based conversion coating on the AZ91D magnesium alloy substrate and the other is calcium phosphate coating on the AM60 magnesium alloy surface, the main works include:(1) Prepared a dense tannic acid conversion coating: The components of the coating with surface density are Zr(HPO4)2 at the bottom of the coating and the tannic acid - magnesium chelate upper part ofthe coating, as well as a small amount of MgF2. There is no clear dividing line between the two layers and the close integration between the deposition of particles, this may result from the hydrogen bonds of Zr(HPO4)2 and OH in the tannic acid-magnesium complex form intermolecular hydrogen bonds. Corrosion resistance is superior to the traditional chromate Dow-7 coating.(2) Determined favorable production condition to obtain good tannic acid conversion coating: When pH value is in range of 3.9~4.5, we can get good coating with higher corrosion resistance. Inorganic acid has no effect on the coating forming, but the organic phytic acid have a strong complexing ability with the metal ions, but they could not deposit in the tannic acid based solution and it frustrated deposition of chelate complex of the magnesium and tannic acid.We can obtain good coating by treated magnesium alloy in the tannic acid based solution for 5min. When we prolonged treatment time to 10min, we get thick coating with the highest corrosion resistance. But at this time the micro cracks formed on the coating surface and self-body of coating does not crack. The coating and the substrate have good adhesion; meanwhile the porosity of the coating is small. The gap is smaller than the volume of deposited particles themselves. When the time exceeds 600s, surface of the coating became rough and corrosion resistance decreased.When the temperature of tannic acid based solution is in range of 10~35℃, we can get high-quality coating with higher corrosion resistance.If the temperature of tannic acid based solution is too low, the caoting can not form.However ultra-high temperature leads to corrosion product of magnesium accumulating on the surface of magnesium alloy.(3) High efficiency of tannic acid based solution: We can get 4.08m2 area of coating with high corrosion resistance on magnesium alloy in one litre of the tannic acid based solution, Less consumption of chemical improve the applicability of industrial production.(4) Raised the tannic acid chemical conversion coating forming mechanism of film: After being soaked in tannic acid for around 50s, magnesium alloy start to corrode and magnesium ofαphase became magnesiun ion. K2ZrF6 in the trestment solution decomposed to Zr ion, which react with HPO42- ion to form Zr(HPO4)2 deposited on theβphase. In the next period of time the first layer deposit continously and the amount of Zr(HPO4)2 increased. In the acidic circumstance, tannic acid hydrolyzes to gallic acid and glucose. The oxygen ions of gallic acid reacted with Mg to form the bond of tannic-magnesium organic complex, which deposited on the Zr(HPO4)2 layer. The composition of TBC (tannic acid based conversion) coating treated for 600s is the same as the coating for 300s, but only differs in thickness of the coating. This suggests that thickness increases from 300s to 600s. Organic complex uniform distributs on the magnesium alloy surface. The coating-forming process can be divided into three stages: the dissolution of Mg, Zr(HPO4)2 formation and the organic chelate complex layer formation.(5) Prepared calcium-phosphate coating: pH value had a great impact on the coating-forming. When pH value of phosphate solution is 2.45, we can receive calcium phosphate coating with the dense and uniform surface. The main components are Ca(HPO4)2 and Ca(HPO4)2·H2O. The corrosion resistance of coating is higher than that of substrate in the 3% NaCl solution.(6) Function of three different additives: It promotes the formation phosphate film. Calcium nitrate can refine the grains to prevent their local growth. The m-nitrobenzene sulfonic sodium can increase the amount of nucleation activation points on the magnesium alloy surface, so that phosphate coating distributes uniformly on the surface. The concentration of sodium molybdate has no impact on the formation of calcium-phosphate coating.(7) Prepared hydroxyl phosphate calcium coating: The hydroxyl phosphate calcium coating obtained by treated in alkaline liquor for 1hour is dense and has high corrosion resistance in a simulated biological fluid.
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