| In this thesis, adopting the traditional phosphating technology and introducing Ca2+ in phosphating liquid, calcium phosphate film has been deposited on the surface of magnesium alloys AZ60, based on the mature zinc phosphating technology of magnesium alloy. Preparation process, formation and corrosion mechanism of calcium phosphate film and dihydrate calcium phosphate film has been studied in detail. The growth mechanism of phosphate film has been introduced that phosphate film will firstly deposite on theβ-phase of magnesium alloy, then extend to theαphase. "Chemical and electrochemical" joint mechanism has been summarized. In 3% NaCl and SBF solution, filiform corrosion happened easily on magnesium alloys covered with calcium phosphate film. On the basis of calcium phosphate film, dicalcium phosphate dihydrate with good bioactivity has been got after the alkali treatment.The major research contents are as follows:1. Orthogonal designing method for the optimal design of calcium phosphate filmOn the basis of phosphating recipes of CaO and phosphoric acid, combined with calcium nitrate, m-nitrobenzene sulfonic sodium salt, sodium molybdate and PH value as factors, nine groups of experiments have been designed. With drip experiment as the assessment standards of the phosphate films' corrosion resistance, combined with polarization curves, and SEM observation on the morphology, the influence level has been summarized and the decreasing order of arrangement is PH, calcium nitrate, m-nitrobenzene sulfonic sodium salt, sodium molybdate. To judge from the morphology of the films, PH value has the greatest influence on film morphology, If PH is too low, the film is not complete. The film is most complete and its corrosion resistance is best when the PH is 3.1. According to orthogonal experiment results, the final optimal experimental component is as follows: calcium nitrate: 11g / L, inter-nitrobenzene sulfonate:0.5g / L, sodium molybdate:0.8g / L, PH = 3.1.2. Dynamics of calcium phosphate film on magnesium alloysAccording to potential - time curve, the forming process of phosphate film is divided into three stages. The first stage is mainly phosphate film deposition. Linear relationship between potential and time, voltage increasing sharply, indicates filming successfully. The second stage is mainly the film's slow growth, and voltage increases smoothly. In the third stage, homeostasis is achieved between the dissolution of magnesium alloy and the film growth. Over time, thickness of phosphate film doesn't have significant change, but coarse structure appears, and then micro-cracks appear in the underlying organization.3. "Chemical-electrochemical" joint mechanismBy means of X-ray, scanning electron microscopy, power spectrum and other testing methods, the growth process of phosphate film deposited on the magnesium alloy has been studied. The filming mechanism is "Chemical and " joint mechanism, in which the electrochemical reaction is dominant. On the surface of the magnesium alloy, H+ in the solution migrates to theβ-phase, the micro-cathode region and H2 precipitates, so that pH value increased.The micro-anodic dissolution ofαphase and H2 precipitation result in the decreased acidity of interface between metal and solution.So that the dihydrogen phosphate salt transform into insoluble phosphate. In addition, phosphate film will firstly deposite on theβ-phase, the micro-cathode region of magnesium alloy, then extend to theαphase,the micro-anode area, and finally the entire magnesium alloy substrates.4. Corrosion mechanism of the calcium phosphate film There mainly occurred filiform corrosion on magnesium alloys covered with calcium phosphate film, and it mainly happened in the lower layers. The main reason is that Cl+ will reach the magnesium alloy substrate through the layer because of the porous film,then in certain local small area of the magnesium alloys high concentration forms, which result in the corrosion. And corrosion products will form a packet set, then the internal hydrogen would be released when it reaches a certain concentration. This process would constantly repeat so that corrosion began to expand,and the filiform corrosion forms finally.5. The corrosion products of calcium phosphate film on magnesium alloysThe main corrosion products are Mg(OH)2,AlPO4 and the basic product of magnesium alloys is Mg(OH)2 in 3% NaCl solution. In the Mg-Al-Zn alloy system, Mg and Al dissolve and Zn does not. Al will be combined with PO43 - to form AlPO4, and Mg2+ will adsorb Ca3(PO4)2 off from the corrosion to form [Ca,Mg]3[PO4]2. Formation of phosphate film on the surface of the magnesium alloy, can be confirmed from the corrosion products above.6. Corrosion mechanism of the calcium phosphate film after the alkali treatmentThere exists polar groups and non-polar groups in alkaline solution. Polar groups adsorbs on the surface of calcium phosphate film by certain physical and chemical adsorption, then the energy of layer will decrease, reducing the corrosion tendency of the film. In addition, Non-polar groups would form a protective layer of hydrophobic qualitative on the surface, and thus impede the corrosion transition to the film. So corrosion resistance of the film increases via the adsorption of the polar groups and non-polar groups.7. Bioactivity of Dicalcium phosphate dehydrateIn 1mol / L NaOH solution, Ca3 (PO4) 2 (TTCP), previously formed on Magnesium alloy substrate, would transform into CaPO3(OH)·2H2O(DCPD)after heating at 80℃for 2 hours. DCPD is the precursor of HAP and the Ca/P ratio is nearly 1:1.67, so that it has good biocompatibility. Soaked in High Calcium Silicon(SBF), DCPD would transform into Carbonated Hydroxyapatite(CHA). Ca2+ and PO43- would adsorb on the surface of DCPD via electrostatic attraction and chemical bonding, and then hydroxyl radical and PO43- would constantly be replaced by CO32- in SBF, Finally forms the new phase of carbonated hydroxyapatite, which structure and composition are similar to natural bone, so it is called bone-calcium phosphate. When hydroxyl radical or PO43- is replaced by CO32-, because of the lattice distortion caused by the different forms, the crystallinity of DCPD film decreases and tiny organizations are easy to form, which increase specific surface area and solubility in the body, making it easily to be absorbed. So DCPD has a good bioactivity.
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