| An attempt has been made to improve the poor corrosion resistance of magnesium alloys,a conversion film technology was employed to obtain the calcium phosphate coating through preheating the magnesium alloy in a calcium phosphating bath which was improved by introducing calcium ions into the bath solution,then aklie treating the calcium phosphate coating to obtained the hydroxyapatite(HA) coating.The technique of preparing coating,mechanism of formationg and the performance of the coatings were investgated in the paper. Moreover,based on many emperiments,we proposed the "chemicalelectrochemical" combination mechanism,which may give an insight in developing the new calcium phosphate(CP) coatings and the hydroxyapatite(HA) coatings.The main works include:(1) Dense and crack-free complex calcium phosphate coatings have been preparedDense and crack-free complex calcium phosphate coatings have been prepared in phosphating bath containing mainly H3PO4,CaO by controlling nucleation sites of phosphate crystals and improving the nucleation rate.The phosphate coating consisted of AlPO4·2H2O,CaPO3(OH) and CaPO3(OH)·H2O phases.The adhesion of magnesium alloy and the zinc phosphate coating was better than that of chromate conversion coating,due to the uneven and micro-hole structure of the phosphate coating.(2) "Chemical-electrochemical" mixed formation mechanismThe growth process of phosphate film on the magnesium alloy substrate was investigated by SEM observation and EDS and XRD analysis.The results indicated that the phosphate crystals only deposited atβphase in the phospahating bath without additive,however,additives are absorbed on the surface of AM60 magnesium alloy made the phosphate crystals deposite at bothαandβphases in the additive phospahating bath simultaneously.Differente effection caused by different additives:the crystals were increased significantly,due to combinating the reducing and oxidative additive viz.alcium nitrate and M-Nitrobenzene sulphonice acid sodium.In the mean time,the chemical mechanism in the phosphating process was investigated by electrochemical technology.(3) Orthogonal optimization of the phosphatingNine teams tests was designed based on the additives(alcium nitrate and M-Nitrobenzene sulphonice acid sodium and Na2Mo2O4) in the phosphating bath and pH being considered as the factors.The anti-corrosion performance was evaluated by drop test.The results indicated that the degree of these four factors is in the sequence of Na2Mo2O4,M-Nitrobenzene sulphonice acid sodium,alcium nitrate and pH.However,the M-Nitrobenzene sulphonice acid sodium presented the most evident effect on the morphology of the calcium phosphate(CP) coating, the morphology of phosphate coating obtained in the phosphating bath with more M-Nitrobenzene sulphonice acid sodium was finer and denser,but anti-corrosion property was not always perfact.Based on the result of orthogonal optimization, the optimum composition is Ca(NO3)2:11g/L;M-Nitrobenzene sulphonice acid sodium 0.5g/L;Na2MoO4:0.2g/L,pH=2.15.And the excellent corrosion resistance properties was testified by polarization curves.(4) The resultes of XRD,SEM and EDS revealed that the morphologies and phases are divers among different calcium phosphate(CP) coatings.And the anti-corrosion presented discrepance among different coatings.The dense coating was not always better than the cracked coating.(5) The hydroxyapatite(HA) coating can be prepared by aklie treating the CP coating for different periods of time.So far,there were not many studies on the HA coating deposited on the magnesium alloy,meanwhile our technology is more economy and convenient than the electrodeposition or ion spay deposition. Consequently,the calcium phosphate can be used not only in anti-corrosion of industry,but also in preparing the HA coatings on the magmesium alloy surface. The CaPO3(OH) and CaPO3(OH)·H2O phases were transformed into HA during aklie treatment,thus obtained the HA coating.(6) Electrochemical Analysis and immersion test indicated that the hydroxyapatite(HA) coating obtained through alkali treating CP coating for 1.0 hour exhibited excellent anti-corrosion performance.However,the overlng aklie treatment time caused the decrease of anti-corrosion property.(7) The variation of Ca/P is invested by EDS,the result indicated that the optimum alkali treatment time is about 1 hour,during that period,the majority of CaPO3(OH) and CaPO3(OH)·H2O phases were transformed to HA,thus the pretreatment must be controlled CaPO3(OH) and CaPO3(OH)·H2O as major phase, and void AlPO4·2H2O phase.If the alkali treatment time lasted 2.5 hours,HA may be dissolved,and the exposed magnesium substrate reacted with NaOH and formed the Mg(OH)2.(8) The FTIR date revealed that some PO43- groups were replaced by CO32- groups,which caused the aberrance of crystal lattice due to the difference between PO43- and CO32-.Crystallization can be hindered by the aberrance,and lessen the crystallinity,which goes against the growth of crystal and benefits to forming crystallite.The crystallite can increase the specific surface area,thus increase the solubility and be prone to degrade in the human body,consequently,it was testified to be a bioactive coating. |
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