Preparation And Corrosion Protection Performance Of Composite Coating On AZ31B Magnesium Alloy

Coronary artery stent implantation has been a major advance in the treatment of occlusive vascular disease.Some inert metal materials are commonly used to manufacture coronary stents,such as stainless steel,titanium alloy and cobalt-chromium alloy,but these materials may easily result in postoperative complication and secondary surgery.In contrast,magnesium alloys has caused widespread concern as potential stent materials due to good biocompatibility and biodegradation.However,the high corrosion rate of magnesium alloys inhibits the biological function.Therefore it is significant to get the suitable degradation rate of magnesium alloys as biodegradable coronary stents.In this paper,firstly,the protective properties of three kinds of organic modified silicate(ORMOSIL)layer were investigated for AZ31 B magnesium alloy,prepared by changing the molar rate of KH-570 and BTMSE.Secondly,the preparation process of the degradable poly(methyl carbonate ester racemic lactic acid)(P(TMC-DLLA))coating was optimized through orthogonal experiment.Finally,the corrosion-proof performance was measured for the optimal ORMOSIL and P(TMC-DLLA)composite coating.The results are as follows:Different ORMOSIL layer was prepared on AZ31 B surface by sol-gel method,which was obtained by changing the molar ratio of KH-570 and BTMSE.The Si-O-Si bonding is formed in the ORMOSIL layer,the Si-O-Mg bonding is obtained by the polymerisation.Compared with the single P(TMC-DLLA)coating,the existence of ORMOSIL layer led to the increase of adhesive strength between composite coating and AZ31 B,reaching threelevel.The open circuit potential of specimens treated with composite coating shifted100-120 mV in the positive direction,and corrosion current density decreased by 2-3orders of magnitude.When the molar ratio was 2:1 between KH-570 and BTMSE,the corrosion resistance of modified specimen reached 2.82x107Ω·cm~2.After 12 d immersion in test solution,the P(TMC-DLLA)/ORMOSIL-2/1 coating suffered the lowest corrosion attack,and showed the highest corrosion resistance.Therefore,When the molar ratio was2:1 between KH-570 and BTMSE,the ORMOSIL layer provides the best bonding effect between the composite layer and AZ31 B substrate,and the best corrosion protection for AZ31 B Mg alloy.By orthogonal test,the preparation process of P(TMC-DLLA)polymer coating wasstudied.The result shows that the best process parameters are as follows: 5wt.% for the concentration of P(TMC-DLLA),5 times for the spin number,1.8 dl/g for the intrinsic viscosity for P(TMC-DLLA).The optimal of ORMOSIL and P(TMC-DLLA)composite layer on AZ31 B was uniform and compact without any cracks and holes.Compared with the naked AZ31 B,in the simulated body fluid at room temperature,the open circuit potential of the modified AZ31 B Mg alloy shifted about 110 mV in the positive direction,the breakdown potential shifted from-1.41 V to-1.30 V positively,and the corrosion current density decreased about3 orders of magnitude.After 20 d immersion,the composite coatings remained intact,and the film resistance reached 1.0x106Ω·cm~2.Therefore,the controllable corrosion degradation of modified AZ31 B magnesium alloy can be achieved through the ORMOSIL and P(TMC-DLLA)composite layer,which can be applied extensively as the potential protective layer on the Mg alloy stent surface.