Construction And Corrosion Behavior Of Superhydrophobic Coating On Magnesium Alloy

In recent years,the research of Mg alloys as biomedical materials has attracted much attention.However,the poor corrosion resistance of Mg alloys has limited its wide application in this field.In order to improve the corrosion resistance of Mg alloys,the cathodic electrodeposition was used in the thesis to build hydrophobic/superhydrophobic coatings on magnesium alloy.Prior to the coating,pre-treatment was used to improve the interface between the coating and the alloy.The influence of surface pre-treatment of magnesium alloy on the coating was systematically studied.And the choice of anode plate during deposition process was analyzed.The influence of different anions in electrolyte on the coating morphology was investigated.In addition,the effect of electrodeposition parameters on the coating was researched as well.Furthermore,the surface state of coating was controlled and the formation mechanism of the coating was put forward.The effect of process parameters on corrosion behavior of the coatings was studied in detail.The corrosion degradation of superhydrophobic coating in simulated body fluid and its corrosion inhibition on Mg alloy was illustrated.The results indicate that the surface pre-treatment of magnesium alloy strongly affects the surface morphology,thickness and wettability of the coating,without affecting its phase composition.Due to the high shielding property of plasma electrolytic oxidation,hydrophobic coating can not be deposited directly on the surface with the plasma electrolytic oxidation pre-treatment.However,hydrophobic coating can be easily deposited on the surface with etched plasma electrolytic oxidation,anodizing pre-treatment or hydrothermal pre-treatment.The pre-treatment layer has a great influence on the corrosion resistance of the whole coating system,and the etched plasma electrolytic oxidation pre-treatment layer can provide the best corrosion resistance for the whole coating system.The reason is that the etched plasma electrolytic oxidation layer owns a compact inner layer and it is the thickest pre-treatment layer on Mg alloy.It improves the stability at coating/pre-treatment layer and pre-treatment layer/alloy interfaces and prevents the penetration of ions into the coating.In the electrodeposition process,the current density shows two stages: decline and remain constant.They correspond to the coating nucleation and growth,respectively.The coating surface shows micro/nanostructure,and the change of surface structure has a direct influence on the wettability of the coating.The fabrication parameters obviously affect the coating morphology,thickness and wettability,without affecting the phase composition.The coating is degradable in simulated body fluid.The coating phase composition changes from C36H70 Ca O4 to C36H70 Ca O4·H2O when immersion in simulated body fluid.The fabrication parameters significantly affect the corrosion resistance of coatings.The time constants evolve from two to three for the coating formed at 15 min,60min and 120 min with direct current voltage 50 V.The third time constant corresponding to the electrochemical double layer appears after immersion for 12 h,24h and 12 h,respectively.Furthermore,the coating formed at 60 min has the highest coating resistance,anodized layer resistance and charge transfer resistance,indicating it owns the best corrosion resistance.When applied a pulse voltage(duty cycle 50%),coatings formed at 100 V and 20 V with the deposition time 60 min shows three time constants after immersion for 1h and 12 h,respectively.The coating formed at 50 V shows three time constants after 48 h,indicating this coating owns the best corrosion resistance.The loading mode of the voltage strongly affects the corrosion resistance of the coating.At the deposition voltage 50 V and deposition time 60 min,the coating formed with direct current voltage shows the time constant corresponding to the electrochemical double layer after 24 h,while with pulse voltage it is after 48 h.For the coating formed at the same deposition voltage and time,the coating deposited with the pulse voltage shows better corrosion resistance.The corrosion resistance of coatings increases with the voltage(or time)first and then decreases,whether for the direct current voltage or pulse voltage.And the coating formed at 60 min and 50 V shows the best corrosion resistance.This is due to the change of surface morphology,coating thickness and wettability with the change of deposition voltage(or time).The combination of these factors contributes to the corrosion resistance.In physiological environment,both anodized layer and superhydrophobic coating show similar behavior for the p H value and osmolality value.They have the higher degradation resistance.Compared to the anodized layer,superhydrophobic coating can provide better degradation resistance for the Mg allo y.Human osteoblast proliferation,but not differentiation is enchanced by anodized layer.Contrary,the superhydrophobic coating reduces the proliferation,but enhances differentiation of osteoblast.