Research On Dissolution Kinetics Of Electrodeposited Magnesium Hydroxide Coating

Magnesium alloy has ideal mechanical compatibility,biocompatibility and degradability,however,the rapid corrosion rate has become the restricting factor for its medical application.It is very important to master the biodegradation mechanism of magnesium alloy and the effect of degradation products on the degradation process.Magnesium hydroxide is the intermediate degradation products of magnesium alloy in physiological environment.This research includes the electrodeposition of magnesium hydroxide coating on magnesium surface,the degradation and dissolution kinetics of the coating,comparing with the dissolution kinetics of magnesium hydroxide powders.The effect of the coating on the degradation process of magnesium matrix was further studied.It was very significant to understand the degradation mechanism of medical magnesium in physiological environment.It is found that:A white Mg(OH)2 coating with a thickness of several microns can be prepared on a pure magnesium surface by cathodic potentiostatic deposition,the composition is same as the intermediate degradation products of pure magnesium.The optimum electrodeposition conditions of magnesium hydroxide coating were obtained by studying the electfodeposition process.When the deposition potential is 5V and the deposition time is 2h,the Mg(OH)2 coating obtained by electrodeposition is uniform and compact,with no turtle crack,more closely binding with the matrix.The potentiodynamic polarization curves show that the corrosion potential of the sample with the coating in SBF solution was positive by 470mV than that of pure magnesium,and the corrosion current density was reduced by an order of magnitude,this phenomenon also occured in high concentration of NaCl solution.The difference between the dissolution behaviors of Mg(OH)2 and MgO powders was small.The dissolution kinetics established shows that between 37? and 50?,the activation energy of Mg(OH)2 powders in the soaking solutions with three kinds of chloride ions concentration(148mmol/L,300mmol/L,500mmol/L)were 62.161kJ/mol,75.109kJ/mol,72.789kJ/mol,respectively.With the increase of chloride concentration,the concentration of magnesium ions increased slightly when two powders reached the dissolution equilibrium,and the dissolution rate was accelerated.When the concentration of chloride increased to 500mmol/L at 37 ?,the dissolution rate of Mg(OH)2 powders is 1.62 times higher than that of 148mmol/L chloride ion solution.With the increase of temperature,the dissolution rates of two powders are accelerated.The results of the chronoamperometry indicate with the decrease of the coating area,the corrosion rates of pure magnesium treated by electrodeposition was different in the three kinds of immersion liquids.EIS results indicate that the Nyquist diagram of pure magnesium treated with electrodeposition in the chloride ion solutions has a capacitance arc in high-frequency region and an inductive arc in the low frequency region.Combining with the equivalent circuit model(ECM),it was found that in 148mmol/L and 300mmol/L NaCl solutions,with the increase of time,the radius of resistance decreases first and then increases,the charge transfer resistance decreases first and then increases,and the corrosion resistance decreases first and then increases.And the charge transfer resistance in the higher concentration solution is always lower than that in the lower concentration.Capacitance arc radius decrease continuously in 500mmol/L NaCl solution,the corrosion resistance decrease continuously and the corrosion rate is accelerated.The kinetic analysis of the dissolution of pure magnesium treated with electrodeposition and Mg(OH)2 powders indicates in three different soaking solutions,the release rates of magnesium ions are different,and with the prolongation of soaking time,the difference of magnesium ion release rate is more obvious,which is related to the corrosion of pure magnesium.Finally,the effect mechanism of Mg(OH)2 film on the degradation behavior of magnesium matrix in the chloride ion solution was discussed.