Preparation Of Micro-arc Oxidation Coating On Magnesium Alloys And Study On Its Corrosion Degradation Behavior

Magnesium alloys have been one of the hot topics in the field of degradablebiomaterials due to their many advantages, such as low elastic modulus, similardensity to natural bone and in vivo degradation via corrosion. However, very rapiddegradation rate of magnesium alloys in body fluids restricts their clinicalapplications. Micro-arc oxidation (MAO) is an effective surface treatment technology,which can improve the corrosion resistance and biocompatibility of magnesium alloys.Calcium phosphate salts have good biological activity, but the corrosion resistance ofthe oxide film prepared by MAO in the solution containing inorganic phosphate isunsatisfactory. In this paper, the coatings were obtained in a solution that mainlyconsisted of sodium phytate, a food additive. The influences of different calcium saltson the performance of anodic coatings were investigated. The calcium-containingelectrolytes that could generate calcium phosphorus-containing oxide layer wereselected. The process parameter was optimized by orthogonal experiments. Inaddition, the formation mechanism of the MAO film and the corrosion degradationregularity in simulated body fluid were discussed. Results were shown as follows:1. Four kinds of calcium salt, such as CaCO3, Ca(OH)2, Ca(CH3COO)2andCa(H2PO4)2were separately added into the solution of6g/L NaOH and15g/LNa12Phy. The effect of calcium salt on the property of anodic coatings formed onmagnesium alloys was investigated. The results showed that the calcium content inthe oxide film was improved. The calcium content in the oxide film prepared byadding CaCO3was the maximum. The dropping test and electrochemical polarizationcurve analysis showed that CaCO3could significantly improve the corrosionresistance of anodic coatings. CaCO3could not only improve the surface morphologyand corrosion resistance of anodic coatings but also slightly increase the calciumcontent, so CaCO3was the best calcium salt additives.2. The effects of NaOH concentration, Na12Phy concentration, CaCO3concentration and treatment time on the corrosion resistance, thickness, final voltageand compositions (Ca and P content) of the coatings were studied by the orthogonalexperimental method. The results showed that the increase of NaOH concentrationreduced the oxide film thickness, corrosion resistance, the final voltage and Ca, Pelement content of the coatings. The addition of Na12Phy concentration increased thecontent of P element, reduced the final voltage. With the treatment time prolongation,the P content of the coatings reduced but the content of Ca elements increased. Theincrease of CaCO3concentration made the thickness, corrosion resistance, final voltage and the Ca content of the coatings firstly increase and then decrease. Theoptimum processing of Micro-arc oxidation was NaOH3g/L, Na12Phy15g/L,CaCO35g/L and treatment time5mins.3. The coating thickness, morphology, elemental distribution and phasecomposition were characterized by eddy current instrument, SEM, EDS and XRD,respectively. The results showed that the surface of the coatings achieved a uniformpore distribution, and the pore size was about1.5m-2.5m. The coating thicknesscould reach20m, wherein the loose layer was approximately15m, the dense layerof approximately5m. Mg-Ca alloy substrate consisted of Mg and Mg2Ca, while thecoating on the magnesium alloy mainly consisted of Mg, MgO, CaHPO4, MgCO33H2O,(Ca, Mg) CO3. The EDS maps of Ca and P elements showed that Na12Phy tookpart in the formation of the coating and the coating firstly grew up at the junction ofthe Mg2Ca and the-Mg phase, then developed into-Mg phase.4. The corrosion resistance and degradation rate of anodic coatings wereevaluated by electrochemical polarization curve and immersion corrosion test. Theresults showed that the current density and the corrosion potential of the Mg-1.0Casubstrate was1.033×10-5A/cm2and-1.655V, respectively. The current density andthe corrosion potential of the MAO coatings were1.627×10-6A/cm2and-1.47855V,respectively. MAO treatment significantly reduced the corrosion degradation rate ofthe magnesium alloy. With the immersion time prolongation, the weightlessness rateof the treated samples firstly decreased and then weight gain occurred, while thehydrogen evolution volumes and the solution pH value gradually increased. Afterimmersion for eleven days, the hydrogen evolution rate reduced and gradually leveledoff, while the solution pH value changed slowly.5. The corrosion degradation products were characterized by SEM, EDS, XRD,respectively. The results indicated that the corrosion degradation products in Hank’ssolution consisted of MgO, Mg(OH)2,(Ca,Mg)CO3, CaHPO4. With immersion timeprolongation, CaHPO4,(Ca,Mg)CO3, Mg(OH)2content gradually increased. Afterimmersion for ten days, the intensity of Mg substrate peak gradually enhanced.