Preparation And Degradation Behavior Of Corrosion-resistant Films Of Magnesium Alloy Biomaterials

With the development of society and the progress of the times,the aging of the population is becoming more and more serious,and the aging of the body will increase the probability of bone tissue lesions,in addition to young and middle-aged sports injuries,industrial injuries and traffic accidents are also the main source of new bone injury patients every year,so the demand of new materials for medical bone implants is growing.Traditional bone repair materials do not biodegrade in the body and must be removed through secondary surgery,which not only increases the pain of patients but also increases their economic burden.Therefore,the development of bone implant materials with degradable properties has significant economic and social benefits.In recent years,magnesium alloy has been favored by many researchers because of its degradability in vivo,non-toxic substances,mechanical properties and other characteristics are close to human bone.However,the degradation rate of magnesium alloys is too fast and the biocompatibility is not enough,which still restrict its further development and application.In order to improve the corrosion resistance of magnesium alloy substrate,the use of surface treatment is a cost-effective way.Among many surface treatment techniques,microarc oxidation is a new technology that can directly generate ceramic film layer in situ on the metal surface.By adjusting the electrolyte composition and electrical parameters,the purpose of introducing new elements to improve the corrosion resistance and biocompatibility of magnesium alloy can be achieved.This techology has been widely used due to its advantages of simple operation,economic and environmental protection,no toxic substances,and adjustable electrolyte composition.In this paper,AZ31 magnesium alloy was selected as the substrate,and the film layer was prepared on its surface using microarc oxidation as the surface modification technique.Firstly,a four-factor three-level orthogonal experiment was designed in this paper to study the effect of the content of four electrolyte additives(Na2PO4,C6H12O6,C3H8O3,NaOH)on the corrosion resistance of the film layer,and the coating performance of each group of specimens was examined by means of tissue structure,film thickness,surface morphology,electrochemical tests,and in vitro simulated immersion,etc.By analyzing the effect of each additive By analyzing the influence of each additive on the corrosion resistance of the film,the electrolyte composition with the best corrosion resistance was finally determined to be 12 g/L of Na2PO4,4 g/L of C6H12O6,6 g/L of C3H8O3,4 g/L of NaOH.To further improve the corrosion resistance of magnesium alloy and its surface microarc oxide coating,this paper introduces fluorine F-containing electrolyte into the electrolyte composition and investigates the effect of F-concentration on the corrosion resistance of the film.Based on the base electrolyte with the best performance obtained from the orthogonal test,1.5 g/L,3 g/L,4.5 g/L,6 g/L,and 7.5 g/L KF were added,and then the corrosion resistance,microscopic morphology,and phase composition of each group of specimens were examined.The results showed that the addition of KF to the electrolyte could enhance the discharge reaction during the preparation of the film,and the best corrosion resistance of the coating was achieved when the KF addition was 6 g/L.However,when the KF addition was greater than 6 g/L,the specimens started to show ablation.As the KF addition gradually increased to 6 g/L,the thickness of the film layer became larger and the pore size in the film layer became smaller and more dense.Due to the increasing thickness and decreasing pore size,the corrosion resistance of the specimen was also improved.The porous nature of the micro-arc oxidized surface limits the further improvement of the corrosion resistance of the coating,therefore,this paper next investigated the sealing of the micro-arc oxidized specimens in hydrothermal solutions with different pH values and studied the effect of the pH value of the hydrothermal solution on the sealing effect of the micro-arc oxidized specimens.In this paper,the specimens were hydrothermally treated in Ca(NO3)2 with acidic(6),neutral(7)and alkaline(8)pH values at 90℃ and 120 min,and the film thickness,surface morphology,corrosion resistance and phase composition of each group of specimens were examined.The results showed that the film thickness and corrosion resistance of the specimens under acidic conditions were significantly reduced and were not favorable for sealing after micro-arc oxidation.The difference in corrosion resistance between the alkaline and neutral environments was not significant,and the film thickness and corrosion resistance were similar,indicating that the alkaline and neutral environments had little effect on the overall performance of the specimens.After the hydrothermal treatment in these two environments,the pores on the surface of the specimens were reduced or disappeared,but the thickness did not change much,which was due to the successful formation of sealing substances at the pores on the surface of the specimens by the hydrothermal treatment in alkaline and neutral environments.