Study On Corrosion Behavior Of Mg-5Sn(-In-Ga) Alloys

Mg-Sn system alloys have good application prospect in transportation,3C electronics and biomedicine fields,due to the advantages of desirable strength,high temperature stability,rapid extrusion ability and biocompatibility.At present,Mg-Sn alloys still face many challenges for practical application,one of which is that the alloy cannot take into account both mechanical properties and corrosion resistance.The Mg₂Sn phase precipitated in Mg-Sn alloy is conducive to the improvement of mechanical properties,but it greatly deteriorates the corrosion resistance of the alloy.This limits the practical application of Mg-Sn alloys.In this work,in order to improve simultaneously the mechanical properties and corrosion resistance of Mg-Sn alloy,a new Mg-5Sn-In（-Ga）alloy was designed.The effects of In and Ga on the mechanical properties and corrosion resistance of as-cast and as-extruded Mg-5Sn alloys were studied.The application environment of the alloy in industrial and biomedical fields was simulated by 3.5 wt%Na Cl and Hank’s solution,respectively.The corrosion resistance and corrosion mechanism of the alloy were revealed in two different solutions.In addition,the biocompatibility of the alloy was analyzed.The results provided theoretical data for broadening the application field of Mg-Sn alloys.The main research contents and findings of this thesis were divided into three parts:（1）Firstly,in order to reduce the effect of Mg₂Sn on the corrosion rate of Mg-5Sn alloy,In element with large solid solubility and which is difficult to form the second phase was added to Mg-5Sn alloy.The effect of In element on the microstructure and corrosion resistance of as-cast Mg-5Sn alloy was studied.The corrosion behavior of as-cast Mg-5Sn-x In alloy in 3.5 wt%Na Cl solution was systematically evaluated by means of static immersion weight loss,hydrogen evolution,electrochemical test,surface morphology observation and corrosion products analysis.The results of weight loss tests after immersion for 72 hours showed that the corrosion rate of as-cast Mg-5Sn-x In alloy first decreased and then increased with the increase of In element content.The as-cast Mg-5Sn-3In alloy had the lowest weight loss corrosion rate of 0.99 mm/y,and the corrosion rate of Mg-5Sn alloy was 3.51 mm/y.The corrosion resistance of Mg-5Sn alloy was enhanced by ca.3.5 times.The corrosion mechanism of In element improving the corrosion resistance of as-cast Mg-5Sn alloy was as follows:In element increased the stability of the corrosion film on the alloy surface and improved the protective effect of the corrosion film on the substrate.After the corrosion film was broken,the dissolved In in theα-Mg matrix reduced effectively the micro-galvanic corrosion betweenα-Mg matrix and Mg₂Sn,and thus increased the corrosion resistance of the alloy.However,with the addition of excessive In（In>3wt%）,the interdendritic regions due to the higher In content will act as the main cathode phase to weaken the corrosion resistance of the alloy,and thus,the corrosion rate for Mg-5Sn-4In alloy was increased.Moreover,electrochemical and hydrogen evolution experiments showed that an appropriate amount of In element can significantly weaken the cathodic reaction kinetics and reduce the cathodic current density of as-cast Mg-5Sn alloy.（2）Secondly,in order to further optimize the corrosion resistance of Mg-5Sn-x In alloy and expand its application field,Mg-5Sn-x In alloy was extruded and its corrosion behavior in Hank’s solution was analyzed.Taking Mg-5Sn-3In alloy as an example,the effects of extrusion deformation on the microstructure and corrosion behavior of Mg-5Sn-3In alloy in Hank’s solution were studied.After extrusion,the grain of Mg-5Sn-3In alloy was refined,accompanied by a large number of Mg₂Sn particles with a size of 100-200 nm precipitated at the grain boundary.It was found that extrusion does not reduce the corrosion rate of Mg-5Sn-3In alloy in Hank’s solution,but greatly reduced the local corrosion sensitivity of the alloy.Based on the results of corrosion behavior of extruded Mg-5Sn-3In,and the effect of In content on the corrosion behavior for extruded Mg-5Sn-x In in Hank’s solution was further studied.It was found that the protective effect of the corrosion film on the alloy surface was strengthened with the increase of In content,which improved the corrosion resistance of the as-extruded Mg-5Sn-x In alloy.And the dynamic recrystallization volume fraction of as-extruded Mg-5Sn-x In alloy increased with the increase of In content,which makes the precipitated Mg₂Sn particles distribute more uniformly,and promotes the formation of a more uniform corrosion morphology after corrosion.In addition,NIH3T3 and HUVECs cells were cultured in as-extruded Mg-5Sn-x In alloy extracts for 5 days.The cytotoxicity tests showed that the alloy had no cytotoxic effect on NIH3T3 and HUVECs cells,suggesting that Mg-5Sn-x In alloy exhibited good cytocompatibility.NIH3T3 cell and HUVECs cell activity was grade 0 and grade 1,respectively.（3）Finally,in order to increase simultaneously the corrosion resistance,mechanical properties and biocompatibility of the alloy,Ga element with low corrosion potential,strong precipitation strengthening and good biocompatibility was further added to replace part of In element in as-extruded Mg-5Sn-1In-2Ga alloy.The effect of Ga element on the microstructure,mechanical properties and corrosion resistance in different solutions was studied.The Mg₅Ga₂ particles were dynamically precipitated due to Ga addition,which significantly improved the mechanical properties of as-extruded Mg-5Sn alloy（tensile strength 309 MPa,yield strength 253 MPa and elongation 14.9%）.The corrosion test results showed that the corrosion products of Ga element were preferentially deposited on the alloy surface and enriched in the inner layer of corrosion products,which greatly improved the protection of corrosion layer and the corrosion resistance of the alloy.The corrosion rate of Mg-5Sn-1In-2Ga alloy after immersion for 72 h in 3.5 wt%Na Cl solution was 1.58 mm/y,which was much lower than 7.01 mm/y of Mg-5Sn alloy.As-extruded Mg-5Sn-1In-2Ga alloy still exhibited the low bio-corrosion degradation rate,the corrosion degradation rate was only0.18 mm/y after immersion for 15 days in Hank’s solution.Moreover,as-extruded Mg-5Sn-1In-2Ga alloy was implanted into rats for 6 weeks,and the results showed that Mg-5Sn-1In-2Ga alloy had no adverse effects on the implanted surrounding tissues and organs such as lung,liver,spleen,kidney and heart,indicating that Mg-5Sn-1In-2Ga alloy had good biocompatibility.