Degradation Behaviors Of Biomedical Magnesium Alloys And The Coatings Under Different Simulated Physiological Environments

Biomedical degradable magnesium（Mg）alloys have broad application prospects in interventional medicine fields such as orthopedics and cardiovascular stents.The implant materials have to face complex physiological factors when serving in the body,such as the flow fields and stress environments.At present,static immersion test is mostly used to study the in vitro degradation behaviors of Mg alloys and their surface coatings,which is different from the real in vivo environments.Besides,the mechanism of degradation process and the quantitative relationships are rarely reported.To solve these issues,this paper designed the in vitro simulated experimental platforms of physiological flow fields and stress environments（PFFSE）.On this basis,the degradation behaviors of different Mg alloys and their coatings under the influences of flow field factors（flow velocity,medium type,etc.）and stress factors（static stress magnitude,loading form,dynamic frequency,etc.）are systematically studied and analyzed.The influenceing mechanisms of different PFFSE factors on the corrosion process and degradation behavior are discussed,and the theoretical numerical relationships between the PFFSE factors and the corrosion rate(v_corr)of Mg alloys are established and verified.The effects of flow velocity（v）and corrosive medium type on the degradation behaviors of AZ31 were studied.The results showed that the increase of v accelerated the v_corrof AZ31.The v_corrof the edge part of the sample was higher than the center,resulting in a marginal effect.That was because there were differences in the distribution of flow patterns near different parts of Mg alloy and leading to changes in flow-induced shear stress（FISS）.Thus affect the diffusion coefficient and mass transfer process.Mg alloys in NS mainly presented as pitting corrosion,while the corrosion in PBS and SBF were more uniform.The v_corrvalues from high to low obeyed the order of NS>PBS>SBF.Under low v conditions,the corrosion of AZ31 would be controlled by the mass transfer process,and the relationship between the corrosion current density i_corrand v accorded with the linear relationship of i^-1_corr∝v^-1/2.The degradation behaviors of different Mg alloys and surface coatings in flow fields were investigated.The results indicated that the corrosion resistances of Mg alloys in the flow field environments from high to low were:WE43>Mg-Nd alloy>AZ31.The v_corrof WE43responded more obviously to the change of v,while the change of Mg-Nd was smaller.Surface coatings can significantly improved the corrosion resistance of Mg alloys.Among them,the change of corrosion resistance of Mg F₂coated WE43 was the most obvious.The v_corrof MAO coated AZ31 first rose up and then gradually decreased.The protective effects of the coatings gradually weakened with the increase of v,and the coatings failure time were also earlier.The electrochemical test results indicated that the i_corrof other Mg alloys and v also exhibited the linear relationship of i^-1_corr∝v^-1/2under the low-velocity flow field,which was similar to the relationship of AZ31 Mg alloy.The effects of static stress magnitude and loading form on the degradation behaviors of Mg alloys and the coatings were studied.The results indicated that the greater the stress,the faster the v_corrof Mg alloys.However,the v_corrgradually decreased with time and tended to be stable.WE43 was more affected by stress in the early stage of corrosion,while AZ31 was more sensitive to stress at the later period.The v_corrof MAO coated AZ31 first increased and then decreased,and finally changed with fluctuation,while the v_corrof Mg F₂coated AZ31gradually increased with time,and presented better protective effect.In terms of loading form,static tensile loading have showed a more obvious acceleration effect on corrosion than compressive loading.The experiment results indicated that there existed a quantitative linear relationship of ln v_corr∝σbetween v_corrof different Mg alloys and static stress valueσ.The effects of loading frequency（f）on the degradation behaviors of Mg alloys and the coatings under low-frequency tension-compression alternating cyclic dynamic stress were studied.The results showed that the acceleration effect of dynamic stress was more obvious than that of static stress,and the greater the f,the faster the v_corrof Mg alloy.The v_corrof AZ31gradually decreased with time.The initial values of v_corrof MAO coated WE43 and AZ31were lower and then gradually accelerated.The corrosion behavior of Mg F₂coated AZ31 was similar to that of MAO treatment,but the change was unconspicuous.Low f dynamic loading can promote the deposition of corrosion products to a certain extent,but cannot affect the acceleration effect on corrosion.There was a linear relationship of ln v_corr∝f in the early stage of corrosion.The results of electrochemical and weight loss experiments showed good agreement,indicating that the linear relationship had a certain degree of universality under the experimental conditions.This research can provide a scientific in vitro evaluation method for the study of the degradation behaviors of biodegradable orthopedic fixators and vascular stent materials in PFFSE,and provide an in vitro experimental basis for the design and development of biomedical Mg alloy materials and functional surface coatings,which has important theoretical significance and application value.