Microstructure Optimization And Performance Evaluation Of Electrodeposited Fe-Zn Alloy

Stent implantation is an effective method for the treatment of vascular occlusivediseases.Within 6-12 months after stent implanted,vascular remodeling and healing will complete itself.Then,there is no necessity for the stent to continue existence.The current treatment is mainly using drug-eluting stent.Compared to the first generation of bare metal stents that made of stainless steel,cobalt-chromium alloy and other corrosion resistant materials,drug-eluting stent can reduce the incidence of postoperative restenosis by controlling the release of drugs.But after a period of time,the absorbent drug layer coated the surface will be gradually absorbed by the body and eventually exposed bare stents.The problems caused by long-term stent in vivo can not be completely solved.Therefore,it is urgent to develop a biodegradable endovascular stent to satisfy the requirement of clinic.At present,there are two directions in the research of materials for biodegradable endovascular stent,polymer and metal materials.In the early year,studies was mainly focused on biodegradable polymers,and numerous polymer materials were evaluated.But the polymer as stent material,due to its weak support,X-ray not developed,the uneven degradation leading to localized fracture and degradation products stimulating vessels it will cause inflammatory response and other issues,making its application limited.Researchers begin to turn their attention to biodegradable metal materials.Magnesium,Iron,Zinc and their alloys have become a research hot-spot due to their excellent mechanical properties,bio-compatibility and easy corrosion characteristics.In this study,the Fe-Zn alloy was prepared by electrodeposition technology.It is expected that the constituent element Zn can decrease the standard electrode potential of solid solution and precipitating of Fe-Zn inter-metallic compound can increase the amount of micro cells in alloy,and these two functions may have a dual accelerated effect on its corrosion.The earlier work showed that when the zinc content is below 20%,the corrosion and degradation performance of Electrodeposited Fe-Zn alloy is better than that of the electrodeposited pure iron,which is much better than that of the conventional pure iron.However,the problems such as large internal stress and poor compactness of Fe-Zn deposited layer need to be solved.For the purpose of optimizing the micro-structure,the Fe-Zn alloy was prepared in this paper,under low current density reduced from 3.0A/dm2 to1.0A/dm2 and controlled p H value of the electrolyte around 2.3.Buffer pool was added to reduce the influence of the change of electrolyte main salt concentration on the micro-structure of Electrodeposited Fe-Zn alloy.Before and after the cathode current density decreased,Electrodeposited Fe-Zn alloy are both single solid solution phase（bcc）,but after the current density decreased,the deposited alloy ishomogeneous with finer and denser micro-structure and has strong preferred grain orientation of（112）.It is found that the effect of p H value of electrolyte on the composition and micro-structure of Electrodeposited Fe-Zn alloy is direct and obvious due to the phenomenon of micro-structure delamination in electrodeposited Fe-Zn alloy.When the p H value of electrolyte is below 2.0,the sodium citrate added as complexing agent is difficult to complexing with Fe²⁺,so that the precipitation potential of Fe²⁺ can not be shifted negatively to close to the precipitation potential of Zn²⁺.The co-deposition cannot be achieved,and Fe²⁺with positive precipitation potential is deposited preferentially,lead to the change of composition and micro-structure of the deposited layer.As the increase of Zn content,the Fe-Zn alloy deposited under optimized process parameters has more and more strong preferred grain orientation of（112）,and the solution to a-Fe lattice of Zn atoms increases leading diffraction peak position slightly to the left at the same time.When subjected to recrystallization annealing at 800℃ under argon protection for1 hour,there are new oxygenated phases formed in the deposited layer,and their corrosion resistance is higher than that of the ferrite substrate,which can promote corrosion degradation of materials as micro cathodes.Because of the O element deposited during electrodeposition,a large number of iron oxide（black particles）diffuse distribution on white ferrite matrix.As the increase of Zn content,iron-zinc oxide（the gray bar）was significantly increased,and the corrosion resistant Zn O phase begins to form.At the same time,the heat treatment process eliminates the internal stress of the deposited layer,and the micro-hardness of the deposited layer are obviously reduced,which is expected to improve the toughness of the Electrodeposited Fe based material.The corrosion degradation rate of Fe-5Zn is similar to Electrodeposited Fe,and the corrosion degradation rate of Fe-10 Zn alloy is about two times as much as Electrodeposited iron,which means that Zn components can help improve the degradation performance of iron-based materials;but because of the electrodeposition process of O dissolved,with the increase of Zn content,will generate corrosion resistance in heat treatment in the process of Zn O phase,which is not conducive to the improvement of performance degradation.But the formation of corrosion resistant Zn O phase during heat treatment is unfavorable to the improvement of degradation performance.