Iron-based alloys have been reported previously as biodegradable metals for cardiovascular stent applications due to their biocompatibility and comparable mechanical properties to those of stainless steel[1]. Iron has been alloyed principally with manganese[2] and addition of other minor elements such as palladium[3], carbon[4], and silicon[5] to tailor its strength and degradation rate that match tissue-healing process. New fabrication techniques such as powder metallurgy[2], electroforming[6], and magnetron sputtering[7] have been reported for the same objectives. Despite all the approaches taken to enhance the degradation rate of iron-based alloys, there are limited studies focused on the factors that influence their degradation rate. It was previously reported that the change of grain size influenced the degradation behaviour of pure iron since superior number of grain provides higher density of matrix and grain boundary where corrosion reaction takes place[1],[6]. Therefore, the objective of the current study is to investigate the effect of grain sizes towards the corrosion behaviour of pure iron. In order to do so, pure iron sheets (Armco soft ingot, purity >99.8%, Goodfellow, UK) with various grain sizes were produced by thermomechanical treatments comprising cold rolling (75-85% thickness reduction) and annealing (550oC-1000oC). The resulting sheets composed of pure iron sheets with grain size varied from 14 um to 168 um with degradation rate between 0.20 to 0.24 mmpy. In the effort of producing iron sheet with grain size smaller than 14 um, electrodeposition of pure iron on titanium cathode was used resulting iron sheet (99.7% purity) with the average grain size of 6 um following annealing at 550oC. The results showed that smaller grain size iron sheet produced by electrodeposition exhibited superior degradation rate (0.51 mmpy) compared to those of the iron sheets with modified grain size by thermo-mechanical treatments. Moreover, it showed superior mechanical properties (YS=270MPa, UTS=292MPa) compromising its elongation at break (e=18.4%) when compared to other iron sheets. Additionally, when manganese was added (35%wt.) to enhance the degradation behaviour of pure iron by powder metallurgy, it gave average grain size of 50 um with degradation rate close to that of electrodeposited iron. The results suggested that the grain size influences the degradation rate of pure iron. Specific window of grain size was observed, in which the degradation rate of pure iron decreased along with the reduction of the grain size (range 10-160 um of grain size). Moreover, alloying was observed to excite the degradation rate of pure iron since in the range where degradation rate of pure iron was low. Suggestively, manganese as the alloying element contributes to the galvanic corrosion process that accelerates the degradation rate of pure iron. This study provides evidence in different approaches to modulate the degradation behaviour of iron-based alloys by changing the grain size and alloying technique.
This work was partially supported by NSERC-Canada, CIHR-Canada, CFI-Canada, FRQ-NT-Quebec, and MRI-Quebec. The authors would like to acknowledge the prestigious NSERC Post Doctoral Fellowship (NSERC-PDF) awarded to Agung Purnama; Commonwealth Scholarship Program (Government of Canada) and University of Nigeria for scholarship award to CSO for the scholarship of Camillus Sunday Obayi; ACDI and AUCC for providing scholarship to Essowè Mouzou.
References:
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