Galvanic corrosion induced by heterogeneous bimodal grain structures in Fe-Mn implant

Highlights: • A heterogeneous bimodal micro/nano-structured Fe-Mn alloy was prepared by mechanical alloying and laser powder bed fusion. • Galvanic current flow generated between the active γ-austenite grains and the negative α-ferrite grains. • Bimodal grain structures enlarged the electrochemical heterogeneity characteristics and promoting degradation propagation. Fe is a promising implant material but its application is obstructed by the slow degradation rate. The introduction of Mn in Fe offers potential because the Fe-Mn alloy has improved degradation rate and antiferromagnetic behaviour. Nevertheless, the problem of slow degradation rate persists in that alloy. In this study, a heterogeneous bimodal micro/nano-structured Fe-Mn alloy was prepared by mechanical alloying (MA) and laser powder bed fusion (LPBF). In detail, the Fe and Mn powders were mechanically milled, of which the impact energy exerted plastic deformation not only refined the grains to nanometres, but also promoted the solid solution of Mn (austenite forming elements) in Fe, and thus obtaining the nano γ-austenite grains. And then the nano-structured powders were blended with unmilled Fe-Mn powders which had micro-structured grains to prepare the bimodal-structured powders. Subsequently, the bimodal powders were prepared by LPBF, in which the fast cooling rate maintained the heterogeneous bimodal grain structure. Results showed that the prepared bimodal alloy had a mixture of micro α-ferrite grains and nano γ-austenite grains. The active γ-austenite grains were predominately anodic while the negative α-ferrite grains act as cathodes, between them a galvanic current flow generated to accelerate degradation. More significantly, the mixture of micro- and nano-sized grains enlarged the electrochemical heterogeneity characteristics of the alloy, which not only encouraged the corrosion susceptibility but also increased the destabilization of the passive film, and thus further promoting the degradation propagation. The electrochemical experiments showed that the bimodal alloy corroded at a higher current density of about 50.2 μA/cm² compared with the micro (11 μA/cm²) and nano (18 μA/cm²) groups. In summary, the heterogeneous bimodal Fe-Mn alloy could be a reliable alternative for repairing bone defects.