Corrosion in Hank's Solution and Mechanical Strength of Ultrafine-Grained Pure Iron

In recent years, there has been a growing interest in developing biodegradable implants which are absorbed by the body after fulfilling a task. Pure iron is a candidate for such applications because it is considered biocompatible and it does not passivate in physiological media. Herein, the potential to increase the strength of pure iron through grain refinement is evaluated to allow a reduction in size of implants and its effect on corrosion behavior. High-pressure torsion is applied to process pure iron and to refine the grain size of less than 1 mu m. Annealing at different temperatures is used to produce samples with different grain sizes. Compression tests show a significant increase in flow stress to over 1 GPa in samples with very small grain sizes. However, such structure is associated with negligible strain hardening and strain-rate sensitivity. Electrochemical and immersion tests in Hank's solution show that the grain refinement reduces the corrosion rate significantly. Samples with grain sizes smaller than 1 mu m display uniform corrosion and develop a homogeneous surface layer of corrosion products. Thus, severe plastic deformation followed by annealing produces mechanically stronger pure iron with reduced biodegradability.