Quality aspects of high-nitrogen stainless steel for surgical implants

High-nitrogen stainless steel has been used in orthopedic implants because of its high strength, good corrosion resistance and biocompatibility. In this study six annealed wrought high-nitrogen stainless steel sample bars from five different manufacturers were characterized in terms of chemical composition, mechanical properties, hardness, metallurgical structure and corrosion resistance. The metallurgical investigation, which determined the microstructure, grain size and inclusion content, was made with a light microscope, image analysis and SEM with EDX. The potentio-dynamic corrosion behaviour in the cross-sectional plane was measured at pH 4 and 40degreesC. No major differences were found in the chemical compositions, mechanical properties, hardness or metallurgies of the six sample materials. The only difference between the materials was established in the corrosion test, where the breakdown potential varied between -150 and +900 mV((SCE)). However, no correlation could be established between the corrosion results and chemical composition, microstructure, or type / number of the inclusions. Additional heat treatment improved the material with low breakdown potential. It is recommended that minimum requirements should be established for the breakdown potential and incorporated in the current material standards.