The area of biomaterials is a continuous research area. Among others, stainless steels are also used in medical applications such as fracture plates, wires, sutures and implants etc. Surgical grade 316 L stainless steel is specifically used for medical application. It primarily contains nickel, chromium and molybdenum wherein the purpose of nickel is to retain the FCC structure of stainless steel almost at any temperature unlike conventional steel which has ferrite (BCC structure) in ambient temperature. Also, nickel imparts a polished and glossy finish to surgical grade stainless steel which is an important factor to keep it hygienic. But, the harmful effect of nickel on human body has been investigated and reported for quite some time now. The adverse effect of nickel (Ni) ions that release from stainless steel upon crevice and pitting corrosion has led to the evolution of Ni-free stainless steel with high nitrogen content. This paper briefly reviews the effect of nickel release in human body and let go the presence of nickel in stainless steel by uplifting nitrogen content in it. Furthermore, the groundbreaking findings on Nifree stainless steel in last 10 years are presented and the possibilities of further developments have been hypothesized.

Introduction

Metals are predominantly used as implants in biomedical industries. The foremost criterion for choosing any implant material is its biocompatibility. The reason for choosing metallic implant is its superior mechanical strength and corrosion resistance. The most commonly used biomedical alloys are grade 5 Ti6Al4V, CoCrMo and stainless steel (AISI 316L) [1,2]. The fabrication of stainless steel is comparatively easier than Ti-alloys and CoCr alloys. Although stainless steel is used as implants at places but its application is confined to bone healing, screws and fixations. Stainless steel is present in three microstructures which are austenite, martensite and ferrite. There is one other kind which is the duplex stainless steel; it basically contains both austenite and ferrite structure [3]. These structures are attributed to proper adjustments in its chemistry which later reflects in its properties. For instance, austenitic steel doesn’t possess ferromagnetism properties like its counterpart martensitic and ferrite steel. One of the preferential attribute of implants is the non-existence of ferromagnetic property which is why austenitic stainless steels are preferred over martensitic and ferrite steels for implant material. The other advantages of austenitic steels are their FCC structure, low yield strength to tensile strength ratio, better formability and corrosion resistance among others. 316 L Stainless steel is basically Fe-based alloys containing carbon (C of maximum 0.03%), chromium (Cr of atleast 16%) and nickel (Ni of atleast 10%). Homogenous corrosion resistance is the most important characteristics of stainless steel. Corrosion resistance is not an intrinsic property but the behavior of material surface in interaction with environment.