Corrosion Behavior of Nickel-Free High Nitrogen Austenitic Stainless Steel in Simulated Biological Environments

Abstract

The corrosion resistance of the nickel-free high nitrogen austenitic stainless steel without manganese, Fe-23Cr-2Mo-1.5N (mass%) (HNS) as biomaterials, was evaluated by the polarization test in various electrolytes: 0.9%NaCl solution (saline), phosphate buffered saline (PBS(-)), Hanks' solution (Hanks) and Eagle's minimum essential medium (E-MEM). Conventional austenitic stainless steel, 316L, was also polarized for comparison. The both alloys were spontaneously passivated in all electrolytes. The HNS didn't show pitting corrosion in the polarization range in all electrolytes although the 316L showed pitting corrosion. Passive current densities of the HNS in all electrolytes were lower than those of 316L. Therefore, the HNS shows higher passivity and resistance to pitting corrosion than 316L. The passive current density in Hanks of HNS was lower than that in saline, indicating that the protectiveness of surface oxide film increased with the existence of inorganic ions such as phosphate and calcium ions. On the other hand, the passive current density in E-MEM was higher than that in Hanks, but was lower than that in saline. Consequently, the HNS must show high corrosion resistance in vivo and be a promising biomaterials. Nickel-free austenitic stainless steels having a large amount of chromium, manganese, nitrogen and a small amount of molybdenum, such as Fe-15Cr-(10-15)Mn- 4Mo-0.9N (mass%), Fe-18Cr-18Mn-2Mo-0.9N (mass%), Fe-(15-18)Cr-(10-12)Mn-(3-6)Mo-0.9N (mass%) and Fe- (19-23)Cr-(21-24)Mn-(0.5-1.5)Mo-(0.85-1.1)N (mass%) (BioDur 108), are currently developed to enhance the strength, corrosion resistance and biocompatibility of stain- less steels. 7-10) In those cases, nitrogen and manganese are employed instead of nickel to obtain austenitic phase, be- cause nitrogen and manganese also stabilize austenitic phase and control mechanical properties of stainless steel as same as nickel. The strength of nickel-free austenitic stainless steels depends on nitrogen content, while corrosion resistance is strongly related to the gross amount of chromium, molyb- denum and nitrogen. Ultimate tensile strength and elonga- tion to fracture of Fe-(19-23)Cr-(21-24)Mn-(0.5-1.5)Mo- (0.85-1.1)N (mass%) are over 931 MPa and less than 49%, respectively, in annealed condition. 9) Corrosion resistance of the alloy is higher than that of nickel containing austenitic stainless steels. 9) Those results indicate that nickel-free stain- less steel seems to be a mechanically and chemically biocom- patible material. 7-10) However, the reports of rather high tox- icity of manganese and its salts have recently increased. 2, 4, 5) Thus the nickel and manganese-free stainless steel is neces- sary for biomedical use. Nickel-free austenitic stainless steel without manganese, Fe-23Cr-2Mo-1.5N (HNS) (mass%), is also developed. 11, 12) The nitrogen content of the HNS is higher than that of other nickel-free austenitic stainless steels leading to the removal of manganese. Ultimate tensile strength and elongation to fracture of the HNS are over 1200 MPa and over 40%, re- spectively, in annealed condition. 12) In addition, corrosion re- sistance of the HNS in artificial seawater is higher than that of 316L. 11-13) Therefore, HNS may be a promising biomate- rial because of its nickel-free and manganese-free composi- tion and high strength and corrosion resistance. However, the corrosion resistance of HNS in biological environment has never been characterized. In this study, anodic polarization test of HNS and 316L in various aqueous solutions was performed to evaluate the cor- rosion behavior of HNS as biomaterials. Effects of inorganic ions and amino acids on the corrosion behavior of HNS were also evaluated.