Electrochemical and XPS surface analytical study on the reactivity of Ni-free stainless steel in artificial saliva

Nowadays, there is a strong demand to replace CrNi stainless steels used for biomedical applications such as brackets in orthodontics with a new generation of more biocompatible austenitic, nickel-free alloys. The aim of this work is the investigation of the growth and stability of the surface films formed on DIN 1.4456 Ni-free stainless steel (18% Cr, 18% Mn, 2% Mo) in artificial saliva (pH 7.9) at 37 degrees C by electrochemistry and XPS surface analyses and the assessment of the ions that leach from the steel. These results show that the open circuit potential asymptotically increases and the corrosion current density decreases from 1 to 24 h exposure time to the artificial saliva. XPS provides evidence that the surface film present on the surface after mechanical polishing transforms into a passive film that becomes progressively enriched in Cr (III) oxy-hydroxides whereas iron and manganese oxides are depleted. It is found that the instantaneous corrosion rate decreases exponentially with the amount of Cr (III) in the passive film. The results are discussed regarding the biocompatibility, thus the release of ions into solution, of the new nickel-free stainless steel. Based on the results presented in this paper and literature evidence, a comprehensive model is proposed that substantiates the biocompatibility of these alloys.