The effect of nitrogen saturation on the corrosion behaviour of Ti-35Nb-7Zr-5Ta beta titanium alloy nitrided by ion implantation

Abstract

Corrosion behaviour of Ti-35Nb-7Zr-5Ta β titanium alloy nitrided by ion implantation was examined. Nitrogen ions were implanted with different fluences in the range from 1·1017 to 9·1017 cm−2 and an accelerating voltage of 90 kV. The specific effects of the nitrogen fluence on structural changes of the implantation zone showed the relationship between nitrogen concentration, nitride formation and corrosion behaviour. Nitrogen depth profiles measured by glow discharge optical emission spectroscopy showed the saturation limit approx. 60 at.% leading to the exo-diffusion of nitrogen. X-ray photoelectron spectroscopy indicated increasing formation of surface nitrides with the fluence of implanted nitrogen, up to the saturation fluence of 4 · 1017 cm−2. Oversaturation induced a decrease in nitrogen concentration, accompanied by a reduction in the amount of surface nitrides, and accompanied by blistering. The corrosion tests in physiological saline solution show high resistance of the implanted surfaces to corrosion attack. The corrosion resistance increased with increasing fluence up to saturation fluence. Fluences higher than saturation one caused decrease of the corrosion resistance to the level of lower fluences. All nitrogen implanted surfaces showed higher resistance than non-implanted surface. The surface nitriding has no influence on the susceptibility to the localized corrosion attack. The formation of surface nitrides and their concentration, rather than the defects of an oversaturated surface, affect the corrosion behaviour.