Steel Contamination of Biomedical Titanium Alloys during Hot Isostatic Pressing Treatments

Abstract

Hot isostatic pressing (HIP) is an industrial treatment for the consolidation of metallic components. In the biomedical field, HIP processes are applied to titanium alloys fabricated by other sintering techniques to reduce residual porosity and improve the mechanical characteristics. Stainless steel sheet encased HIP processes are used when surface open porosities are present, in particular for powder metallurgy products or when nitrogen absorption is a major concern. In these circumstances, localized steel contamination may occur on the titanium surface. Contamination could be easily avoided using titanium encasing sheets, but these are rarely applied in industrial practice. In this work, Grade 2 and Grade 5 titanium have been treated with a standard HIP process. During treatment both alloys have been kept in contact with stainless steel casement, causing diffusion of elements inside the titanium. Samples have been investigated using two-dimensional profilometry and scanning electron microscopy. Qualitative and semi-quantitative compositional analyses have been performed using energy-dispersive x-ray spectroscopy and glow discharge optical emission spectrometry (GDOES). Microstructural analyses have been obtained after GDOES plasma etching in order to avoid any further surface preparation and easily obtain comparable results from the two different alloys. Localized potentiodynamic polarization curves were obtained on the different regions of the samples using microcell equipment. Both alloys were contaminated by stainless steel elements in the stained regions. A reduction in the corrosion resistance of the alloys has been observed due to the contamination. The experimental setting demonstrated the corrosion susceptibility of stainless steel contaminated biomedical titanium alloys after HIP, even if the passive behavior of titanium was not completely compromised.