Corrosion behaviour in physiological fluids of surface films formed on titanium alloys

Abstract

The properties of the surface films formed on Ti and its Ti5Al4V and Ti6Al3.5Fe alloys by chemical (immersion in 10?M NaOH) and thermal treatment (500 degrees C) were studied in this paper. Their corrosion behaviour in Ringer solution was investigated. After treatments, the sample microhardness was measured. Also, the sample surfaces were observed by an optical microscope before and after immersion in Ringer solutions. The electrochemical behaviour of the surface films obtained by the chemical and thermal treatments was studied using the methods of the electrochemical impedance spectroscopy (EIS) and monitoring of open circuit potentials at different immersion periods (up to 60 days) in Ringer solution. Microhardness increased with the increasing loads for the studied materials, showing the existence of a compact protective layer. Microscopic observations exhibited more compact coatings after 60 days of exposure in Ringer solution, due to the formation of an apatite layer. EIS spectra revealed surface films with two layers: an inner, barrier protective layer and an outer, porous apatite layer. The impedance increases in time denoting that the films have grown by apatite nucleation. Also, EIS spectra showed that the complex treatment by chemical?+?heat method is the most efficiently. For the samples treated by the two processes (chemical?+?heat) the values of the open circuit potentials are nobler than of the chemical treated samples, denoting better protective, bioactive films.