The electrochemistry of titanium corrosion

Thumbnail Image
Noel, James J.
Journal Title
Journal ISSN
Volume Title
This Thesis describes investigations of the corrosion resistant behaviour of titanium and several of its dilute _-phase alloys in aqueous solutions under conditions (temperature, pH, solution composition, 'etc'.) expected to be relevant to titanium alloy containers buried in a nuclear fuel waste disposal vault. A range of electrochemical experiments (open circuit potential, polarization, electrochemical impedance spectroscopy, coupled electrode crevice corrosion and combined electrochemistry/neutron reflectometry measurements) and post-electrochemistry, 'ex situ' analyses (X-ray photoelectron spectroscopy, hydrogen extraction, optical microscopy, and Auger electron spectroscopy) were used to study the nature and growth of the passive oxide film on titanium, its breakdown and dissolution, the absorption of hydrogen into the titanium, and the influence of ilute alloying components on some of these behaviours. The major conclusions are mentioned briefly below. The data generated in these experiments support published papers that suggest the passive film on titanium is composed of TiO2 with a rutile-like packing density; however, when grown in aqueous solution, significant amounts of hydrogen are incorporated into the oxide in the form of Ti-OH and bound water molecules. On alloy materials containing Ti-noble metal intermetallic precipitates, the passive film seems to be discontinuous, presumably because it does not form as well over exposed intermetallic particles as it does over grains of titanium. This observation helps explain the enhanced corrosion resistance of these alloys over unalloyed titanium--the exposed particles are believed to catalyze the cathodic half-reaction and reinforce passivity by anodically polarizing the alloy. The passive oxide film on titanium can be penetrated by dissolution in acidic solution. This requires a pH less than ~1 in deaerated HCl solution at room temperature. It appears that catalysis of the hydrogen evolution reaction by exposed intermetallic particles, or even by titanium hydride spontaneously formed at the metal surface, anodically polarizes the metal under some circumstances, thereby limiting the amount of corrosion occurring. The data presented here also support published claims that the passive oxide undergoes mechanical breakdown induced by increasing temperatures above 60-70C. The underlying cause of the breakdown was suggested to be crystallization of the passive film from an amorphous state. Thermally induced breakdown of the passive film was found to be a necessary, but not sufficient, condition for the initiation of crevice corrosion on Ti. The nature of the electrolyte species present, in particular the type of anion, was found to play an important role in crevice corrosion initiation. The passive oxide film was found to help protect the underlying titanium from absorption of electrolytic hydrogen at potentials positive of a certain threshold potential. This threshold potential was found to be close to the flat band potential of the semiconducting oxide film. The experimental results suggest that, at potentials below the threshold, band bending in the semiconductor results in electronic degeneracy, accompanied by an increase in the electrical conductivity of the oxide, oxide reduction, incorporation of hydrogen into the oxide, and eventually hydrogen ingress into the metal. This work focuses on understanding the underlying physical and chemical reasons for many of the empirically observed corrosion and hydrogen absorption properties of titanium and its alloys that have evolved into practical guidelines for the industrial service conditions of these materials.