Hydrogen absorption into alpha titanium alloys under aqueous conditions

This thesis is a study of hydrogen absorption into alpha titanium alloys under aqueous conditions. The study was focused on the hydrogen absorption behavior of Grade 2, 7, and 12 (Ti-2, Ti-7, and Ti-12) under active (simulated crevice or real crevice) conditions, and on the passive film properties of Ti-7 which will influence absorption under passive conditions. Experiments were designed to determine the influence of practical parameters (e.g. time, temperature, and salinity) on these three titanium alloys with the eventual aim of developing a hydrogen absorption model to predict the lifetime of the alpha titanium alloys under nuclear waste disposal conditions.;First, hydrogen absorption into Ti-2 under simulated crevice conditions at 25°C was studied in detail. Initially, hydrogen absorption occurs rapidly on Ti-2 surfaces free of a passivating oxide film to form a hydride layer. The surface hydride layer can catalyze the reduction of protons but acts as a barrier to further absorption. For a surface hydrided for a short time (≤ 2 hours), the cathodically active hydrided sites can coexist with anodically active alpha-Ti sites. SIMS imaging shows that proton reduction and hydride formation occur preferentially on iron-containing beta-phase and intermetallic particles (TixFe) which tend to be located in grain boundaries.;Hydrogen absorption under actual crevice corrosion conditions was studied on Ti-2 using a previously developed galvanic coupling technique. The results show that the iron content and its distribution in Ti-2 have a marked effect on the accumulation of crevice corrosion damage and the maximum depth of penetration. Iron present in intermetallics appears to suppress the propagation rate and to limit the maximum depth of penetration by catalyzing the proton reduction reaction inside the crevice. Crevice corrosion initiates more rapidly and the rate of activation of propagation increases as temperature is increased over the range 85°C to 135°C. However, the extent of overall crevice corrosion is suppressed with increasing temperature. Both the total amount of damage accumulated and the maximum penetration depth increase with chloride concentration over the range of 0.27 mol·L-1 to 2.5 mol·L-1, while in 5.0 mol·L-1 NaCl solution, the amount of crevice corrosion damage was significantly suppressed. Additionally, both a high temperature and a high Cl- concentration can lead to the localization of crevice propagation at a small number of active sites. A relatively constant hydrogen absorption efficiency of 5% to 8% was obtained, indicating that hydrogen absorption into Ti-2 under crevice corrosion conditions is insensitive to duration of propagation, temperature and Cl - concentration. This can be attributed to a steady state balance between hydrogen absorption by crevice propagation and chemical dissolution of the hydride within the extremely acidic crevice.;Finally, the influence of temperature, chloride concentration, and anion species (e.g., Cl- and SO42-) in the solution on the passive film properties has been investigated. These experiments were the first step in determining whether hydrogen absorption is possible under passive conditions. Experiments were performed on the Ti-7 alloy. Despite the presence of Pd, Ti-7 shows some general corrosion activity in NaCl solution as indicated by oxide film breakdown/repassivation transients. For T ≤ 80°C, the properties of the oxide films formed in chloride solutions improved with temperature, while for T ≥ 90°C, the oxide film became more defective than at low temperatures. The direction of the potential transients (positive-going or negative-going) changed with the value of Ecorr prior to breakdown. When Ecorr prior to breakdown was very positive, negative-going transients were observed, indicating a cathodically controlled process in the breakdown site. By contrast, when Ecorr prior to breakdown was very negative, positive-going transients were observed, indicating an anodically controlled process in the breakdown site. (Abstract shortened by UMI.);Keywords: Hydrogen absorption, Hydride, Ti-2, Ti-7, Ti-12, Alloying elements, Galvanostatic polarization, Catalyzing effect on proton reduction, SIMS, Crevice corrosion, Crevice initiation, Crevice propagation, Maximum penetration depth, Damage function, Passive oxide film, Potential transients.