Hydrogen Absorption into Alpha Titanium Alloys under Aqueous Conditions

Abstract

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 α-Ti sites. SIMS imaging shows that proton reduction and hydride formation occur preferentially on iron-containing β-phase and intermetallic particles (TixFe) which tend to be located in grain boundaries. The influence of temperature on hydrogen absorption into Ti-2 under simulated crevice conditions was then investigated. A temperature of 45°C was identified for hydride layer fracturing based on the potential transients obtained in galvanostatic polarization experiments and optical micrographs. The formation of a surface hydride layer can catalyze the formation of oxide and enhance the passivation of Ti-2 under open- iii circuit conditions at T ≤ 45°C. However, active corrosion conditions are maintained on hydrided surfaces at higher temperatures. The repassivation process on a hydrided surface at T < 45°C has been shown to occur in three steps, TiHx MPL> TiTiY Hl O2 step2 >TiivTim Hl O2 step3 >TiO2 The first two steps involve the formation of a defective oxide (a mixture of Tim and Tiιv), and a more stable and resistive oxide (TiO2) forms in step 3. SIMS images show that absorbed hydrogen can diffuse into Ti-2 through the iron-containing β-phases and intermetallic particles (TixFe). The effects of alloying elements on hydrogen absorption under simulated crevice conditions have been studied. The alloying element Pd has a strong catalyzing effect on proton reduction, which forces the polarization potential to positive values and leads to oxide formation on the electrode surface of Ti-7. The hydrided patches observed on Ti-7 imply that the oxide only partially covers the surface of the electrode and that the existence of “windows” within the oxide allows hydrogen evolution and absorption. Compared to Ti-2 and Ti-12, hydrogen absorption into Ti-7 is greatly suppressed by the formation of the oxide on the electrode surface. The alloying elements Ni/Mo catalyze proton reduction, but also strongly enhance hydrogen absorption into Ti-12. In this titanium alloy, the high density of β-phase and Ti2Ni intermetallic particles provide active sites for hydrogen absorption and diffusion. 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 iv 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 CΓ 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 Ci^ 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., C1' 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 V 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 Ecθιτ prior to breakdown was very negative, positive-going transients were observed, indicating an anodically controlled process in the breakdown site. In the most concentrated chloride solution (5 moi L"1 NaCl) the film resistance was noticeably reduced at high temperatures (T > 130°C) showing repassivation was not as effective as for lower chloride concentrations. Due to the ability of SO42 to complex Ti4+, more corrosion damage occurred in sulphate solutions compared to chloride solutions, especially for temperatures > 80°C. It was found that temperature, not chloride concentration or anion species, was the main factor causing film fracture events. A temperature of > 60°C was required to initiate fracturing in both chloride and sulphate solutions.