The Mechanism of Titanium Hydride Formation on Grade-2 Titanium

Abstract

ASTM Grade-2 titanium (Ti-2) is a commercially pure grade widely used for its corrosion resistance in extreme environments. Therefore, Ti-2 has found applications within industries such as oil and gas, chemical processing, and nuclear power, where it is used for components such as reaction vessels, piping, and heat exchangers. Challenges with the fabrication process result in relatively high amounts of iron in commercial materials such as Grade-2, Grade-7, and Grade-12, which all have a maximum tolerance of 0.3 wt.% iron.The amount of iron present in titanium has been shown to have a significant effect on the microstructure and corrosion behaviour[1]. For example, Fe will precipitate and form TixFe intermetallic particles (IMPs) along grain boundaries if the local solubility limit is exceeded. Many of the corrosion processes (e.g., crevice corrosion) that determine the long-term integrity of Ti are strongly affected by the presence of IMPs[1]. Another detrimental corrosion scenario is the formation of titanium hydride (TiHx), which can precipitate on the surface and/or in the bulk material when the hydrogen concentration is high enough. Formation of TiHx phases increases the susceptibility of the metal to cracking, as the hydride is less ductile than the metal and its formation results in a strain-inducing volume increase within the matrix. Despite the proven importance of IMPs to the corrosion behaviour of Ti, and the detrimental effects of hydride formation on the material’s integrity, a direct relationship between IMPs and TiHx formation is not well established. There is some evidence that titanium hydride formation initiates at IMPs[2], but a complete mechanistic understanding is missing.In this study, the initiation and propagation mechanism of TiHx formation on Ti-2 is being investigated over a 24-hour period. Hydrides were grown galvanostatically in a simulated crevice corrosion environment. The surface was analyzed at various times using field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), focused ion beam (FIB) milling, and X-ray diffraction (XRD). By visualizing TiHx within the matrix using FE-SEM, we determined that titanium hydride formation initiates at IMPs and then grows both laterally and vertically around the IMP. After reaching a depth of ~ 4 µm below the surface of the IMP, the hydride stops growing vertically but continues to spread laterally across the Ti grain face until full surface coverage is reached. Finally, a steady-state condition is reached, in which hydride is present as a uniformly distributed layer across the surface with a thickness of ~ 4 μm. Additionally, the electrochemical potential values during the galvanostatic hydride growth process and the amount of hydride detected using XRD both support the hypothesis of the hydride growing rapidly before reaching a limited thickness.[1] X. He, J. J. Noël, D. W. Shoesmith, Corrosion 2004, 60, 378–386.[2] Q. Tan, Z. Yan, H. Wang, D. Dye, S. Antonov, B. Gault, Scr. Mater. 2022, 213, 114640.