Titanium Hydride Growth and Tix Fe Intermetallic Particle Dissolution on Grade-2 Titanium Under Simulated Crevice Corrosion Conditions
Authors: Morgan Adam M. , Dehnavi Vahid , Zagidulin Dmitrij , Shoesmith David W. , Noel James J. 
Published: December 2024

ASTM Grade-2 titanium (Ti-2) is a commercially pure grade of titanium with a combination of high strength, excellent ductility, and elite corrosion resistance. That determines its use in diverse and demanding applications, including use as components in flue-gas desulphurization plants, exhaust systems in the automotive industry, and multiple components in marine equipment.
Crevice corrosion and hydrogen embrittlement are the common failure mechanisms for Ti-2 components. Crevice corrosion involves the development of occluded cell corrosion conditions characterized by a solution consisting of low [O2] and low pH. Under these conditions, the hydrogen evolution reaction (HER) becomes the dominant cathodic reaction. The HER drastically affects crevice propagation and leads to the formation of titanium hydride (TiHx) phases. Such phases are responsible for Ti-2 hydrogen embrittlement by decreasing Ti ductility, leaving it susceptible to hydrogen-induced cracking under tensile stress even after cessation of crevice corrosion. Due to the roles of microstructural features in these processes, the hydrogen absorption and hydride formation behaviour of Ti-2 will vary on the surface and in the bulk material. Improving our understanding of the HER and hydride formation on titanium is crucial due to their importance in material failure scenarios.
Iron, present as an impurity in Ti-2, has been shown to affect titanium's corrosion behaviour drastically1. Due to the formation of TixFe intermetallic particles (IMPs), iron affects crevice corrosion behaviour, HER kinetics, and hydride formation. With the much higher exchange current density of iron (io = 2.0-3.0 x 10-6 A/cm2)2,3 compared to titanium (io = 5.0-6.3 x 10-9 A/cm2)2,4, it is probable that the HER and hydride formation will preferentially occur on IMPs. Given the influence of TiHx on corrosion behaviour and the extreme applications of Ti-2, it is crucial to develop a relationship between our understanding of TiHx formation and microstructure.
This research aims to develop a relationship between Fe content in Ti, the resulting microstructure, and their effect on hydride formation, leading to improved material selection criteria, alloy development, and performance. This study investigates the initiation and propagation mechanism of TiHx formation on Ti-2 over a 24-hour period. Hydrides were grown galvanostatically in a simulated crevice corrosion environment. The surface was analyzed at various time intervals using field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), focused ion beam (FIB) milling, X-ray diffraction (XRD), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). By visualizing TiHx within the matrix using FE-SEM, we determined that titanium hydride formation initiates at IMPs and then the hydride grows in all directions around these sites. After reaching a depth of ~ 5 µm below the surface of the IMP, the vertical hydride growth ceases, but the hydride continues to spread laterally across the Ti grain face until it reaches full surface coverage. Under steady-state conditions, the hydride is present as a uniformly distributed layer across the surface, with a thickness of ~ 5 µm. The electrochemical potential values during the galvanostatic hydride growth process and the amount of hydride detected using XRD support the hypothesis that the hydride proliferates before reaching a limited thickness. Elemental maps obtained using ToF-SIMS validate the observation that hydrides initiate at IMPs and confirm that the features observed in FE-SEM are hydrides. Furthermore, the amount and type of hydrogen-containing species were measured to elucidate the dynamics of TiHx formation.

References

He X, Noël JJ, Shoesmith DW. Effects of iron content on microstructure and crevice corrosion of Grade-2 titanium. Corrosion. 2004;60(4):378-386.

Trasatti S. Work function, electronegativity, and electrochemical behaviour of metals. III. Electrolytic hydrogen evolution in acid solutions. J Electroanal Chem. 1972;39(1):163-184.

Abd Elhamid MH, Ateya BG, Weil KG, Pickering HW. Calculation of the Hydrogen Surface Coverage and Rate Constants of the Hydrogen Evolution Reaction from Polarization Data. J Electrochem Soc. 2000;147(6):2148.

Bockris JO, Reddy AKN. Modern Electrochemistry. Plenum Press; 1970.