Hydrogen Absorption into Titanium under Cathodic Polarization: An In-Situ Neutron Reflectometry and EIS Study

Abstract

Hydrogen (deuterium) absorption into sputter-coated titanium (Ti) film electrodes during cathodic polarization in heavy water (D 2 O) was monitored using in-situ neutron reflectometry (NR) and electrochemical impedance spectroscopy (EIS). The scattering length density (SLD) of Ti metal increased with increasing cathodic polarization, due to the penetration of deuterium through the surface oxide and into the underlying metal. The rate of D absorption estimated from the NR data showed a pattern with four distinctive regions separated by potential boundaries between −0.35 and −0.4 V SCE and around ∼−0.6 V SCE . EIS results support division of the behavior into these potential ranges. Hydrogen absorption by Ti was observed at potentials <∼−0.35 V SCE , where the capacitance and resistance of the TiO 2 layer dramatically changed. At this point, the D content of the film quickly achieved a level of ∼900 ppm by weight (atom ratio D:Ti ∼ 0.04). Decreased absorption kinetics were observed over the potential region from ∼−0.40 V SCE to −0.6 V SCE , indicating that D absorption was controlled either by a diffusion process through the TiO 2 layer or by the formation of blocking hydrides at the Ti/TiO 2 interface, at the base of the defective locations in the oxide through which the hydrogen was entering. Significant increases in the current density and SLD of the Ti film at potentials more negative than −0.6 V SCE were assigned to widespread hydrogen absorption and TiH x growth within the metal. These observations are consistent with hydrogen ingress through the oxide film, probably via weak points containing electronic defects and disorder, such as grain boundaries and triple points, at potentials as mild as ∼−0.4 V SCE , and with hydrogen penetration through continuous, intact oxide via the previously published redox transformation mechanism, at potentials more negative than −0.6 V SCE . Titanium corrosion processes are often accompanied by hydrogen production. For example, 80% or more of the crevice corrosion process on Ti is driven by the reduction of protons inside the crevice:This leads to the absorption of atomic hydrogen, and can result in extensive hydride formation. 1,2 Cathodic polarization and galvanic coupling with active metals can also cause hydrogen evolution on the Ti surface and hydrogen penetration into the bulk metal. Since hydrogen absorption into Ti structures can lead to their failure by brittle cracking, once the hydrogen concentration achieves a critical level, 3 safe operation of such structures requires a detailed knowledge of the mechanism of hydrogen entry into Ti so that exposure conditions that put the structure at risk of hydrogen-induced cracking can be avoided. A number of studies have been done on the physicochemical properties of Ti/H systems, as well as on hydrogen-induced cracking of Ti.3,4 Here we explore the role of the electrochemical potential in determining whether adsorbed hydrogen atoms generated by water reduction can penetrate the native oxide on Ti.The surface oxide layer play...

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