| dc.description.abstract | Nickel (Ni) being a member of the transition metal series, possess excellent corrosion-resistant properties that it has been widely used as key alloying elements for stainless fabrication as well as other nickel-based alloys with high corrosion and temperature resistance. Additionally, as a group 10 metal, it has the comparable catalytic performance to the more rare and precious Palladium (Pd) and Platinum (Pt). Due to its abundance by weight in the earth crust and readiness to be mined and refined, Ni becomes a low-cost substitution catalyst for many catalytic reactions such as hydrolysis, cross-coupling chemical reactions, and reforming reactions. Because of all these mentionings above, Ni is frequently exposed to the aqueous environment that contains hydrogen. Ni as a cathode under cathodic charging is a very typical situation and gradually drawing more and more attention and interest to its research during the recent decades.
In this dissertation, the goal of the research presented here is to investigate the structure and properties of Ni surface under cathodic charging as there was remarkably little information about the surface on Ni under cathodic charging was known from previous studies. Only XRD and SEM observations were commonly conducted by previous investigations, not to mention that the XRD detection range is also relatively narrow that surface changes fall outside this range may be overlooked. Therefore, in the first part of the dissertation, more advanced, thin-film specialized 2D-XRD measurements are conducted, along with a wide range of surface characterization techniques. As a result, the surface phases that are rich in Ni-O-S-H have been successfully observed. These phases partially cover the Ni surface with ~150 nm thickness and have surface potential that is cathodic compared to the surrounding Ni surface. These phases are proposed to be the decay product of the oxidation reactions of some Ni surface hydride with the electrolyte. Such unique crystallographic structure, electrochemical and mechanical properties are highly likely to cause various degradation and performance breakdowns such as localized corrosion, electrode deactivation, and hydrogen embrittlement.
Following the findings of the first part, hydrogen concentration measurements of cathodically charged Ni are conducted as an indirect way to infer the surface hydrogen concentration and seek the existence of surface hydride. It shows that the amount of hydrogen that can diffuse into the Ni is not governed by electrochemical parameters such as overpotential or current. In fact, it is surface H coverage dependent. Additionally, the un-restricted current density at full hydrogen surface coverage is found to be related to the switching of the hydrogen evolution reaction pathway. The full hydrogen coverage at the Ni surface under cathodic charging provides necessary but not sufficient conditions for the surface hydride formation.
Finally, the third part of the thesis demonstrates observation of localized corrosion pitting that initiated on Ni surface under cathodic charging. These localized corrosion features are found to occur when surface hydrogen, as well as surface phases introduced in the first part, reach their full coverage. The change of semiconducting behavior and existence of Aluminum impurity inclusions that are preferentially located inside the corrosion pittings are hypotheses to be related to the surface passive film breakdown and corrosion initiation. | en |
