Non-Linear Effects of Solution Parameters and Gamma Radiation on Nickel Oxidation Dynamics

Nickel is the main component in nickel-based superalloys which are known for their superior corrosion resistance and mechanical properties. These alloys are used in nuclear power plants, mainly for thin-walled heat exchanger tubing where these materials are exposed to a continuous flux of ionizing radiation. The long-term corrosion behaviour of these alloys at high temperatures and in the presence of gamma-radiation is not well understood. Moreover, the mechanism by which nickel improves the corrosion resistance of these alloys is also not known. Therefore, a mechanistic understanding of the nickel oxidation process is required to develop a predictive corrosion model for nickel-containing alloys. This thesis presents a systematic study of the corrosion dynamics of pure nickel in different solution environments. Aqueous corrosion is an electrochemical process involving many elementary steps, which include the interfacial transfer of electrons and metal atoms, solution reactions of metal cations (hydrolysis), mass transport processes, and precipitation of metal oxide. The kinetics of the elementary reactions are strongly coupled, which leads to systemic feedback between different elementary steps. To predict the long-term corrosion behaviour of nickel-containing alloys, it is important to identify the key elementary processes that control the overall corrosion rate. In this thesis, the effect of gamma-radiation and solution environment (temperature, pH, ionic strength) on the short-term and long-term corrosion behaviour of pure nickel is investigated. A combination of electrochemical techniques and coupon-exposure tests combined with post-test analyses is used to study the nickel oxidation process. The findings of this work indicate that the overall nickel oxidation dynamics evolve through different phases as corrosion progresses. The elementary steps that control the overall metal oxidation rate in each phase were identified. Phase I involves net electron/metal atom transfer at the metal-solution interface (Ni0(m) ⇌ Ni2+(solv) + 2 e-), diffusion of Ni2+ from the interfacial region into the bulk solution, and hydrolysis of Ni2+ to produce Ni(OH)2, which can condense as colloids. In Phase II, in addition to the elementary steps that comprise Phase I, additional elementary steps involving the Ni2+ hydrolysis product Ni(OH)2, and interfacial electron transfer (Ni2+/Ni(OH)2 + OH- ⇌ Ni(OH)3 + e-) occur that accelerate the precipitation of mixed NiII/NiIII hydroxide, growing a hydrogel network. Phase III occurs after sufficient mixed NiII/NiIII hydroxide colloids have precipitated to allow nucleation and growth as solid crystalline particles on the surface. This study also reveals the non-linear nature of nickel corrosion dynamics over long time periods. It is the combination of solution parameters that dictates how fast the system can transition through different dynamic phases, and which final dynamic phase the overall metal oxidation process can reach. The kinetics of the elementary steps are strongly coupled and describing the overall metal oxidation dynamics based on linear dynamics is not valid. The mechanism proposed in this thesis deviates from the corrosion mechanisms that underpin most existing corrosion rate models. The new mechanism presented here is capable of accounting for many phenomena observed (in relation to the effects of solution parameters on nickel corrosion) that are not explainable using existing mechanistic models. The proposed mechanism can also explain the corrosion behaviour of nickel in the presence of gamma-radiation. From the time-dependent electrochemical studies, different metal oxidation rate parameters can be extracted which are required to develop a predictive corrosion model for nickel-containing alloys