Hydrogen embrittlement is a mechanism by which hydrogen enters a metal, causing a loss in strength and ductility. This phenomenon is of great concern to the oil and gas industry as deep-sea wells operate in high temperature, highly acidic and high stress conditions. Nickel-based superalloys are ideal for use in such environments due to their high strength and exceptional resistance to both corrosion and hydrogen embrittlement. Alloy 945X is a newly developed nickel-based superalloy that has been specifically designed for use in downhole applications. This thesis compares the performance of hydrogenated Alloy 945X with the more established oil and gas grade Alloy 718. The hydrogenating environment of an oil well was simulated via cathodic polarisation. The effect of hydrogen content on the tensile performance of both alloys was studied, alongside fracture and microstructural analysis. A new video-recording technique was employed to investigate the crack initiation and propagation behaviour of both alloys, alongside in-SEM tensile testing. The diffusive nature of hydrogen in Alloy 945X and Alloy 718 was explored. With the use of a ppm-sensitive hydrogen analyser, it was possible to measure the rate at which hydrogen enters and outgassed from both materials as well as the saturation conentrations. Outgassing behaviour was also examined using X-ray diffraction and nano-indentation. The depth of brittle fracture in cathodically charged tensile specimens was correlated with Fick’s diffusion calculations and the critical concentration for embrittlement calculated. In a similar method, a parameter (based on diffusion coefficient calculations) that describes the rate of embrittlement in a material was proposed.

The development of technologies that enable efficient and reliable energy inter-conversion and storage is of key importance for tempering the intermittent availability of renewable energy sources, and thus for developing an energy economy based on sustainable, clean energy production. Solid oxide electrolysis cells (SOECs) may be used to store excess electrical energy as hydrogen, while solid oxide fuel cells (SOFCs) could convert back hydrogen into electricity, thus balancing energy availability and demand. However, the current state-of-the-art hydrogen electrode used in both SOECs and SOFCs, the Ni-yttria-stabilised zirconia cermet (Ni-YSZ), is unreliable in conjunction with intermittent energy sources, in particular due to its innate redox instability. This thesis explores the fundamental properties of various inherently redox stable A-site deficient titanate perovskite systems (A1-αBO3, B = Ti), seeking to uncover the principles that enhance their properties so that they may be used to replace Ni-YSZ. In particular, this work demonstrates that the versatility of perovskites with respect to the introduction of lattice defects such as vacancies and cation substitutions enables considerable improvements in the extent of reduction, electronic conductivity and overall electrochemical activity. Most importantly, the defect chemistry context set by the presence of A-site vacancies was found to trigger the exsolution of electrocatalytically active nanoparticles from the parent perovskite, upon reduction. This is an entirely new phenomenon which was explored and exploited throughout this study to produce perovskite surfaces decorated with uniformly distributed catalytically active nanoparticles. As demonstrated in this study, the exsolution phenomenon excels in terms of producing nanoparticles with uniform size, distribution, diverse composition and ‘unconventional' surface anchorage. The resulting enhanced properties, and especially the exsolution phenomenon, contributed coherently towards improving the suitability of the perovskites developed here towards their application as hydrogen electrode materials. Consequently, when integrated into SOEC button cells as hydrogen electrodes, they exhibited a step-change increase in performance compared to other perovskites considered to date. Many of the principles and perovskite defect chemistry explored and exemplified in this study on perovskite titanates may be extended to other perovskites as well. In particular the advanced control and understanding achieved in this work over the exsolution phenomenon may inspire the formulation of new and sophisticated oxide materials with advanced functionality.

Thesis (M.S.); Submitted to Iowa State Univ., Ames, IA (US); 19 Dec 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 2498" Seymen Murat Aygun. US Department of Energy 12/19/2004. Report is also available in paper and microfiche from NTIS.