The research reported in this doctoral thesis involves constructing physiochemical models that reproduce the transport behaviour of aqueous chemical species present in environments relevant to the oil and gas industry to gain an improved insight into the local electrochemistry near the electroactive surface (uniform corrosion) or inside the pit (pitting corrosion). The first part of the project involved constructing physiochemical models with one dimensional geometry with aqueous chemical species and chemical and electrochemical processes observed in oxygen (O2) containing brine environments to determine the changes in the local electrolyte composition and the potential within an initiated pit for a variety of external physical and chemical conditions. It was determined that the bottom of the pit suffers greatly from the effects of iR drop (Ohmic drop) if the pit geometry is taken to be macroscopic. The model was extended to include additional aqueous chemical species in conjunction with the chemical and electrochemical processes observed in carbon dioxide (CO2) rich environment to investigate the effects of CO2 on the local electrolyte chemistry at the bottom of the pit. It was found that the proton reduction electrochemical process on its own was incapable of supplying the high currents experimentally measured in CO2 environments via the buffering effect. The second part of the project was to investigate the influence of different experimental conditions on the polarisation behaviour of near static carbon steels in CO2 saturated brine electrolyte via multiple electrochemical measurement techniques. The key observation from this study was the presence of two distinct mass transport limited regions on the cathodic polarisation curve at natural pH (3.775). From the physiochemical model fitted to the experimental cathodic curve, the first mass transport limited region, occurring at lower cathodic potentials, was identified to be the direct reduction of carbonic acid while the second wave, occurring at slightly higher cathodic potentials, was shown to be the direct reduction of aqueous carbon dioxide. Based on the polarisation scans under forced convection, the rate of the direct reduction of carbon dioxide was determined to be under neither potential nor mass transport control. The third part of the project involved extending the existing one dimensional models to include the precipitation of salt films (iron chloride – FeCl2(s) and iron carbonate – FeCO3(s)) in O2 and CO2 saturated brine electrolyte respectively along with the capability to track their respective thickness. Furthermore, the ability of the underlying metal to undergo a change in its state from active to passive is implemented in the model via a set of rules based on the Pourbaix diagram. It was determined that the precipitation of salt films is greatly influenced by the mass transport with no or minimal thickness observed under even natural convection conditions. Furthermore the successful precipitation of salt film was determined to be a precursor step to the metal attaining passivation.

The need to inject treated seawater to enhance reservoir pressure and secondary oil recovery is increasing in the oil field, so also is the reservoir souring potential caused by the activities of Sulphate Reducing Bacteria (SRB) generating H2S in the reservoir. The total cost of SRB mediated corrosion in the United States alone is estimated to be 1-2 billion US dollars per year. In the last few years, a number of potential souring mitigation and prevention tools have been studied. These include: sulphate-reduction using membranes, biocide injection and nitrate injection. Out of all the various methods used for the mitigation and prevention of reservoir souring, the use of nitrate injection in conjunction with waterflood projects is becoming more popular because of its economic benefits and least environmental impact. However, nitrate injection is still widely considered as an emergent technology because there are still many unknowns. One of the major unknowns, of great concern is the susceptibility of subsea hardware components to nitrite, which is a by-product of nitrate anti-souring treatment. Any detrimental effect can compromise the technical integrity of subsea installations. The objective of this research is to study the corrosion susceptibility of CRA (13Cr- Martensitic, 22Cr, and 25Cr super duplex stainless steel) to pitting and stress corrosion cracking in the presence of nitrite. Research hitherto, has investigated corrosion susceptibility of carbon steel to nitrite and found out that nitrite causes pitting in carbon steel. This research work built on previous studies and extensively investigated the effect of nitrite on CRA materials in terms of pitting and stress corrosion cracking. Using electrochemistry techniques in conjunction with C-ring test and slow strain rate test, with variables such as temperature, and nitrite concentration all under anaerobic conditions. Metallographic examination and further evaluation using scanning electron microscopy confirmed pitting and intergranular stress corrosion cracking of 13Cr-L80 and 25Cr due to presence of nitrite.Test data confirmed that sodium nitrite is an anodic inhibitor; it shifts the corrosion potentials to more noble potential and also shifts the anodic curve to lower current, given a net reduction in corrosion rate. A critical concentration of 400ppm is required for inhibition to be effective on 13Cr-L80 and 25Cr. However, below the critical concentration, nitrite significantly increases the corrosion rate. The experimental data generated from this research work provides very valuable information that will tremendously assist the materials selection process for subsea and subsurface hardware components and also serve as a guide in the corrosion management process in existing systems.

The localised corrosion behaviour of various grades of Austenitic Stainless Steels has been demonstrated by optical and electron microscopy. The effect of sensitisation upon subsequent corrosive attack has been investigated. A theoretical model based upon thermodynamic and kinetic considerations has been proposed to account for the observed experimental results. / Thesis / Master of Engineering (ME)

Cette thèse a porté sur la conception d'un capteur de corrosion passif, sans contact de moyenne portée. Les solutions existantes sans fil concernent soit des capteurs à architecture classique, soit des solutions passives. Dans le premier cas, le capteur de corrosion est actif et peut être interrogé à longue portée. Dans le second cas, les solutions passives existantes ne fonctionnent qu’avec des distances de lecture de quelques centimètres du fait des basses fréquences utilisées. L’objectif de ce travail était de répondre à ce besoin. Pour cela, nous nous sommes inspirés de la technologie RFID (Radio Frequency Identification) passive chipless pour le développement d’un capteur basé sur une fonction hyperfréquence. La première structure réalisée a été une ligne microruban, dont le ruban est constitué d’une couche mince d’un élément sensible à la corrosion. Dans ce cas, la corrosion de la ligne s’est traduite par une variation d’amplitude du fait de l’apparition de pertes expliquées principalement par l’effet de peau ou la création de défauts. Une deuxième structure hyperfréquence a été élaborée en se basant sur un stub (circuit ouvert) qui a permis de suivre le processus de la corrosion via une variation de fréquence. Comme le cas de la ligne microruban, cette structure nous a permis de distinguer la corrosion uniforme et la corrosion localisée. Grâce à la mise en évidence de ces fonctionnalités, diverses stratégies de contrôle de la corrosion peuvent être imaginées et un démonstrateur a été réalisé. Le point clé du démonstrateur proposé est une augmentation de la distance de lecture dans la technologie RFID chipless, ceci a été rendu possible en considérant l’isolation Tx / Rx du lecteur. Dans ce contexte, trois types d’antennes ont été étudiées. Afin d’augmenter encore la distance de lecture, d’autres techniques d’isolation ont été proposées : l’utilisation d’un déphaseur mais aussi l’isolation temporelle par l’utilisation d’une ligne à retard SAW. Grâce à ces 2 méthodes, une distance de lecture de deux mètres a été obtenue. Suite aux caractérisations RF des métaux soumis à la corrosion discutées, nous avons aussi élaboré une sonde RF à champ proche permettant de diagnostiquer la corrosion de surfaces métalliques. / This thesis focused on the design of a passive wireless corrosion sensor. Existing wireless solutions concern either classic architecture sensors or passive solutions. In the first case, the corrosion sensor is active and can be interrogated at long range. In the second case, the existing passive solutions only work with reading distances of a few centimeters because of the low frequencies. The objective of this study was to respond to this need. That’s why; we were inspired by the RFID( Radio Frequency Identification) passive chipless technology for the development of a sensor based on a microwave function. The first structure was a microstrip line, of which the strip is composed of a thin layer of an element sensitive to corrosion. In this case, the corrosion of the line is proven by an amplitude variation due to losses principally explained by the skin effect or the creation of defects. A second microwave structure was developed based on a stub (open circuit) which allowed us to follow the process of corrosion via a frequency variation. Much like the microstrip line, this structure allowed us to distinguish between uniform corrosion and localised corrosion. Due to the highlighting of these features, different corrosion control strategies can be imagined and a demonstrator was executed. The key point of the proposed demonstrator is an increase in the reading distance in the chipless RFID technology; this was made possible by taking into consideration the isolation TX / Rx of the reader. In this context, three types of antennas were studied. In order to increase the reading distance, other isolation techniques were proposed: the use of a phase shifter but also a temporal isolation using a SAW delay line. Thanks to these two methods, a reading distance of two meters was obtained. Following the RF characterizations of metals exposed to the discussed corrosion, we also developed a near field RF probe, which allows corrosion diagnostic of metal surfaces.

The majority of UK’s intermediate level radioactive waste is currently stored in 316L and 304L austenitic stainless steel containers in interim storage facilities for permanent disposal until a geological disposal facility has become available. The structural integrity of stainless steel canisters is required to persevere against environmental degradation for up to 500 years to assure a safe storage and disposal scheme. Hitherto existing severe localised corrosion observances on real waste storage containers after 10 years of exposure to an ambient atmosphere in an in-land warehouse in Culham at Oxfordshire, however, questioned the likelihood occurrence of stress corrosion cracking that may harm the canister’s functionality during long-term storage. The more corrosion resistant duplex stainless steel grade 2205, therefore, has been started to be manufactured as a replacement for the austenitic grades. Over decades, the threshold stress corrosion cracking temperature of austenitic stainless steels has been believed to be 50-60°C, but lab- and field-based research has shown that 304L and 316L may suffer from atmospheric stress corrosion cracking at ambient temperatures. Such an issue has not been reported to occur for the 2205 duplex steel, and its atmospheric stress corrosion cracking behaviour at low temperatures (40-50°C) has been sparsely studied which requires detailed investigations in this respect. Low temperature atmospheric stress corrosion cracking investigations on 2205 duplex stainless steel formed the framework of this PhD thesis with respect to the waste storage context. Long-term surface magnesium chloride deposition exposures at 50°C and 30% relative humidity for up to 15 months exhibited the occurrence of stress corrosion cracks, showing stress corrosion susceptibility of 2205 duplex stainless steel at 50°C.The amount of cold work increased the cracking susceptibility, with bending deformation being the most critical type of deformation mode among tensile and rolling type of cold work. The orientation of the microstructure deformation direction, i.e. whether the deformation occurred in transverse or rolling direction, played vital role in corrosion and cracking behaviour, as such that bending in transverse direction showed almost 3-times larger corrosion and stress corrosion cracking propensity. Welding simulation treatments by ageing processes at 750°C and 475°C exhibited substantial influences on the corrosion properties. It was shown that sensitisation ageing at 750°C can render the material enhanced susceptible to stress corrosion cracking at even low chloride deposition densities of ≤145 µm/cm². However, it could be shown that short-term heat treatments at 475°C can decrease corrosion and stress corrosion cracking susceptibility which may be used to improve the materials performance. Mechanistic understanding of stress corrosion cracking phenomena in light of a comprehensive microstructure characterisation was the main focus of this thesis.