Carbon dioxide (CO2) corrosion is a major problem in the oil and gas production industry. The survival of mild steel equipment is to a large extent conditional on the formation and stamina of protective iron carbonate (FeCO3) films. Damage to protective films allegedly leads to accelerated corrosion attacks and increases the risk of failures. In single-phase flows, film removal phenomena are broadly ascribed to two intrinsic mechanisms: mechanical removal by hydrodynamic forces and/or chemical removal by dissolution. The fact that both mechanisms usually act simultaneously in practice puts their combined action in the forefront regarding its significance and relevance for the industry. Yet, virtually no information is available on the exact conjoint mechanism of protective FeCO3 film removal in single-phase environments. The obscurity is largely due to the uncertainty regarding the roles of hydrodynamic forces and mass transfer, where both are closely related to turbulence intensity levels. The aim of this dissertation was to clarify the roles of the two basic FeCO3 film removal mechanisms during the conjoint removal in undisturbed, single-phase flow in terms of their relative contribution and possible synergistic interaction. The proposed aim was accomplished by applying an innovative analytical approach, in which inherently coupled processes of film formation and removal were decoupled. Also, the two intrinsic removal mechanisms were studied separately in the initial stages, before they were combined to provide a complete picture of the conjoint mechanism. An integrated approach to studying film formation/removal mechanisms involved advanced electrochemical techniques for following film growth/removal, complemented by detailedScanning Electron Microscopy/Energy Dispersive Spectroscopy/X-Ray Mapping characterisations of protective/residual films. A single-phase, highly turbulent flow field was attained by employing a rotating cylinder configuration. A standard corrosion experimental setup was extended to accommodate more complex film studies. A comprehensive flow characterisation around the rotating cylinder was carried out by means of flow visualisation and mass transfer measurements under turbulent flow conditions. While the former facilitated proper design of film formation experiments, the latter led to an empirical mass transfer correlation that enabled quantification of film dissolution rates. Furthermore, although some information on film growth kinetics is available, customised experimentation was necessary to identify the key parameters needed to obtain films with desired characteristics. Sound procedures for FeCO3 film growth were established, which led to the reproducible formation of realistic, protective films after a few days. The results of the pure mechanical removal of protective FeCO3 films have shown that its kinetics are rather slow even at high velocities and have caused a delayed, partial macroscopic type of damage. Yet, the findings demonstrate that the currently widely accepted view, that film removal by hydrodynamic forces in the absence of film dissolution in undisturbed, single-phase flows does not occur, is wrong. The strong correlation found between velocity and pure chemical film removal kinetics implicitly followed via corrosion rates suggests that the dissolution of protective FeCO3 films is under mass transfer control. Pure dissolution has faster removal kinetics and is far more detrimental to film integrity even at relatively high pH (just below saturation) than pure mechanical removal at the same Reynolds number. It has been found that the controlled pure dissolution mechanism led to only partial and selective film removal, where the more dissolution-resistant crystalline top film layer and the dissolution-prone inner layer were differently affected both in terms of the type of damage and its severity. A strong synergistic effect between mechanical and chemical film removal mechanisms has been identified during their simultaneous action. The quantified synergistic share in fully established conjoint film removal (during the steady, linear corrosion rate increase) expressed via corrosion rate gradients increased from 19.4% to 29.7% for the corresponding increase in the rotational speed from 7,000 rpm to 10,000 rpm. The synergism comprised two modes of mutual interactions: enhanced mechanical removal due to dissolution (M/D) and enhanced dissolution due to mechanical removal (D/M). In contrast to the independent action of integral removal mechanisms, where dissolution appears to be more destructive, the interaction between the two was primarily dominated by drastically accelerated mechanical film removal kinetics, that is, M/D rather than D/M mode, the latter of which was inferior. A fundamentally improved understanding of film removal mechanisms in single-phase flows has been reached as a result of the present project, thereby creating a solid foundation for future modelling and a more effective prevention and control of flow accelerated corrosion, not only in CO2 corrosive environments, but also in a wide range of industrial settings.
| Identifer | oai:union.ndltd.org:ADTP/252880 |
| Creators | Ruzic, Vukan |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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