Wen, Xiaoqi (2026) Biochemical and microbial interactions with advanced coatings in industrial biotechnology applications. University of Southampton, Doctoral Thesis, 256pp.
Abstract
Anaerobic digestion (AD) is a well-established environmental biotechnology. It can recover energy from biogas by-products and simultaneously convert nutrients into digestate. Traditional AD is typically carried out in reactors made from either enamel or epoxy-coated carbon steel or concrete, designed to serve for decades. Separating into the liquid/slurry phase and the gas phase, the liquid conditions in AD contain chemicals and microorganisms involved in the digestion process, while the gas condition could be aggressive with the presence of CO2 and H2S.
Only a few reliable studies have been found on interactions between in-service AD conditions, material performance, and operational lifetimes. Additionally, anaerobic digestion is a complex phenomenon involving physical, chemical, and biological factors. Material tests, however, are generally focused on a single adverse impact/condition. This test is not sufficiently representative of a real industrial practice where multiple adverse impacts may arise from the synergy of physical, chemical, and biological aspects; however, they provide a rapid overview of coating resistance. From the perspective of industrial practice, more reliable data is required to link AD conditions and general material tests and establish a suite of standards and guidance on epoxy coating applications when applied to the AD industry. A combination of test methods involving conventional material tests and AD performance data can make it possible to identify a reliable application standard.
To study the key factors in AD operations affecting metal corrosion and coating degradation and their mechanisms, electrochemical investigation and immersion tests of short term and long term of the targeted materials were conducted. Materials, including carbon steel, fusion-bonded epoxy (FBE) coating and glass-fused-to-steel material, were tested in different conditions. Combined with material morphology characterisation technology, for example scanning electron microscope (SEM), and X-ray diffraction (XRD), the evolution of the material surface and composition of the residue layers of the exposed samples were explored. By electrochemical tests using the conventional three-electrode cell system, the electrochemical behaviour, including electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation curves, of bare carbon steel in the centrifuged supernatant of the digested sludge without dissolved oxygen has been studied. The corrosion kinetics of carbon steel in the deaerated supernatant suggested to be governed by a mixed process and controlled by charge transfer and mass transfer processes. In the supernatant medium, the anodic polarization response exhibited an S-shaped profile, consistent with inhibition by organic acids and/or the formation of surface films (e.g., bicarbonate species). A three-month static immersion test of bare steel and scribed fusion-bonded epoxy panels at room temperature, 35 °C, or 55 °C. Retrieved samples were evaluated by mass change, ex-situ EIS in the centrifuged supernatant, and surface analyses, including SEM and XRD. Corrosion increased with temperature. The shifts in the EIS showed diffusion process in the interfacial process, indicating the growth of residue layers. Corrosion product layers formed at 55 °C were more adhered and compact, and accumulated within the scribed zones of the FBE panels. Bare carbon steel samples, partially damaged coated samples, and perfect coated samples were exposed to both the liquid and headspace phases in the continuously stirred tank reactor, with samples removed after 3 months, 6 months, and 12 months. The H2S concentration in the headspace affected the corrosion rate and mechanisms of corrosion layer formation on the carbon steel. The glass-fused-to-steel showed better physical and chemical resistance than the FBE coating, while the FBE coating showed early deterioration sign when they were exposed in the headspace at higher temperature.
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