Browsing by Author "Pidcock, Andrew"
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Item Open Access Assessment of Coating Performance on Waterwalls and Superheaters in a Pulverised Fuel-Fired Power Station(Springer, 2017-01-11) Simms, Nigel J.; Seraffon, Maud; Pidcock, Andrew; Davis, ColinProtective coatings offer one route to increase the lives of heat exchangers in pulverised fuel power plants. A range of candidate coatings have been exposed on the waterwall and superheaters of a 500 MWe UK power station unit for periods of up to ~4 years (24,880 operating hours), during which time this unit was fired on a mixture of UK and world-traded coals. Both nickel- and iron-based candidate coatings were included, applied using high velocity oxy-fuel or arc-wire process; a selection of these also had a surface sealant applied to investigate its effectiveness. Dimensional metrology was used to evaluate coating performances, with SEM/EDX examinations used to investigate the various degradation mechanisms found. Both the waterwall and superheater environments generated their characteristic corrosion damage morphologies which depended on the radial positions around the tube. Coating performances were found to depend on the initial coating quality rather than composition, and were not improved by the use of a sealant.Item Open Access Control of Magnesium Alloy Corrosion through the Use of Engineered Intermetallics(Cranfield University, 2014-12) Pidcock, Andrew; Robinson, M. J.; Impey, Susan A.The low density and high relative strength of Mg alloys means they can offer engineering benefits over steels or Al alloys. However, the susceptibility of Mg alloys to corrosion has limited their exploitation and restricted their use to more benign environments. An Mg-Al intermetallic surface layer is a good candidate for a robust corrosion protection method. This work demonstrates their development by using a novel ionic liquid electroplating process to deposit Al on to Mg substrates that when heat treated diffuses to form discrete intermetallic layers. Examination of three Mg-Al-Zn alloys showed that the amount Mg-Al intermetallic phases in their microstructures was linked to the quantity of Al they contained. Subsequent self-corrosion measurements using electrochemical impedance spectroscopy demonstrated that their performance was connected to the amount of intermetallic present, and in particular the strength of the micro-galvanic couples generated between the anodic and cathodic phases. Measurements of the self-corrosion behaviour of manufactured samples of the Mg-Al intermetallics confirmed that they could provide significant improvements, but it was acknowledged that their noble nature compared to an Mg substrate would encourage galvanic corrosion if a surface layer was damaged. As such the galvanic activity of the Mg-Al-Zn alloys and Mg-Al intermetallics was compared against a pure Mg standard using zero resistance ammetry and the resistance box technique. Galvanic models of alloy self-corrosion and a damaged intermetallic surface layer were also used to assess the potential problem. These measurements demonstrated that the intermetallics could act as strong cathodes, but further discussion on the nature of the behaviour suggested means by which galvanic corrosion might self-limit or self-repair. The galvanic corrosion experiments also revealed how the combination of current flow and a solution saturated with Mg2+ ions could lead to the formation of a highly protective Mg(OH)2 film with promising characteristics.Item Open Access Protection of magnesium alloys from corrosion using magnesium rich surfaces.(Cranfield University, 2021-06) Wang, Yading; Impey, Susan A.; Pidcock, AndrewMg alloys have great potential in engineering applications for saving energy consumption due to the high strength to weight ratio. Mg alloys are also biocompatible and biodegradable with biomedical applications such as orthopaedic and vascular implants. Controlling the corrosion of Mg alloy components is necessary to sustain their performance over the design lifespan. A low corrosion rate is also preferred for implants to mitigate negative effects such as hydrogen evolution during corrosion. Surface films can be used to control the corrosion of an Mg alloy effectively. In this work, Mg(OH)₂ coatings were deposited on Mg alloy substrates (AZ31 and ZM21) by hydrothermal (H.T.) steam treatment as a benchmark and subsequently by novel processing using electrochemical (E.C.) and additive treatment with an Mg²⁺ rich solution. The microstructures and compositions of the alloys are characterised both with and without coatings. Corrosion tests were conducted in various test solutions, including 3.5% NaCl, 0.9% NaCl and Hank's solutions. Electrochemical techniques and mass change measurement are used for the corrosion testing of initial exposure and longer-term immersion, respectively. The processing parameters of the electrochemical and additive methods were optimised based on the corrosion behaviour of the coated samples. This research shows that the Mg(OH)₂ based coating can enhance the corrosion protection of the Mg alloy substrates, with at least a 3 fold reduction in corrosion rates compared to uncoated substrates. Comparing hydrothermal coatings, the electrochemical and additive (EC+Additive) coatings not only show similar corrosion performance but also greater manufacturing flexibility and repairability with potential for further enhancement.