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Proceedings Papers
ITSC2016, Thermal Spray 2016: Proceedings from the International Thermal Spray Conference, 654-659, May 10–12, 2016,
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This work assesses the behavior of thermally sprayed corrosion-resistant alloy (CRA) coatings in an aqueous solution containing supercritical CO 2 . 316L stainless, Ti, alloy 625, and alloy C-276 powders were sprayed on carbon steel using a HVOF torch and 8 mm holidays were drilled in the coatings to expose the substrate. The samples were divided into two sets and placed in autoclaves for 30 days, where they were exposed to a salt solution, bubbled with 10 MPa CO 2 , at temperatures of 40 and 80 °C. Sample cross-sections showed that wherever the coating was intact, it protected the substrate from CO 2 corrosion, but in holiday regions, where bare steel was exposed, a siderite scale had formed and severe undercutting occurred, possibly due to galvanic interactions with the CRA coating.
Proceedings Papers
ITSC2016, Thermal Spray 2016: Proceedings from the International Thermal Spray Conference, 933-938, May 10–12, 2016,
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Thermally sprayed aluminum (TSA) coatings have been successfully used to mitigate corrosion of carbon steel in offshore service, but concerns regarding its suitability in CO 2 -containing solutions have kept it out of the running for emerging carbon capture and storage applications. This paper presents the results of a 30-day test in which carbon steel specimens protected by TSA coatings were immersed in deionized water at ambient temperature in 0.1 MPa CO 2 . Acidity and corrosion potential were monitored during the test and dissolved Al 3+ ion content was analyzed at the completion. Based on experimental results, thermally sprayed aluminum is a viable candidate for corrosion mitigation in CO 2 -containing water as would be encountered in carbon capture and storage applications.
Proceedings Papers
ITSC2016, Thermal Spray 2016: Proceedings from the International Thermal Spray Conference, 939-945, May 10–12, 2016,
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Commercially pure aluminum was arc sprayed on low-carbon manganese steel and electrochemical impedance spectroscopy (EIS) was carried on coating samples in a simulated marine immersion environment at 35 °C. A simple pore network circuit model was used to analyze the data and calculate the corrosion rate, which was estimated to be 5-15 μm/y from the charge transfer resistance value. After 9 months of exposure, the actual corrosion rate was found to be ~5 μm/y. The mechanism of protection offered by thermally sprayed aluminum (TSA) coatings is discussed.
Proceedings Papers
ITSC2016, Thermal Spray 2016: Proceedings from the International Thermal Spray Conference, 91-96, May 10–12, 2016,
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This study assesses the viability of three suspension spray processes for producing photocatalytic TiO 2 . In the experiments, flame, plasma, and HVOF torches were used to spray TiO 2 suspensions onto stainless steel substrates, varying process parameters in order to gauge their effect on phase composition, crystal size and, in turn, photoactivity. The TiO 2 samples were characterized by means of XRD, SEM, and UV-Vis analysis and photocatalytic hydrogen-production testing. Suspension flame spraying proved to be the most effective method, producing phase-controlled nanostructured titania 32% more photoactive than the SPS samples and up to five times more active than analogous coatings produced by CVD.
Proceedings Papers
ITSC2016, Thermal Spray 2016: Proceedings from the International Thermal Spray Conference, 109-114, May 10–12, 2016,
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Thermally sprayed aluminum (TSA) has been used in offshore applications for decades, protecting steel structures from seawater corrosion. However, very little work is reported on the performance of TSA when damaged, particularly in deep sea applications. This paper presents the results of a study in which an arc-sprayed aluminum-coated steel sample was subjected to synthetic seawater at 5 °C for 30 days in an autoclave at 50 MPa to simulate 5000 m of water pressure. Discontinuities or “holidays” amounting to 3% of the sample area were drilled into the coatings, exposing the underlying steel to direct attack by the synthetic seawater. After testing, SEM and EDX analysis revealed the formation of a protective Mg-based layer on the exposed steel with negligeable calcium content and no visible corrosion products. The results indicate that TSA coatings can protect steel in deep sea environments even when damaged.
Proceedings Papers
ITSC2016, Thermal Spray 2016: Proceedings from the International Thermal Spray Conference, 382-387, May 10–12, 2016,
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This study demonstrates a low-pressure coating (LPC) technique for the deposition of titania films. In this process, compressed air is used to accelerate angular titania feedstock into a substrate such as stainless steel, producing a photoactive surface consisting of embedded titania particles. The method is relatively inexpensive, requiring only the basic materials and an air supply. A range of LPC titania coatings were produced, varying nozzle stand-off distance, air pressure, and substrate exposure time. Samples are characterized via SEM and EDX analysis, photocurrent measurements, and NOX removal testing.
Proceedings Papers
ITSC 2015, Thermal Spray 2015: Proceedings from the International Thermal Spray Conference, 135-141, May 11–14, 2015,
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Thermally sprayed aluminum (TSA) has been used to protect offshore structures for many years at ambient temperatures. However, the melting point of aluminum, and the limited corrosion rate data indicate that they are likely to perform well in high temperature seawater as they tend to form calcareous deposits. Nonetheless, not much is known regarding the protection mechanism offered by TSA at elevated temperatures. This paper reports micro-scale surface characterization data on damaged TSA-coated carbon steel exposed to boiling synthetic seawater. The corrosion rate, calculated using linear polarization resistance (LPR) technique of TSA was ~10µm/year. This is very similar to the TSA corrosion rate observed at room temperature. The formation of a calcareous deposit which covers the exposed steel seems to reduce the corrosion rate. Detailed microstructural characterization of the calcareous was also carried out and the kinetic information regarding the rate of formation of calcareous deposits is also presented.
Proceedings Papers
ITSC 2015, Thermal Spray 2015: Proceedings from the International Thermal Spray Conference, 321-328, May 11–14, 2015,
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The application of aluminum coatings onto steel for corrosion mitigation is governed by standards specifying surface cleanliness and roughness prior to coating, and minimum coating bond strength. Controlling the surface preparation and spray parameters to achieve the specified surface condition and coating bond strength is challenging, particularly for manual on-site work. In this research, the process parameters were varied and the effect on surface quality and coating adhesion determined. It was found that blasting at angles as low as 30° from the surface, and varying stand-off distances up to 100 mm from the optimum, produced an acceptable surface; whilst spray angles of 60° to 90° and stand-off distances up to 50 mm from the optimum produced acceptable coatings with adhesion above 20 MPa. Adhesion appeared unaffected by a limited amount of remaining mill scale, but was reduced to ≈15 MPa when the surface chloride content was increased from 2 to 20 μg cm-2.
Proceedings Papers
ITSC 2015, Thermal Spray 2015: Proceedings from the International Thermal Spray Conference, 964-970, May 11–14, 2015,
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Corrosion rate determination is often based on linear polarization (LPR) technique, carried out on small specimens in a controlled environment. The current is measured when the specimen is polarized away from its corrosion potential and the corrosion current gives the corrosion rate using Faraday's law. An important parameter in this calculation is the specimen area exposed. When testing rough, porous specimens such as thermally sprayed aluminum (TSA), the geometrical surface area does not represent the real surface area and hence the corrosion rate is not the actual corrosion rate. To measure the actual corrosion rate aluminum was thermally sprayed onto glass and the surface was characterized using a confocal microscope. These specimens were exposed to seawater at 25, 50 and 100°C, and the linear polarization resistance was determined using a potentiostat at room temperature. This was converted to a corrosion rate based on geometric and the actual surface area.
Proceedings Papers
ITSC 2015, Thermal Spray 2015: Proceedings from the International Thermal Spray Conference, 1072-1077, May 11–14, 2015,
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This paper reports the use of HVOF-sprayed corrosion resistant alloy (CRA) coatings to protect carbon steel in supercritical CO 2 -containing environments. 316L, Ti, alloy C- 276 and alloy 625 were sprayed onto carbon steel substrate using a JP-5000 HVOF system and tested in 3.5wt.% NaCl solution at 10 MPa in a mixture of 95% CO 2 and 5% H 2 S at 40°C for 30 days. Microstructural characterization revealed that the coating protected the steel substrate from supercritical CO 2 /H 2 S corrosion. The bare steel formed a sulfide scale while no such scale was seen in the case of CRA-coated steel. It can be concluded that thermally sprayed CRA coatings provide a cost-effective corrosion mitigation method for infrastructure likely to be in contact with wet supercritical CO 2 /H 2 S. However, care must be taken to ensure that the thermally sprayed layer does not have any through-thickness porosity; else, such coatings may accelerate corrosion of the underlying steel due to galvanic interactions.
Proceedings Papers
ITSC2014, Thermal Spray 2014: Proceedings from the International Thermal Spray Conference, 830-835, May 21–23, 2014,
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This study assesses the effectiveness of wire arc sprayed aluminum coatings for protecting welded super duplex stainless steel (UNS S32750) in subsea applications. Pitting and crevice corrosion tests were conducted at different potentials in recirculated synthetic seawater maintained at 90 °C with an acidity of 7.5-8.1 pH. After 90 days, the samples were examined, showing no signs of corrosion even in areas where coating defects were present.
Proceedings Papers
ITSC2012, Thermal Spray 2012: Proceedings from the International Thermal Spray Conference, 184-189, May 21–24, 2012,
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This paper reports the corrosion behaviour of coatings deposited by high velocity oxy-fuel (HVOF) spraying and representative boiler substrate alloys in simulated high temperature biomass combustion conditions. Four commercially available oxidation resistant Ni alloy coating materials were selected: NiCrBSiFe, alloy 718, alloy 625 and alloy C-276. These were sprayed onto P91 substrates using a JP5000 spray system. The corrosion performance of the coatings varied when tested at 525, 625 and 725°C in K 2 SO 4 - KCl and gaseous HCl-H 2 O-O 2 containing environments. Alloy 625, NiCrBSiFe and alloy 718 coatings performed better than alloy C-276 coating at 725°C, which had very little corrosion resistance resulting in degradation similar to uncoated P91. Alloy 625 coatings provided good protection from corrosion at 725°C, with the performance being comparable to wrought alloy 625, with significantly less attack of the substrate than uncoated P91. Alloy 625 performs best of these coating materials, with an overall ranking at 725°C as follows: alloy 625 > NiCrBSiFe > alloy 718 >> alloy C-276. Although alloy C-276 coatings performed poorly in the corrosion test environment at ~725°C, at lower temperatures (i.e. below the eutectic temperature of the salt mixture) it outperforms the other coating types studied.
Proceedings Papers
ITSC2012, Thermal Spray 2012: Proceedings from the International Thermal Spray Conference, 544-549, May 21–24, 2012,
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In this paper evaluation of sealed and unsealed thermally sprayed aluminum (TSA) for the protection of 22%Cr duplex stainless steel (DSS) from corrosion in aerated, elevated temperature synthetic seawater is presented. The assessments involved general and pitting corrosion tests, external chloride stress corrosion cracking (SCC), and Hydrogen induced stress cracking (HISC). These tests indicate that DSS samples which would otherwise fail on its own in a few days do not show pitting or fail under chloride SCC and HISC conditions when coated with TSA (with or without a sealant). TSA-coated specimens failed only at very high stresses (>120% proof stress). In general, TSA offered protection to the underlying or exposed steel by cathodically polarizing it and forming a calcareous deposit in synthetic seawater. The morphology of the calcareous deposit was found to be temperature dependent and in general is of duplex nature. The free corrosion rate of TSA in synthetic seawater was measured to be ~5-8 µm/year at ~18°C and ~6-7 µm/year at 80°C.
Proceedings Papers
Effect of Heat Treatment on Pore Architecture and Associated Property Changes in Plasma Sprayed TBCs
ITSC 2007, Thermal Spray 2007: Proceedings from the International Thermal Spray Conference, 411-416, May 14–16, 2007,
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Plasma sprayed Thermal Barrier Coatings (TBCs) exhibit many interlamellar pores, voids and microcracks. These microstructural features are primarily responsible for the low global stiffnesses and the low thermal conductivities commonly exhibited by such coatings. The pore architecture thus has an important influence on such thermophysical properties. In the present work, the effect of heat treatment (at temperatures up to 1400°C, for times of up to 10 hours) on the pore architecture in detached YSZ top coats has been characterised by Mercury Intrusion Porosimetry (MIP) and gas-sorption techniques. While the overall porosity level remained relatively unaffected (at around 10-12%) after the heat treatments concerned, there were substantial changes in the pore size distribution and the (inter-connected) specific surface area. Fine pores (<~50 nm) rapidly disappeared, while the specific surface area dropped dramatically, particularly at high treatment temperatures (~1400°C). These changes are thought to be associated with intra-splat microcrack healing, improved inter-splat bonding and increased contact area, leading to disappearance of much of the fine porosity. These microstructural changes are reflected in sharply increased stiffness and thermal conductivity. Measured thermal conductivity data are compared with predictions from a recently-developed analytical model, using the deduced inter-splat contact area results as input parameters. Good agreement is obtained, suggesting that the model captures the main geometrical effects and the pore size distribution measurements reflect the most significant microstructural changes.
Proceedings Papers
ITSC 2007, Thermal Spray 2007: Proceedings from the International Thermal Spray Conference, 423-427, May 14–16, 2007,
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It was found that the content of impurity oxides in 7YSZ, such as SiO 2 and Al 2 O 3 , has a significant effect on the coating sintering resistance and phase stability of 7YSZ thermal barrier coatings (TBCs). The reduction of the impurity content will significantly improve the sintering resistance and phase stability of 7YSZ TBCs and thus allow the 7YSZ TBCs to be used at higher temperatures.