Materials used in utilizing geothermal energy can be subjected to corrosion and scaling due to elevated temperature and fluid chemistry in the systems. This can result in high costs associated with materials and decreased efficiency in the production of geothermal power. Carbon steel materials are often used in heat exchangers due to good heat transfer properties but have poor anti-corrosion and -scaling properties. Thus, corrosion-resistant alloys with less favorable heat transfer properties but better anti-corrosion and -scaling properties are commonly used in geothermal power production. In this study coatings developed in the GeoHex project to enhance corrosion, scaling and heat transfer performance were tested in experimental Organic Rankin Cycle (ORC) heat-exchanger (HE) test equipment to evaluate the anti-corrosion and -scaling properties in heat transfer experiments. Different types of coatings developed for simulated brine and working fluid sides of the ORC HE were tested at RT to 80°C in hot brine with a low temperature boiling working fluid. These included electroless nickel coating, and amorphous metal coatings. The microstructure and chemical composition of the coatings were analysed before and after the tests in hot brine with Scanning Electron Microscope and X-Ray Energy Dispersive Spectroscopy to evaluate the durability of the coatings. The morphology and hydrophobic properties (wettability) were also examined with roughness and water contact angle (WCA) measurements. The coatings were observed to increase the WCA in most of the coatings, however, higher angles were measured after the LT test influenced by the morphology of the coatings after the exposure. The findings showed that the Ni-P-PTFE samples were highly hydrophobic and had better corrosion properties in hot brine compared to the Amorphous coatings due to cracks and peeling of the coating peeling in the Amorphous samples. The Ni-P-PTFE coating was concluded to be the most promising Geo-Hex coating material candidate tested in hot brine in the simulated ORC-HE environment.

Subject
Scanning electron microscopy, Nickel coatings, Test methods, Steel surfaces, Geothermal energy, Coatings, Materials, Substrates, Roughness, Substrate surfaces, Brines, Deposit corrosion, Samples
Keywords:
coatings, corrosion testing, ORC heat exchanger, geothermal utilization
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2023
Association for Materials Protection and Performance (AMPP)
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