Geothermal power is an attractive and environmentally friendly energy source known for its reliability and efficiency. Unlike some renewables like solar and wind, geothermal energy is available consistently, making it valuable for mitigating climate change. Heat exchangers play a crucial role in geothermal power plants, particularly in binary cycle plants, where they represent a significant portion of capital costs. Protecting these components from deterioration is essential for improving plant profitability. Corrosion is a common issue due to direct contact with geothermal fluid, which can lead to heat exchanger failure. Additionally, temperature changes within the heat exchanger can cause scaling, reduce heat transfer efficiency, or even block the tubes. This review critically examines the challenges posed by corrosion and scaling in geothermal heat exchangers, with a primary focus on three key mitigation strategies: the application of corrosion-resistant alloys, the utilization of protective coating systems, and the introduction of anti-scaling agents and corrosion inhibitors into the geothermal fluid. The paper discusses recent strides in these approaches, identifying promising advancements and highlighting impending obstacles. By bridging existing knowledge gaps, this review aims to offer valuable insights into material selection, heat exchanger design, and the progression of geothermal energy production. Ultimately, it contributes to the ongoing endeavor to harness geothermal energy as a sustainable and enduring solution to our energy needs.

Geothermal energy is likely to be a significant contributor in achieving sustainable energy goals and net-zero emissions targets. Within geothermal power plants, heat exchangers play a critical role in harnessing this renewable energy source. However, these heat exchangers encounter significant challenges when exposed to geothermal fluids, including erosion, corrosion, and scaling, which adversely affects their performance and longevity. The current review focuses on surface engineering techniques, particularly coatings, as a highly effective and economically viable solution to address these challenges in geothermal heat exchangers. The review begins by providing an overview of geothermal energy, its significance in the context of sustainability and the important role played by heat exchangers in geothermal power generation, followed by the challenges and their impact on heat exchangers. The subsequent section focuses on surface engineering by coatings and its types employed to enhance the performance of heat exchangers. In the final part, the reader is presented with an overview of the challenges associated with the application of coatings in geothermal heat exchangers and potential future directions in this field. This review offers a detailed understanding of the critical role coatings play in improving the efficiency and service life of heat exchangers in geothermal power plants.

Geothermal energy generation faces challenges in efficiency, partly due to restrictions on reinjection temperatures caused by scaling issues. Therefore, developing strategies to prevent scaling is critical. This study aims to simulate the scaling tendencies and corrosion effects of geothermal fluids on various construction materials used in scaling reactor/retention tank systems. A range of materials, including carbon steel, austenitic stainless steel, duplex stainless steel, two proprietary two-part epoxy coatings, and thermally sprayed aluminium (TSA), were tested in a simulated geothermal brine. Experiments were conducted in a laboratory vessel designed to replicate the wall shear stress conditions expected in a scaling reactor. The tests revealed varying scaling tendencies among the materials, with minimal corrosion observed. The dominant scale formed was calcium carbonate, consistent with geochemical modelling. The findings suggest that despite the high operating temperatures, the risk of corrosion remains low due to the brine’s low chloride content, while the wettability of materials after immersion may serve as a useful indicator for selecting those that promote scaling.

In recent years, there has been growing interest in thermal spray techniques using suspension or solution-based coatings. These techniques offer precise control over particle size and microstructure, improving feedstock flowability and allowing for high-quality coating customization. Spray parameters, such as stand-off distance (SOD) and feedstock flow rate, can alter the performance and characteristics of these coatings. Geothermal power plant heat exchangers often face issues like corrosion, scaling, and fouling. The literature suggests that these issues could be mitigated, at least in part, by the use of spray coatings. In this study, TiO₂ coatings were applied on a carbon steel substrate using suspension plasma spray (SPS) to enhance the performance of geothermal heat exchanger materials. The impact of SOD (50, 75, and 100 mm) and feedstock flow rate (10, 20, and 30 mL/min) on these coatings was examined through various techniques, including scanning electron microscope (SEM), profilometry, X-ray diffraction (XRD), and adhesion testing. The results demonstrated that coatings deposited using a 10 mL/min feedstock flow rate were well adhered to the substrate due to the efficient melting of the coating material, but as the SOD and feedstock flow rate increased due to poor thermal and kinetic energy exchange between the torch and feedstock particles, adhesion between the coating and substrate decreased.

This study investigates the corrosion and scaling behaviour of Extreme High-speed Laser Application (EHLA)-fabricated corrosion-resistant alloy (CRA) claddings under simulated geothermal brine conditions. EHLA 316L stainless steel and alloy 625 coat-ings were produced and tested in simulated brine (Chloride–carbonate–silica geo-thermal brine) at 70°C for 720h to evaluate the influence of additive manufacturing microstructures on corrosion performance. The EHLA coatings exhibited dense, met-allurgically bonded microstructures with minimal porosity. Microstructural analysis revealed Nb- and Mo- rich segregation in EHLA 625 and fine columnar dendritic morphology in all coatings. EHLA 625 also displayed stable passive film formation but had a thin deposit of Mg-O-containing compounds from the brine. EHLA 316L on the other hand exhibited localised pitting and heavy Si- and Mg-containing scale accumu-lation, especially in as-built conditions. Surface finishing reduced corrosion activity by minimizing roughness and defect-driven localised attack. Critical pitting temperature tests confirmed the superior localised corrosion resistance of EHLA 625 compared to EHLA 316L. However, further process improvements and perhaps post-deposition thermal treatments might be required to bring the performance of the coatings in line with the wrought alloys. The results demonstrate that EHLA-fabricated coatings offer corrosion and scaling resistance. This coating technique has the potential to provide cost-effective solutions to produce corrosion resistant coatings on geothermal components.

This study investigates the corrosion and scaling behaviour of Extreme High-speed Laser Application (EHLA)-fabricated corrosion-resistant alloy (CRA) claddings under simulated geothermal brine conditions. EHLA 316L stainless steel and alloy 625 coatings were produced and tested in simulated brine (chloride–carbonate–silica geothermal brine) at 70 °C for 720 h to evaluate the influence of additive manufacturing (AM) microstructures on corrosion performance. The EHLA coatings exhibited dense, metallurgically bonded microstructures with minimal porosity. Microstructural analysis revealed Nb- and Mo-rich segregation in EHLA 625 and fine columnar dendritic morphology in all coatings. EHLA 625 developed a stable passive film with only a thin deposit of Mg-O-containing compounds, whereas EHLA 316L exhibited localised pitting and significant Si- and Mg-containing scale accumulation, especially in as-built conditions. Surface finishing reduced corrosion activity by minimising roughness and defect-driven localised attack. Critical pitting temperature (CPT) tests confirmed the superior localised corrosion resistance of EHLA 625 relative to EHLA 316L under laboratory conditions. While these results indicate promising corrosion and scaling resistance of EHLA coatings, further process optimisation and post-deposition thermal treatments might be required to achieve coating performance comparable to wrought alloys. The results indicate the potential of EHLA-fabricated coatings for producing corrosion and scaling resistance surfaces.