# Corrosion and Scaling in Geothermal Heat Exchangers
## 6 Degradation Mitigation for Heat Exchangers
### Protective Coating Systems
To address this need, the study explored a range of new thermosetting and thermoplastic paint systems designed for exposure to geothermal fluid at 160 \({}^{\circ}\)C and 20 bar for 6 months, including an epoxy mixed with PTFE. However, the paper did not report the results of these tests. Metallic coatings, particularly electroless nickel plating-based systems, are intensively investigated for geothermal applications. This process entails the chemical deposition of a nickel-phosphorus alloy onto a substrate, offering a cost-effective solution that does not require a power supply, unlike electrodeposition. Studies have shown that Ni, Ni-P, and Ni-W-P coatings effectively reduce calcium carbonate scaling when compared to surfaces such as PVC, CS, 316 stainless steel, and copper in heat exchangers [95; 96]. Interestingly, Ni-based coatings predominantly exhibited aragonite scaling, while non-Ni surfaces were primarily covered in calcite. [PERSON] et al. [73] explored the impact of a Ni-W-P coating on fouling formation in a heat exchanger, reporting a significant reduction in fouling. Exposure  
Figure 3: Polymeric coating of an ORC heat exchanger at the Soultz-sous-Forets (France) geothermal plant: (**a**) ORC evaporator front after water jetting; (**b**) coating blistering; (**c**) coating spalling. Reproduced with permission from the copyright holder [94]. to boiling tap water for 24 h resulted in a threefold reduction in fouling, attributed to the role of corrosion products and features (e.g., cracks and pits) as fouling catalysts. The presence of the coating effectively mitigated corrosion, subsequently reducing fouling. In this study, agronomic preferentially deposited on uncoated mild steel, whereas calcite dominated on Ni-W-P-coated steel, contrasting with previous observations [95; 96], necessitating further investigation. The addition of PTFE to the Ni-P system enhances corrosion and scaling protection by increasing hydrophobicity. [PERSON] et al. [97] assessed electroless plated Ni-P/Ni-P-PTFE coatings on CS for safeguarding geothermal heat exchangers. In their study, conducted by the Geo-Coat consortium, the corrosion resistance of these coatings was evaluated in a simulated geothermal environment using electrochemical tests at 25 \({}^{\circ}\)C in a 3.5 wt.% NaCl solution with a constant pH of 4. The results indicated that Ni-P/Ni-P-PTFE coatings exhibited exceptional performance, displaying the lowest corrosion rates compared to all other coatings tested. Furthermore, their corrosion rate was comparable to that of the 254 SMO bulk alloy. However, the promising performance observed in the controlled environment did not align with the outcomes of exposure tests conducted in real geothermal environments, as later reported by the same research group requiring further investigation. [PERSON] et al. [74] investigated a Ni-Cu-P-PTFE coating system applied to CS. They identified an optimum PTFE loading of 12 mL L \({}^{-1}\), beyond which pore content increased due to the presence of PTFE particles, outweighing the hydrophobicity benefits. Pores in the coating were shown to decrease thermal conductivity, mechanical properties (microhardness and elasticity modulus), and corrosion resistance. While the corrosion rate of a CS heat exchanger coated with the Ni-Cu-P-PTFE system decreased from 1 mm year\({}^{-1}\) to 0.16 mm year\({}^{-1}\) in 10% HCl, no information was provided regarding localized corrosion resistance. Thus, this coating requires further testing in geothermal environments. Ceramic coatings are commonly used for their chemical stability and good mechanical performance. They can serve as fillers in organic or metallic coatings or act as the primary matrix. Various ceramic fillers, such as Al\({}_{2}\)O\({}_{3}\)[98], TiO\({}_{2}\)[99], SiC [100], and ZrO\({}_{2}\)[101], have been used to enhance wear and corrosion resistance in coating systems. These insoluble particles play a crucial role in reducing corrosion-induced damage by impeding the propagation of corrosion features [67].