# Corrosion and Scaling in Geothermal Heat Exchangers
## 5 Scaling  
The economic viability of geothermal plants is frequently challenged by the precipitation of mineral phases, commonly known as scales, from geothermal fluids [7; 43]. These scale deposits are not to be confused with those formed by corrosion products. In geothermal environments, the most prevalent scale deposits include silica [44], silicates [45], carbonates [46; 47; 48], sulfates [49], sulphides [50; 51], as well as native metals like arsenic (As) [52], lead (Pd) [53], and antimony (Sb) [51]. Scaling occurs due to changes in geothermal fluid thermodynamic properties, such as temperature, pressure, and pH, during the energy production process. Heat exchangers, where fluid temperatures drop significantly, are particularly susceptible to scale formation, as shown in Figure 2. This presents a major challenge as scales act as thermal insulators [54]. Furthermore, scale buildup can reduce effective tube diameters or completely block them, diminishing plant productivity [12]. Scales can also lead to the accumulation of toxic chemicals, introducing additional risks and cost challenges during the operational phase. Notably, the presence of Ra 226 and Pb 210 radionuclides in some scale products classifies them as Naturally Occurring Radioactive Material waste (NORM classification), increasing disposal costs [55]. Additionally, scale formation significantly impacts the corrosion of surface installations in geothermal power plants [11]. Regular maintenance operations are necessary to remove scales, often requiring manual hydrol blasting to clean tubes [54]. For instance, the geothermal plant at Sultz-sous-Forets needs to be stopped three times a year to perform scale cleaning operations [56]. The introduction of oxygen during this process can lead to corrosion issues [26]. Silica scale (SiO\({}_{2}\)) is a common product in geothermal plants due to its abundance in the rock reservoir and high solubility at elevated temperatures [51]. Silica precipitation is primarily influenced by temperature changes reducing solubility or an increase in silica concentration due to partial fluid flashing [50]. Calcium carbonate (CaCO\({}_{3}\)) such as calcite and aragonite, is the predominant carbonate deposit in geothermal plants. The solubility of calcium depends on temperature and CO\({}_{2}\) pressure [51], resulting in calcite deposits near the fluid flashing point or in heat exchangers. Metallic sulphide scaling often occurs in volcanic or high Cl\({}^{-}\) containing environments, such as Reykjanes (Iceland) [57] or Salton Sea (California) [18], due to the interaction between metals and H\({}_{2}\)S present in the geothermal fluid. Common sulphide scales in geothermal environments include bornite (Cu5 Fe54), chalcogenite (Cu2S), barite (BaSO4), galena (PbS), sphalerite (ZnS), and pyrite (Fe5\({}_{2}\)) [51; 53]. In geothermal power plants, pressure primarily diminishes during the vertical transfer of fluids from the reservoir to the surface. Within tube and shell heat exchangers, the pressure is typically maintained between 18 and 25 to avoid detrimental degassing [59]. The pressure drop across the heat exchanger is influenced by the presence of rough scale deposits, leading to drag effects, yet it remains constrained within the range of approximately 3.5-5 bar [54]. Under these conditions, temperature predominantly governs mineral solubility, with pressure exerting a secondary influence. Nevertheless, pressure significantly impacts gas solubility, and any alterations in pressure conditions can result in CO2 degassing. CO2 degassing is highly undesirable since it raises acidity levels by forming carbonic acid and promotes precipitation of calcite [58]. To mitigate CO2, it is advisable to operate geothermal systems at higher pressures (18-25 bar). However, turbulence-induced localized pressure fluctuations can still cause degassing and exacerbate calcite precipitation, as reported by [PERSON] et al. [58].