# Corrosion and Scaling in Geothermal Heat Exchangers
## 6 Degradation Mitigation for Heat Exchangers
### Anti-Scaling Agents and Corrosion Inhibitors  
Inhibitors for scaling (anti-scaling agents) and corrosion are commonly employed in the oil and gas industry. Anti-scaling agents are classified into two main categories based on their function: inhibitors, which hinder crystallization, and dispersants, which disperse mineral crystals once formed. Inhibitor-type anti-scaling agents, including phosphonates and polycarboxylates [105], are particularly effective in controlling barite formation. On the other hand, dispersant-type anti-scaling agents excel in reducing metal sulphide scaling [56]. Corrosion inhibitors predominantly consist of organic compounds that interfere with either the anodic or cathodic corrosion reactions, forming a protective barrier on metal surfaces [24]. Among these, nitrogen-based organic surfactants like imidazolines and amines, commonly used in the oil and gas industry to protect mild steel [106], have been extensively applied to mitigate the corrosion of CS in geothermal environments [107]. A recent study by [PERSON] et al. [29] examined the performance of commercial inhibitors at temperatures of 80 \({}^{\circ}\)C and 160 \({}^{\circ}\)C in artificial brines. Their effectiveness and resulting corrosion rates were found to depend on factors such as the nature of the active species, temperature, and hydrodynamic conditions. In static conditions, an imidazoline-based product exhibited greater efficiency than an amine-based counterpart, achieving up to 77% efficiency at 200 ppm and 80 \({}^{\circ}\)C. However, when stirred, one amine-based product displayed higher efficiency. Nevertheless, some increase in the corrosion rate was observed after product application, especially at elevated temperatures, suggesting that the products or their dosing methods require further optimization. [PERSON] et al. [108] assessed the efficacy of a methacrylate-structured, grafted co-polymer known as PEGPHOS-LOW in preventing the precipitation and deposition of common geothermal scales, such as silica, silicates, sulphides, and calcium carbonate. The selection of this inhibitor was based on previous tests involving similar co-polymers containing polyethylene glycol (PEG) and phosphonate grafts. The study employed four artificial geothermal brines designed to simulate saturated geothermal well fluids. PEGPHOS-LOW, comprising 34 PEG20 grafts and 14 phosphonate grafts, served as a dual-scale control additive, inhibiting scale formation and dispersing scale particles. While PEGPHOS-LOW exhibited dose-dependent scale inhibition and dispersion properties, high concentrations were required for effective performance due to the pronounced scaling propensity of the brine. A comprehensive testing campaign was initiated in the Upper Rhine Graben geothermal plants to efficiently mitigate scaling and corrosion using anti-scaling agents and corrosion inhibitor additives [109]. In the initial phase of the project (SUBITO project), anti-scaling agents were assessed to prevent barite deposition [110; 111]. Several anti-scaling agents involved phosphonic acid-based formulations, while others remained undisclosed. Seven sulphide anti-scaling agents and four corrosion inhibitors were evaluated at two geothermal power plants: Insheim (Germany) [53] and Soutz-sous-Forets (France) [56]. The results demonstrated that antiscalant compatibility varied between the two plants. The most suitable antiscalant at Soutz proved incompatible at Insheim, leading to system clogging attributed to the higher temperature and calcium concentration of the Insheim geothermal fluid [109]. While sulfate scaling, such as barite, was effectively reduced using fluid-compatible sulfate anti-scaling agents at both plants, the surface installations encountered the formation of thin, brittle black scales (approximately 3-5 mm thick) on pipe walls. These scales comprised PbS, elemental Pb, As, Sb, and minor oxide minerals, raising con cerns due to the toxicity of Pb, As, and Sb.