# Corrosion and Scaling in Geothermal Heat Exchangers
## 6 Degradation Mitigation for Heat Exchangers
### Protective Coating Systems  
While CRAs are commonly used in heat exchangers, they can be expensive and may require replacement due to corrosion-induced failures. Moreover, their thermal conductivity is notably lower than that of CS, as shown in Table 4, resulting in reduced plant yields [70]. To reduce capital costs and increase the efficiency of heat exchangers, there is a potential to replace CRAs like titanium alloys and stainless steels with CS. However, as shown in Table 2, the corrosion rates of CSs in geothermal brines often exceed the acceptable rate of 0.3 mm year\({}^{-1}\)[23]. In dynamic flow conditions with real geothermal brine, corrosion rates can surpass 1 mm year\({}^{-1}\)[28], rendering such corrosion rates economically unviable due to the frequent need for heat exchanger replacement. In many cases, plain CS cannot be employed for heat exchangers due to its high corrosion rate and the formation of corrosion or scale products that impede heat transfer. As a solution, protective coating systems are applied to mitigate corrosion and scaling while taking advantage of the favorable thermal conductivity and cost-effectiveness of CS. Protective coating systems primarily fall into three categories: organic, inorganic, and hybrid. Organic coatings offer corrosion protection via polymerization and crosslinking but are less effective at high temperatures [80,81]. Inorganic coatings consist mainly of ceramics, metals, metal oxides, or insoluble inorganic salts, exhibiting excellent thermal stability [82,83]. Ceramic coatings exhibit superior resistance to corrosion and wear at elevated temperatures [84]. Inorganic-organic hybrid coatings were developed to combine the superior corrosion protection of organic coatings with the thermal stability and mechanical resistance of ceramics and metals [85, 86, 87, 88, 89]. Numerous protective coating systems are documented in the literature, with [PERSON] et al. [90] identifying 23 organic and 20 inorganic coating systems for geothermal applications. For heat exchanger protection, the coating must possess excellent thermal conductivity, corrosion resistance, and anti-scaling/fouling properties. Achieving superhydrophobicity, often through multi-scale fractal features [84], or with the addition of PTFE fillers [72], is highly desirable to enhance these properties. The utilization of coated CS for heat exchangers in geothermal power plants has been explored since the 1990s. [PERSON] et al. [69,70] conducted tests on polymer-based coatings to safeguard CS tubes within a heat exchanger at the Hoch geothermal power plant in the Salton Sea reservoir, California. Their findings identified two promising systems: polyphenylene sulphide (PPS) and phenolic-based coatings. These coatings exhibited robust corrosion protection and had low bond strength with geothermal scale deposits, facilitating the cleaning process. In a more recent study, [PERSON] et al. [71] further explored phenolic coating systems along with two fluoropolymer-based paints and epoxy/sol-gel systems for the protection of geothermal heat exchangers. Their objective was to identify potential candidates to replace or enhance the existing heat exchanger at the Balmatt geothermal plant in Belgium, constructed using stainless steel. The base metals used were P265G CS and 316L stainless steel. The coatings underwent testing in a synthetic geothermal fluid at 130 \({}^{\circ}\)C and 40 bar for 570 h. The phenolic-based paint system exhibited strong adhesion with limited defects, but its surface energy increased after exposure, possibly reducing its scaling mitigation capability. Furthermore, electrochemical corrosion tests indicated that this coating allowed electrolyte permeation, rendering it unsuitable for corrosion mitigation. Consequently, the phenolic-based coating was not considered promising, while epoxy-based and fluoropolymer-based coatings demonstrated suitability.