# Corrosion and Scaling in Geothermal Heat Exchangers
## 6 Degradation Mitigation for Heat Exchangers
### Corrosion Resistant Alloys  
Using CRAs in heat exchangers is a common approach for mitigating corrosion in geothermal plants. Initially, titanium alloys and stainless steels found application in tube and shell heat exchangers for geothermal binary cycle plants, such as those in the Salton Sea reservoir (California) [65]. More information on materials for geothermal heat exchangers is given in Table 3.  
Among CRAs, stainless steels (SS) are the most cost-effective, with the 316L grade being widely employed in geothermal heat exchangers [75]. However, 316L SS is not suitable for applications combining high-temperature and H\({}_{2}\)S-containing environments [76]. The protective Cr-rich passive film destabilizes as the temperature increases from 30 \({}^{\circ}\)C to 130 \({}^{\circ}\)C. This destabilization was attributed to the replacement of the Cr-oxide film with a less protective Ni-rich film combined with the preferential adsorption of S instead of O at higher temperatures. In contrast, Inconel 625, a corrosion-resistant nickel-based alloy, is recommended for use in sulfur-rich, higher-temperature environments [77]. Stainless steel heat exchangers are also susceptible to environmentally assisted cracking, including hydrogen-induced stress cracking (HISC) and stress corrosion cracking (SCC). For instance, the SCC susceptibility of an HP-13 Cr SS increases with rising tempera \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Material** & **Coating System** & **Key Insights** \\ \hline CS * [52] & - & Failure due to arsenic precipitation leading to galvanic corrosion \\ \hline Titanium alloy [52] & - & Frequent cleaning required due to scaling \\ \hline
316L SS * [10] & - & Failure due to SCC * in the Cl\({}^{-}\) rich environment \\ \hline
904L SS, 254 SMO, DX 2205, SDX 2507, Alloy & - & Scaling independent of alloy used \\
825, TiGr2 [11] & - & Subsurface cracking Titanium not suitable due to erosion \\ \hline Cupronickel (CuNi10 Fe) [12] & - & Failure due to hydrogen embrittlement and sulphide stress cracking \\ \hline
316L SS [67] & Ni-Al\({}_{2}\)O\({}_{3}\) Nanocomposite & \\ \hline
304 SS [68] & SiO\({}_{2}\), SiO\({}_{2}\)-FPS * and TiO\({}_{2}\) & Coating reduces fouling and corrosion \\ \hline CS [69; 70] & PTFE-blended PPS * system with a zinc phosphate (ZnPh) & - \\ \hline CS EN 10028:2, P265G, 316L SS [71] & fluoropolymer-based and phenolic-based composite epoxy/sol-gel & Low impact from coatings on the overall thermal conductivity \\ \hline CS [72] & PPS/PTFE/MWCNT * & Super-hydrophobic surface obtained \\ \hline CS [9] & Ni-P-PTFE & - \\ \hline CS 1015 [73] & Ni-W-P & Calcites and aragonites formation, good antifouling \\ \hline CS 1015 [74] & Ni-Cu-P-PTFE & Good anti-corrosion and mechanical properties. \\ \hline \hline \end{tabular}  
* CS = carbon steel, FPS = heptadecafluorodecyltri-isopropylosevilane compound, MWCNT = multi-walled carbon nanotube, SCC = stress corrosion cracking, and PPS = polyphenylenesulfphide.  
\end{table} 
Table 3: Materials used for geothermal heat exchanger applications. ture and CO\({}_{2}\) pressure, as reported by [PERSON] et al. [42]. [PERSON] et al. [10] also identified SCC as the cause of failure in a 316L SS tube and shell heat exchanger after a year of service. Super austenitic stainless steels like 254 SMO are favored for geothermal applications, offering improved corrosion resistance in aggressive environments, such as those rich in Cl\({}^{-}\)[78] or acidic [79] substances. [PERSON] et al. [66] examined the suitability of four CRAs (316L SS, 254 SMO, titanium ASTM Grade 2, and Inconel 625) for heat exchanger applications in two geothermal environments. Reykjanes (Iceland), a volcanic area, and Chaunoy (France), a sedimentary basin, served as test locations. In the Reykjanes geothermal site, the samples underwent testing within a pressure vessel at conditions of 18 bar and 200 \({}^{\circ}\)C, resulting in a dual vapor/liquid phase fluid.