# Corrosion and Scaling in Geothermal Heat Exchangers
## 4 Corrosion in Geothermal Environments
### Uniform Corrosion  
Uniform corrosion, also known as general corrosion, is characterized by a relatively even and consistent corrosion attack that affects the entire exposed surface of a metal component. This type of corrosion typically occurs when geothermal fluids directly interact with metal surfaces over an extended period. Corrosion rates are commonly quantified in millimeters per year (mm year\({}^{-1}\)). Uniform corrosion is preferred over localized corrosion due to its predictable nature, which can be factored into design considerations. In general, an acceptable corrosion rate falls below 0.3 mm year\({}^{-1}\)[23; 24]. However, even this corrosion rate can be considered fairly high as such high corrosion rates would require significant corrosion allowance. Estimating the corrosion rate is particularly crucial when uniform corrosion poses the primary threat. Under conditions of active dissolution, where soluble corrosion products are continuously exposed to the metal, the corrosion rate depends on thermodynamic factors such as temperature and acidity/alkaninity (pH). The corrosion rate increases with rising temperature and decreasing pH. However, in practical scenarios, even uniform corrosion becomes complex due to the deposition of corrosion products on the metal surface. In geothermal applications, the corrosion rate is influenced by various factors, including fluid \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Class**} & \multirow{2}{*}{**Fluid**} & \multirow{2}{*}{**Characteristics**} & \multirow{2}{*}{**TKS (ppm)**} & \multirow{2}{*}{**Chloride**} & \multirow{2}{*}{**pH**} & \multicolumn{2}{c}{**Volume of**} & \multirow{2}{*}{**Temperature**} & \multicolumn{1}{c}{**Total**} \\  & & & & & & **Phase** & & **Inlet/Outlet (\({}^{\circ}\)C)** & **(ppm CaCO\({}_{3}\))** \\ \hline \multirow{2}{*}{I} & Hyper saline, high temperature & \multirow{2}{*}{\(>\)100k} & \multirow{2}{*}{99} & \multirow{2}{*}{\(<\)5/} & \multirow{2}{*}{\(<\)2.5} & \multirow{2}{*}{326/199} & \multirow{2}{*}{-} \\  & Acid liquid & & & & & & \\ \multirow{2}{*}{II-A} & dominated (Acid & \multirow{2}{*}{1k-60k} & \multirow{2}{*}{15-99} & \multirow{2}{*}{\(>\)3} & \multirow{2}{*}{-} & \multirow{2}{*}{-} & \multirow{2}{*}{-} \\  & sulfate & & & & & & \\ \multirow{2}{*}{II-B} & Adelaide liquid & \multirow{2}{*}{500-40k} & \multirow{2}{*}{20–99} & \multirow{2}{*}{\(<\)3} & \multirow{2}{*}{-} & \multirow{2}{*}{-} & \multirow{2}{*}{-} \\  & dominated (Acid & \multirow{2}{*}{500-40k} & \multirow{2}{*}{20–99} & \multirow{2}{*}{\(<\)3} & \multirow{2}{*}{-} & \multirow{2}{*}{-} & \multirow{2}{*}{-} \\  & chlorine & & & & & & \\ \hline \multirow{2}{*}{III} & High salinity, neutral pH & \multirow{2}{*}{10k-20k} & \multirow{2}{*}{45-99} & \multirow{2}{*}{5–6} & \multirow{2}{*}{-} & \multirow{2}{*}{178–300/149–191} & \multirow{2}{*}{-} \\  & & neutral pH & & & & & \\ \multirow{2}{*}{IV} & Moderately saline, neutral pH, liquid & \multirow{2}{*}{500–10k} & \multirow{2}{*}{45-99} & \multirow{2}{*}{\(>\)5} & \multirow{2}{*}{\(<\)2.5} & \multirow{2}{*}{137–280/121–199} & \multirow{2}{*}{-} \\  & dominated & & & & & & \\ \hline \multirow{2}{*}{V-A} & Low salinity, low pH, neutral pH & \multirow{2}{*}{\(<\)5k} & \multirow{2}{*}{3–72} & \multirow{2}{*}{6.7–7.6} & \multirow{2}{*}{-} & \multirow{2}{*}{49–96/49–96} & \multirow{2}{*}{207–1239} \\  & Low salinity, low pH, alkaline pH & & & & & & \\ \hline \multirow{2}{*}{VI-A} & Vapor dominated & \multirow{2}{*}{\(<\)5k} & \multirow{2}{*}{3–72} & \multirow{2}{*}{7.8–9.85} & \multirow{2}{*}{-} & \multirow{2}{*}{120–205/120–205} & \multirow{2}{*}{\(<\)210} \\  & without HCl & & & & & & \\ \multirow{2}{*}{VI-B} & Vapor dominated & \multirow{2}{*}{-} & \multirow{2}{*}{-} & \multirow{2}{*}{-} & \multirow{2}{*}{-} & \multirow{2}{*}{-} \\  & with HCl & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Classification of geothermal fluids revised by [PERSON] et al. [20], reproduced with permission from Elsevier.