# Corrosion and Scaling in Geothermal Heat Exchangers
## 4 Corrosion in Geothermal Environments
### Localised Corrosion  
Unlike uniform corrosion, localized corrosion is highly unpredictable and characterized by a rapid rate of metal penetration that can lead to premature component failure. Only  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Test Conditions** & **Fluid** & **CR \({}^{*}\) (mm y\({}^{-1}\))** \\ \hline J55 CS \({}^{*}\)[25] & Mass loss, \(\text{P}_{\text{CO}_{2}}\) = 5 MPa, 65 \({}^{\circ}\)C & Synthetic, static & 0.4 \\ \hline API L80 steel [26] & Tafel PP + LPR \({}^{*}\), deaerated, 180 \({}^{\circ}\)C & Synthetic, static & 0.02 \\ \hline API L80 steel [27] & Mass loss, 100 \({}^{\circ}\)C & Synthetic, static & 0.308 \\  & Mass loss, 200 \({}^{\circ}\)C & Synthetic, static & 0.052 \\ \hline CS [28] & Tafel PP \({}^{*}\), 30 \({}^{\circ}\)C & Real (TDS 161 mg/L, pH 4.68), static & 0.23 \\  & & Real, 1.9 m s\({}^{-1}\) flow & 1 \\ \hline C45 CS [29] & Mass loss, CO\({}_{2}\) sat., 80 \({}^{\circ}\)C–160 \({}^{\circ}\)C & Synthetic, liquid & 1.4–3.9 \\  & & Synthetic, vapor & 0.09–0.16 \\ \hline API P110 CS [30] & Topography measurements, CO\({}_{2}\) & Synthetic, 500 rpm stirring & 5 \\  & sat., 80 \({}^{\circ}\)C & Synthetic, static & 0.95 \\ \hline C45 CS [24] & Mass loss, CO\({}_{2}\) purged, 80 \({}^{\circ}\)C & Synthetic, static, with Ca\({}^{2+}\) & 0.67 \\ \hline S235 JR steel [31] & Mass loss, CO\({}_{2}\) purged, 70 \({}^{\circ}\)C & Synthetic, static & 0.16 \\  & Mass loss, CO\({}_{2}\) purged, 150 \({}^{\circ}\)C & 0.24 \\ \hline \hline \end{tabular} \({}^{*}\)CR = corrosion rate, CS = carbon steel, PP = potentiodynamic polarization, and LPR = linear polarization resistance.  
\end{table}
Table 2: Corrosion rates of CSs in geothermal environments.  
a small section of the material is affected, with two primary forms of localized corrosion: pitting and crevicing.  
Pitting corrosion occurs when the protective oxide film or deposit layer is breached at a local point, exposing a small area of the metal to the corrosive environment. The exposed metal dissolves, creating a pit that serves as an anode relative to the surrounding protected outer surface. Fitting is an autocatalytic process, where pit growth creates conditions that further encourage pit development [33]. Detecting pitting corrosion is challenging as it often results in minimal weight loss, making it difficult to identify. The formation of a pit can have severe consequences for the structural integrity of a component, as it represents a stress concentration feature. Under specific conditions, stress and pitting can interact, leading to stress corrosion cracking (SCC). Typically, pitting is associated with alloys like stainless steel, Ni-alloys, and titanium alloys that rely on the formation of a protective oxide film for corrosion resistance. Pitting corrosion is a stochastic phenomenon linked to material defects (such as slip planes, grain boundaries, or inclusions), design issues, or environmental factors.  
Crevicing differs from pitting in terms of its initiation mechanism, which originates from geometric considerations. Crevicing occurs in narrow, enclosed spaces on the metal surface. This type of corrosion is often characterized by accelerated metal loss within the crevice or gap, driven by differences in oxygen (aeration cell) concentration, pH, or other factors between the crevice and the surrounding environment [34]. Crevices can form due to poor component design or after the deposition of scale deposits. Localized corrosion is exacerbated by the presence of chemical species in the geothermal fluid that promote depassivation, such as Cl\({}^{-}\), SO\({}_{4}\)\({}^{-}\), and H\({}^{+}\) (low pH) [35].