# Corrosion and Scaling in Geothermal Heat Exchangers
## 6 Degradation Mitigation for Heat Exchangers
### Protective Coating Systems
These coatings exhibited thermal resistances similar to or lower than the fouling factor currently used for the stainless steel heat exchanger (0.0004 m\({}^{2}\)K W\({}^{-1}\)), with thermal conductivities ranging from 0.25 W m\({}^{-1}\) K\({}^{-1}\) (fluoropolymer-based) to 0.63 W m\({}^{-1}\) K\({}^{-1}\) (epoxy-based), as well as providing excellent corrosion protection and anti-fouling properties due to their low surface tension (<30 mN m\({}^{-1}\)). PPS coatings, initially identified as promising candidates for heat exchanger protection by [PERSON] et al. [69,70], were subsequently enhanced by incorporating carbon fillers [91,92]. The addition of carbon fibers, approximately 7.4 um in diameter and 3 mm in length, at varying concentrations (0.2, 0.5, 1.0, and 1.5 wt.%) to the PPS system, applied on AISI 1008 CS, was tested for 14 days in synthetic geothermal fluid (200 \({}^{\circ}\)C, 20,000 ppm CO\({}_{2}\), 13 wt.% NaCl) using electrochemical impedance spectroscopy (EIS). The optimal carbon fiber content was found to be 0.5 wt.%, which improved corrosion performance compared to pure PPS. This adjustment also enhanced thermal conductivity by 60% and mechanical \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Alloy** & **Carbon** & **Stainless** & **DX Stainless** & **Nickel Alloys** & **Titanium—Grade 2** & **Nickel Alloys** \\  & **Steel** & **Steel—2205** & & & & \\ \hline
**Thermal** & & & & & & \\
**conductivity** & 40–50 & 16–20 & 19 & 9–12 & 16.4 & 9–12 \\
**(W m\({}^{-1}\) K\({}^{-1}\))** & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Thermal conductivity of common metal alloys used for geothermal heat exchangers. properties, with performance improvements ranging from 1.5 to 2.6 times that of pure PPS. However, concentrations exceeding 0.5 wt.% resulted in excessive porosity, leading to solution permeation. This coating system was subsequently applied to protect CS tubes in heat exchangers at the Puna Geothermal Venture power plant in Hawaii [93]. The preparation of the substrate, either zinc phosphate layer application or galvanization, proved to be a critical factor in coating performance. G galvanized tubes experienced paint delamination after four weeks of service, leading to a 70% decline in heat transfer performance compared to its original state, surpassing the decline of 43% observed in uncoated CS. Further developments in PPS/carbon coating systems were carried out by [PERSON] et al. [72] for high-performance heat exchangers in geothermal plants. The study focused on a PPS/PTFE/MWCNT (multi-walled carbon nanotube) composite coating with varying MWCNT loadings (0 to 10 wt.%). The authors achieved a super-hydrophobic surface with a maximum water contact angle of 171\({}^{\circ}\) at 5 wt.% MWCNT, in contrast to 80\({}^{\circ}\) and 103\({}^{\circ}\) for pure PPS and PPS/PTFE, respectively. The optimal loading range for MWCNT was between 3 and 5 wt.%, as concentrations above this range led to detrimental aggregation, affecting mechanical properties. Several organic coating systems have seen long-term service, as exemplified at Soultz-sous-Forets in France. Here, an undisclosed polymeric paint system was employed for the protection of Organic Rankine Cycles (ORC) heat exchangers from 2008 to 2012 [94]. Initially, the paint system displayed promise in reducing scaling. However, in 2012, the appearance of paint blistering and spallation (Figure 3) raised concerns. Chemical analysis of detached paint fragments revealed varying compositions between the surface exposed to geothermal fluid and the underlying steel substrate. The fluid-exposed surface contained strontium-rich barite (Ba1-xSrxSO4), galena (PbS), and minor mixed sulphides ((Fe, Sb, As)Sx). In contrast, the substrate surface exhibited a mixture of iron oxides, hydroxides, and/or carbonates, indicating underlying substrate corrosion. Consequently, it was concluded that paint systems held potential for scaling and corrosion protection but required optimization for specific service conditions.