This study focuses on the corrosion mechanism of carbon steel exposed to an artificial geothermal brine influenced by carbon dioxide (CO₂) gas. The tested brine simulates a geothermal source in Sibayak, Indonesia, containing 1500 mg/L of Cl⁻, 20 mg/L of SO₄²⁻, and 15 mg/L of HCO₃⁻ with pH 4. To reveal the temperature effect on the corrosion behavior of carbon steel, exposure and electrochemical tests were carried out at 70 °C and 150 °C. Surface analysis of corroded specimens showed localized corrosion at both temperatures, despite the formation of corrosion products on the surface. After 7 days at 150 °C, SEM images showed the formation of an adherent, dense, and crystalline FeCO₃ layer. Whereas at 70 °C, the corrosion products consisted of chukanovite (Fe₂(OH)₂CO₃) and siderite (FeCO₃), which are less dense and less protective than that at 150 °C. Control experiments under Ar-environment were used to investigate the corrosive effect of CO₂. Free corrosion potential (E_corr) and electrochemical impedance spectroscopy (EIS) confirm that at both temperatures, the corrosive effect of CO₂ was more significant compared to that measured in the Ar-containing solution. In terms of temperature effect, carbon steel remained active at 70 °C, while at 150 °C, it became passive due to the FeCO₃ formation. These results suggest that carbon steel is more susceptible to corrosion at the near ground surface of a geothermal well, whereas at a deeper well with a higher temperature, there is a possible risk of scaling (FeCO₃ layer). A longer exposure test at 150 °C with a stagnant solution for 28 days, however, showed the unstable FeCO₃ layer and therefore a deeper localized corrosion compared to that of seven-day exposed specimens.

Al₂O₃ has been widely used as a coating in industrial applications due to its excellent chemical and thermal resistance. Considering high temperatures and aggressive mediums exist in geothermal systems, Al₂O₃ can be a potential coating candidate to protect steels in geothermal applications. In this study, γ-Al₂O₃ was used as a coating on martensitic steels by applying AlOOH sol followed by a heat treatment at 600 °C. To evaluate the coating application process, one-, two-, and three-layer coatings were tested in the artificial North German Basin (NGB), containing 166 g/L Cl⁻, at 150 °C and 1 MPa for 168 h. To reveal the stability of the Al₂O₃ coating in NGB solution, three-layer coatings were used in exposure tests for 24, 168, 672, and 1296 h, followed by surface and cross-section characterization. SEM images show that the Al₂O₃ coating was stable up to 1296 h of exposure, where the outer layer mostly transformed into boehmite AlOOH with needle-like crystals dominating the surface. Closer analysis of cross-sections showed that the interface between each layer was affected in long-term exposure tests, which caused local delamination after 168 h of exposure. In separate experiments, electrochemical impedance spectroscopy (EIS) was performed at 150 °C to evaluate the changes of coatings within the first 24 h. Results showed that the most significant decrease in the impedance is within 6 h, which can be associated with the electrolyte penetration through the coating, followed by the formation of AlOOH. Here, results of both short-term EIS measurements (up to 24 h) and long-term exposure tests (up to 1296 h) are discussed.

Seal is one of the most important components in fuel cells which is required to bond the cell stacks and prevent mixing of gases in electrodes. Some of the requirements for such components are able to seal several adjacent cell components, which means compatible in thermal expansion coefficient values, and need to be chemically stable in a long-term operation. In this study, the potential use of natural silica sands from Bancar and Sowan in Tuban, East Java, Indonesia was explored, particularly from phase composition point of view. Six batches of samples were collected from both sites. X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD) data from all samples were complementarily used in this study. XRF and XRD data for each sample showed that quartz (SiO₂) was the most dominant phase, with estimated Rietveld method-based weight fraction content ranged between 70.1 and 98.7%. The second dominating phase is calcite (CaCO₃). According to the results obtained, we found that there is a slightly difference in the value of phases weight fraction due to RIR and Rietveld methods. In this research, PB-01 sands mixed with 17 wt.% magnesia and were calcined at various temperatures. Natural silica-sand-based composite may give promising excellent candidate for seal fuel cell material, because it forms forsterite and enstatite which suit the CTE value of sealant of fuel cell.

Polyaniline has been widely developed for many applications, e.g. sensor, supercapacitor components, electrochromic devices, and anticorrosion pigments. Although the addition of polyaniline pigment in organic coatings has been an alternative for corrosion protection in industrial applications, the protection mechanism is still not fully understood. Herein in this study, as a part of the development of polyaniline/silicon dioxide coating for geothermal application, polyaniline has been deposited electrochemically on carbon steel surface in oxalic acid medium and tested in geothermal solution to understand the contribution of polyaniline to the corrosion protection of a polyaniline-based composite in the geothermal system. To observe the surface/interface reaction between the electrolyte and electrode surface during the electrochemical polymerization, electrochemical impedance spectroscopy (EIS) was applied after each cycle. For corrosion study in the geothermal application, an artificial geothermal solution was used with the composition of 1,500 mg/l Cl^-, 20 mg/l SO₄^2-, 15 mg/l HCO₃^-, 200 mg/l Ca²⁺, 250 mg/l K⁺, and 600 mg/l Na⁺, and pH 4 to simulate a geothermal brine found in Sibayak, Indonesia. An electrochemical measurement was performed by monitoring the open circuit potential over seven days, with the interruption by EIS every 22 hours. The experiments were performed at room temperature and 150 °C (1 MPa) in an oxygen-free environment. Impedance spectra showed a reduction of the total impedance value of approximately 10 times for specimens measured at 150 °C compared to the specimens measured at room temperature, suggesting a less stable layer at high temperature.