Laboratory tests simulating corrosion in geothermal power plants: influence of service conditions

Abstract

One of the main challenges associated with the operation and maintenance of binary geothermal power plants is the degradation of construction materials. In this sense, it is crucial to apply appropriate preventive maintenance in critical components (such as the wellheads, heat exchangers, or pipes), while reducing shutdown times. Based on electrochemical measurements performed in an autoclave corrosion testing setup, we studied the corrosion mechanism of API L80 steel grade as a function of operational and/or maintenance procedures. We used a test fluid representative for a site in Switzerland, but the main observations made may be applicable in a wider context. We found that changes in the fluid temperature (from 200 to 100 °C) or temporary oxygen ingress significantly influenced the corrosion behavior of this carbon steel and increased its corrosion rate (from approx. 20 µm/year to > 120 µm/year). After a few days, the corrosion rate was found to decrease and stabilize around values of 50–70 µm/year, as a result of a porous corrosion product layer formed on the metal surface (approx. 250 µm thick). Electrochemical impedance spectroscopy indicated an increase in capacitance of the double layer over time, most likely due to an increase in the effective surface area of the steel sample, as a consequence of surface roughening due to corrosion. The results from this study may be implemented in the design and operation of future power plants in Switzerland and elsewhere to ensure reliable and cost-effective energy production from geothermal resources.

Geothermal energy resources in Ethiopia: Status review and insights from hydrochemistry of surface and groundwaters

“…The transition zone between these two broad fluid types in the area between Dofan-Fantale and Meteka (Figure 3b) is dominated by Na-mixed waters (typically 40-50% Na). This spatial distribution was first documented through hot springs (UNDP, 1973) and later confirmed with geothermal exploration wells, where the following water types are identified: Na-HCO 3 for Corbetti, Na-Cl for Tendaho, and Na-dominant for Aluto-Langano (Figure 3a). At Aluto-Langano, about 87% of the samples are Na-alkalinity, with the remainder of Na-Cl or Na-mixed type.…”

“…Exploration for geothermal energy in Ethiopia started in 1969 with initial surface geology, geochemistry, air photo interpretations, and infra-red airborne survey investigations (UNDP, 1973). These were later complimented by focused hydrochemical and isotopic studies (Scripps Institute of Oceanography, 1977).…”

“…Hot springs from Dallol and the Danakil Depression appear to be mature in the anion trilinear plot (Figure 4b) but only partly equilibrated in the cation plot (Figure 5b). This apparent contradiction is related to the atypical geochemistry of the fluids (especially at Dallol) due to the dissolution of evaporites which alters their chemical composition (UNDP, 1973). Initially, Na-HCO 3 type geothermal fluids from the MER were believed to result from mixing between fluids of deep origin and shallower bicarbonate waters (UNDP, 1973).…”

“…This apparent contradiction is related to the atypical geochemistry of the fluids (especially at Dallol) due to the dissolution of evaporites which alters their chemical composition (UNDP, 1973). Initially, Na-HCO 3 type geothermal fluids from the MER were believed to result from mixing between fluids of deep origin and shallower bicarbonate waters (UNDP, 1973). Gizaw (1996) noted the wide divergence in the igneous country rock between these regions (felsic in the MER and mafic in the Afar Depression) and concluded that the different chemical signatures result from water-rock interactions with different lithologies.…”

“…Corrosion is driven by electrochemical or chemical reactions with certain concentrated species in geothermal fluids and can lead to the spontaneous degradation of construction materials (Vallejo Vitaller et al, 2020). High concentrations of chloride can be problematic for ferrous-based metals, such as stainless and carbon steel, but the greatest risk of corrosion comes from dissolved acidic gasses (Allahvirdizadeh, 2020).…”