Corrosion Behaviour of L80 Steel Grade in Geothermal Power Plants in Switzerland

Abstract

In Switzerland, deep geothermal energy can give a promising contribution to the future energy scenario. However, the expertise in operational issues of deep geothermal power plants is limited, and technical challenges, such as corrosion, are a determining factor for their reliable and long-term operation. In this work, two representative fluids of optimal geothermal conditions in Switzerland were studied. The corrosiveness of the solutions was assessed using two experimental setups that allow investigating the range of temperatures and pressures that apply to the reservoir and power plant conditions. The corrosion behaviour of API L80 steel was analyzed by means of electrochemical measurements (at 100 and 200 • C) and of gravimetric tests (at 100 • C). After the tests, the morphologies and composition of the corrosion products were obtained by scanning electron microscopy (SEM) coupled with energy dispersive X-Ray (EDX) and X-Ray diffraction (XRD). Results show that corrosion rates are significantly high at 100 • C in environments with a chloride concentration of around 600 mg/L and pH around 7. The corrosion products deposited on the metal surface mainly consist of magnetite and/or hematite that might potentially form a protective layer. This study gives a first insight of the potential corrosiveness of geothermal fluids in Switzerland.

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

“…The two time constants observed might correspond to the corrosion reaction (R ct , CPE dl ) as (Table 3) and the formula for a capacitance with the relative dielectric constant for iron oxide ε r = 12 (Stimming and Schultze 1976) and the surface area A of the sample (11 cm 2 ), the thickness d of the porous oxide layer results in approximately 250 µm. This is in agreement with former studies utilizing surface analysis methods (Vallejo Vitaller et al 2019) showing that at 200 °C and with the same metal and similar fluid chemistry as in phases 1 and 3-5 of the current work, a protective scale (mainly consisting of hematite) was formed on the metal surface. This scale formation may explain the gradual decrease of corrosion rate that was observed within phase 3.…”

“…This increase is reasonable considering that oxygen entered the system and the temperature decreased. Moreover, this result is in agreement with previous studies testing the same kind of fluid and metal (Vallejo Vitaller et al 2019), in which the averaged corrosion rate over 120 h at 100 °C (308 µm/year) was higher than at 200 °C (52 µm/ year). Other studies have also evidenced lower corrosion rates toward higher temperatures (> 120 °C) (Mundhenk et al 2019).…”

“…The increase of around two pH units in this phase might be an indication of the change of temperature from 180 to 90 °C. However, as shown in previous studies that tested the same type of fluid and steel grade, the pH difference at 100 and 200 °C was of only 0.5 units (Vallejo Vitaller et al 2019). Thus, this temporary alkalization of the aqueous solution might be due to a change of partial pressure of the gases.…”

“…In a binary geothermal power plant, components, such as the well casings, pipelines, or heat exchangers, generally consist of metallic materials, and thus, subject to corrosion processes (Nogara and Zarrouk 2018a, b;Clark et al 2010). In Switzerland, previous studies have shown that fluids from regions with optimal conditions for the generation of electricity (and space heating) might lead to corrosion damages in lowalloyed steels, especially at low temperatures (100 °C) (Vallejo Vitaller et al 2019).…”

“…For instance, it is well known that dissolved oxygen might act as an oxidizing agent and initially accelerate the corrosion process (Landolt 2007;Revie and Uhlig 2008). Moreover, it has recently been shown that a difference in temperature may influence the corrosion conditions (Vallejo Vitaller et al 2019). In this context, there is a risk that actions meant as "preventive maintenance" may actually enhance corrosion and thus lead to expensive downtimes to the power plant operators.…”