Contents lists available at ScienceDirect Geothermics journal homepage: www.elsevier.com/locate/geothermics Inhibitor performance on carbon steel in the geothermal environment of the Upper Rhine graben (Central Europe) – A laboratory study Petra Huttenloch a, ⁎ , Roman Zorn a , Linda Makni a , Hagen Steger b , Frank Schilling b , Wolfgang Hater c a Europäisches Institut für Energieforschung, Emmy-Noether-Str. 11, 76131 Karlsruhe, Germany b Institute for Applied Geoscience/KIT, Adenauerring 20b, 76131 Karlsruhe, Germany c Kurita Europe GmbH, Giulinistr. 2, 67065 Ludwigshafen, Germany ARTICLE INFO Keywords: Carbon steel Inhibitor performance Electrochemical studies Weight loss tests CO2 corrosion Artificial geothermal brine ABSTRACT The corrosion behavior of carbon steel in CO 2 -saturated artificial geothermal brine was tested in the absence and presence of an amine-based inhibitor by weight loss tests and electrochemical measurements. Experimental conditions were adapted to the conditions existing on the re-injection sites of geothermal power plants located in the Upper Rhine Graben. The influence of Ca 2+ -ions on the corrosion behavior due to scale formation and its impact on inhibitor performance was investigated. Inhibitor adsorption was strongly dependent on inhibitor concentration, Ca 2+ concentration in the brine, reaction time, and pre-corrosion of the metal surface. 1. Introduction The Upper Rhine Graben (URG) provides a high geothermal gra- dient, which is used for geothermal power production in Germany and France (Agemar et al., 2014; Genter et al., 2013). Deep geothermal fluids of the URG have salinities of up to 200 g/L and a high CO 2 concentration (Sanjuan et al., 2016) resulting in corrosion of the con- struction materials and in scaling problems. Besides being due to the chemical composition of the produced brine, corrosion also depends on the high temperature, the fluid and gas flow, and on the design of the power plant (Carvalho et al., 2005). Corrosion leads to operating pro- blems, necessitates equipment maintenance, and causes process shut- downs and economic losses (Finšgar and Jackson, 2014). Carbon steels are common construction materials, which are highly susceptible to general and localized corrosion in a geothermal environment. As a consequence of the corrosion process, a layer of corrosion products forms on the metal surface in the CO 2 -bearing brine mainly consisting of a mixture of siderite (FeCO 3 ) and magnetite (Fe 2 O 3 )(Farida et al., 2012; Lopez et al., 2003). At the initial stage of steel corrosion in a CO 2 environment the formation of a Fe(OH) 2 layer is also expected (Yevtushenko et al., 2012). The behavior of corrosion scales as pro- tective or non-protective layers depends on the specific conditions in the respective geothermal power plant, e.g. on flow velocity, tem- perature, pressure, and brine composition. Protective scales may pre- vent uniform corrosion, but localized corrosion can also take place beneath the scales (Mundhenk et al., 2013). Small chloride ions can pass through defects of a scale layer onto the metal surface, promoting localized corrosion (Cheng et al., 2016). In a highly aggressive en- vironment, or if non-protective scales are formed, the use of corrosion inhibitors is preferred (Lopez et al., 2005). Corrosion and/or scaling control can help to extend the lifetime of geothermal power plant equipment from downhole to surface like casings, tubings, pumps, or heat exchangers. The use of corrosion inhibitors is a common method and among the most useful ones in the petroleum industry due to low cost and good handling (Álvarez-Bustamante et al., 2009; Finšgar and Jackson, 2014; Wang et al., 2001). However, incorrect choice or use of organic inhibitors can lead to corrosion stimulation. Organic inhibitors have a threshold concentration value, below which no corrosion in- hibition occurs (Schweitzer, 2009). Most organic inhibitors have been tested on fresh samples (Zhang and Zhao, 2017). In practice, the tubes and pipes of power plants operated for some time are covered by a corrosion product layer before inhibitors have been added. The effect of corrosion scales on the inhibitor performance can be antagonistic or synergistic. Corrosion inhibitors are mostly organic substances interfering with the anodic or cathodic corrosion reaction and forming a protective barrier on the metal surface. The efficiency of an inhibitor depends on flow patterns, solution chemistry, temperature and pressure, the mode of interaction with the metal surface, and on the inhibitor properties (Wang et al., 2001). Nitrogen-based organic surfactants – such as https://doi.org/10.1016/j.geothermics.2019.05.007 Received 20 December 2018; Received in revised form 6 May 2019; Accepted 8 May 2019 ⁎ Corresponding author. E-mail address: huttenloch@eifer.org (P. Huttenloch). Geothermics 81 (2019) 198–208 0375-6505/ © 2019 Published by Elsevier Ltd. T