Mapping and delineation of steel corrosion products under geothermal-like conditions

In this paper we attempt to understand the corrosion behavior of carbon (mild) steel (a common structural component in water handling installations) and the factors that influence it, by identifying various corrosion products that form under diverse conditions relevant to geothermal systems. Hence, experiments were performed under variable stressful experimental conditions, by systematically studying the effect of certain important variables, such as temperature and brine composition. Therefore, three brines and four temperatures were selected (ambient, 60, 90, and 130 °C), while the pH was kept constant at ∼ 7. It was found that for all water qualities corrosion rates (quantified by gravimetric methods and soluble Fe measurements) are not directly proportional to the temperature, with the measured values being the lowest at RT and highest at T = 130 °C. In the two intermediate temperatures (60 and 90 °C) the corrosion rates are lower. A possible explanation for this could be the fact that corrosion products can form films on the metal surface, affecting the corrosion aggressiveness, and thus corrosion rates. The full characterization of precipitates collected from these experiments led to the identification of the corrosion products and to the correlation of corrosion aggressiveness (due temperature and water quality) with the identity of each corrosion product. The variability in color of the corrosion products on the metal surfaces was an indication of the formation of lepidocrocite and magnetite films, as demonstrated by the orange and black color of the films on the specimens, respectively. The identification of the corrosion products that formed under the selected experimental conditions was achieved by the physicochemical characterization (ATR-IR and powder XRD) of the iron deposits collected from the control experiments. These studies confirmed the qualitative indications based on the deposit color.