| Abstract |
Corrosion and scaling are the major issues in the exploitation of geothermal sources. Geothermal fluids are very complex mixtures consisting of dissolved gases and high salinity solutions, which creates very aggressive environments. This aggressiveness is primarily due to the presence of high concentrations of carbon dioxide (CO2), hydrogen sulphide (H2S), chlorides and other chemical species in lower concentrations. Besides, the high temperature of the geothermal fluids increases corrosion rates promoting failures related to stress and fatigue corrosion. On the other hand, reinjection of cooled brine exiting the heat-exchanger favours corrosion and scaling onset, since the chemicals dissolved in geothermal waters may tend to precipitate promoting calcite, sulphate and/or silicate depositions on the casing. Corrosion and scaling phenomena are difficult to detect visually or monitor continuously. Standard techniques based on pH, temperature and pressure measurements, chemistry composition and fluid physical properties are habitually applied as indirect methods for corrosion rate control. Other traditional techniques such as mass loss probes or electrical resistance measurements have been introduced more recently for corrosion data registration. These methods, however, lack of enough robustness for accurate and reliable measuring and evaluating corrosion behaviour of materials employed in geothermal systems. In this scenario, a novel system is proposed for online corrosion monitoring of materials due to the corrosive attack of geothermal fluids. The corrosion rate information early-obtained will allow prediction of failures in the critical units of the plant through the correlation of process events to corrosion evolution. Thus, in GECO project a dedicated electrochemical-based test system will be designed and developed for on-line and continuous monitoring of the corrosion/scaling rate of different materials exposed to the real plant conditions at different locations. This system will use non-standard methods based on electrochemical impedance spectroscopy (EIS) for the in-situ and on-line monitoring of corrosion and scaling degradation. With this instrument, the corrosion of the inner side of the casing and geothermal pipes will be followed in real time in three demonstration plants sited in Iceland and Germany. The analysis of the impedance spectra generated by equivalent circuits can determine contribution of each stage affecting to the corrosion process providing useful information about the corrosion mechanism and the scaling growth. |