Performance of coatings developed for protecting steel pipes in geothermal power plants: in-situ corrosion monitoring by EIS-based sensor

摘要

High salinity, acidic gasses, elevated temperature and pressure of geothermal brines create very aggressive environments that make corrosion and fouling major issues for the elements in direct contact with the brines. Currently, the heat exchangers and piping systems of the geothermal plants are constructed of very resistant and costly materials. The aim of this work is to evaluate whether cheaper materials (carbon steel or low alloying stainless steels) with cost-efficient coatings could deliver protection equivalent to that of expensive alloys. Chemistry of geothermal fluids and material solutions depends on the local geology, and MATChING project is focused on the geothermal site of Balmatt (Belgium). Several coatings were selected for testing, cost-effectiveness combinations of commercial and newly developed coatings. Coating evaluation involved exposure tests at lab scale. Results showed that certain coatings effectively protect the substrates and do not deteriorate. The five best performing coatings were selected for demonstration under real operational conditions in pipe sections of a bypass loop installed at the geothermal plant. A novel probe was integrated in the installation for in-situ and on-line corrosion monitoring using electrochemical impedance spectroscopy (EIS) to determine the corrosion rate and the scaling degree of pipes. EIS is introduced as a useful tool for real time estimation of the service life of components and prediction of failures in the critical units of the plant, by correlating the process events to corrosion evolution. New EIS-based sensor will facilitate the decision making and contribute to optimize the operation conditions, the material selection and the plant maintenance costs.