Localized Corrosion Phenomena of Passive Materials in Geothermal Fluids

Geothermal energy has gradually become a significant green energy source. A key point for the engineering design of geothermal power plants concerns undoubtedly materials selection. Service conditions in geothermal facilities from deeply located hydrothermal reservoirs are critical in terms of corrosion. Beside high temperature, geothermal fluids are usually highly saline solutions containing some corrosive gases like CO₂ and H₂S. In order to guarantee a secure and reliable operation, selected materials must provide a corresponding appropriate corrosion resistance. The use of passive materials like stainless steels, Ni-based and Ti-based alloys represents therefore a very good alternative. However, phenomenon of localized corrosion phenomena including pitting and crevice corrosion can take place on these materials.

Currently, a project in Germany deals with the evaluation of long-term corrosion of nine materials including: stainless steels, duplex and super duplex steels, and Ni-based alloys in fluids from three different geothermal areas in Germany: North German Basin, Molasse Basin and the upper Rhine valley. Preliminary findings obtained from electrochemical and exposure tests in an artificial geothermal fluid with a chemical composition according to the aquifer of Groß Schönebeck (North German Basin) showed the limitations of suitability of Alloy 24, Alloy 31 and Alloy 59 concerning pitting and crevice corrosion. In the present paper, the results of these tests are discussed and additionally a brief overview about the further scheduled investigations is presented.
 Keywords: pitting corrosion, crevice corrosion, stainless steel, Ni-based alloy, geothermal energy