Corrosion Behavior of High Alloy Austenitic Stainless Steel in Simulated High Temperature Geothermal Environment

Karlsdóttir, Sigrún Nanna (University of Iceland) | Jonsson, Tindur (University of Iceland) | Stefánsson, Andri (University of Iceland)

OnePetro 

ABSTRACTIn recent years there has been an increased interest in drilling deeper geothermal wells to obtain more energy output per well with the corresponding higher temperature and pressure and increased corrosiveness of the geothermal environment. To explore the potential of the high alloy austenitic stainless steel UNS S31254 in future deep geothermal wells corrosion testing was done in simulated geothermal environment at 180°C and 350°C with a pressure of 10 bar. The simulated environment was composed of steam with H2S, HCl and CO2 gases, with a pH of 3 upon condensation. The testing was done in a flow through reactor for 1 and 3 week exposures. The stainless steel UNS S31254 performed well at 180°C with negligible corrosion rates both for the 1 and 3 week tests and no localized corrosion damage detected. After the testing at 350°C localized corrosion and substantial amount of NaCl crystals were observed on the surface of the samples. Microstructural and chemical composition analysis revealed large cracks in the cross-section of the sample most likely due to chloride induced stress corrosion cracking. The measured corrosion rate for the 1 and 3 week test was 0.024 mm/year and 0.24 mm/year respectively.INTRODUCTIONMaterials used in high temperature geothermal steam can be subjected to corrosion due to the chemical composition of the geothermal fluid. Geothermal fluids contain corrosive substances such as the dissolved gases hydrogen sulfide (H2S) and carbon dioxide (CO2), and chloride ions (Cl-)1-5. The source of chloride ions (Cl-) can be from volatile chloride transported as hydrochloric acid (HCl) in the gas phase from the volcanic system or from salt brine in geothermal areas close to the sea. If localized enrichment of hydrochloric (HCl) acid occurs e.g. due to condensation and/or re-boiling it will cause severe corrosion of materials in the systems6-7. H2S in wet environment, such as in geothermal environment, can also cause severe corrosion damage in materials exposed to the environment, including hydrogen induced cracking (HIC), stress corrosion cracking (SCC) and sulfide stress cracking (SSC)8-14.