Study of al2o3 sol‐gel coatings on x20cr13 in artificial north german basin geothermal water at 150 °c

Abstract

Al2O3 has been widely used as a coating in industrial applications due to its excellent chemical and thermal resistance. Considering high temperatures and aggressive mediums exist in geothermal systems, Al2O3 can be a potential coating candidate to protect steels in geothermal applications. In this study, γ‐Al2O3 was used as a coating on martensitic steels by applying AlOOH sol followed by a heat treatment at 600 °C. To evaluate the coating application process, one‐, two‐, and three‐layer coatings were tested in the artificial North German Basin (NGB), containing 166 g/L Cl−, at 150 °C and 1 MPa for 168 h. To reveal the stability of the Al2O3 coating in NGB solution, threelayer coatings were used in exposure tests for 24, 168, 672, and 1296 h, followed by surface and crosssection characterization. SEM images show that the Al2O3 coating was stable up to 1296 h of exposure, where the outer layer mostly transformed into boehmite AlOOH with needle‐like crystals dominating the surface. Closer analysis of cross‐sections showed that the interface between each layer was affected in long‐term exposure tests, which caused local delamination after 168 h of exposure. In separate experiments, electrochemical impedance spectroscopy (EIS) was performed at 150 °C to evaluate the changes of coatings within the first 24 h. Results showed that the most significant decrease in the impedance is within 6 h, which can be associated with the electrolyte penetration through the coating, followed by the formation of AlOOH. Here, results of both short‐term EIS measurements (up to 24 h) and long‐term exposure tests (up to 1296 h) are discussed.