Studies of Composition and Structure of CO2 Corrosion Film of X56 Steel in the Medium of CO2 and Salt Water

The X56 steel samples was corroded in the medium of salt water solution at the conditions of CO2 partial pressure Pco2 0.5 to 2.0 MPa. temperature 80℃ and flow rate 1.4m/s. Corrosion weigh loss, composition and strutcture, morphology and phase of corrosion films of the samples were investigate by SEM, EDS, XRD and XPS. The results indicated that the corrosion degree was accelerated with increasing Pco2. The intense localised corrosion occurred on the surface of samples. The corrosion films mainly comprised of FeCO3 and complex phase products (Fe. Ca,...)CO3. There exists serious pitting on the metal substrates under the corrosion film. The theoretic and experimental analyses indicate this is caused by existed micropores or micro holes in films, which have the function of mass transportation.