Corrosão do aço API5L X65 por CO2 : propriedades dos filmes de produtos de corrosão em pressões subcríticas e supercríticas

• Autor(a) principal: Rodrigues, Tania Regina Schmitz de Azevedo
• Data de Publicação: 2016
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Biblioteca Digital de Teses e Dissertações da PUC_RS
• Texto Completo: http://tede2.pucrs.br/tede2/handle/tede/7060
• topic: AÇO - CORROSÃO; ENGENHARIA DE MATERIAIS; ENGENHARIAS

This study investigates the influence of pressure on the formation of corrosion product scales on API 5L X65 steel used in the oil and gas industry. Experiments were performed in humid CO2 and water saturated with CO2 environments at 50ºC for periods of 168 and 360 hours in 2 MPa and 5 MPa subcritical conditions and 10 MPa, 15 MPa and 20 MPa supercritical conditions. The thermodynamic model of Duan was used to obtain information about CO2 solubility in water, the water concentration present in the humid CO2 and the pH for the experimental conditions used in this work. Corrosion rates were determined by weight loss tests, and the scales were characterized by scanning electron microscopy, electrochemical measurements and X-ray diffraction. The results showed that for the 168 hours exposure time, the size of iron carbonate crystals formed in CO2 saturated water was bigger in relation to the ones formed in humid CO2 environments, with the exception of the experiment performed at 20 MPa. The scales formed in 168 hours and 360 hours exposure time, in both environments and pressures, showed typical morphology of iron carbonate. It was also observed that the films tend to be more compact with increased immersion time, but not necessarily more protective. Corrosion rates were classified as severe according to NACERP- 07-75 for both subcritical to supercritical pressures in CO2 saturated water environment and for both exposure times. In the case of 168 hours of exposure and CO2 saturated water environment, the highest corrosion rate occurred at 15 MPa. Already for 360 hours exposure times, performed at supercritical pressures, the corrosion rates increased with increasing pressure and the higher corrosion rate values were observed in water saturated with CO2. Electrochemical impedance tests showed no significant differences in resistance to polarization in subcritical pressures to 168 hours for both environments. As for supercritical pressures, higher polarization resistance was 15 MPa for also both environments. As for supercritical pressures, higher polarization resistance was 15 MPa in CO2 environment. Potentiodynamic polarization tests indicated that at 168 hours of immersion, there was a reduction in the rate of dissolution of the films formed at 10 MPa and 20 MPa in both environments. For the 360 hours of immersion, the corrosion potential was most active at 5 MPa for water saturated with CO2, with no significant differences of humid CO2 environment.