Recent technological advances in the field of pipeline steel have underpinned the basis of evolution of electrolyte infiltration and electrochemical impedance for identifying specific corrosion processes that take place in a disbonded area. Past publications have shown that the outer coatings protect the pipeline steel from corrosion processes while transporting goods such as petroleum, gas and coal over a long distance. However, most studies have shown that the coating loses adhesion, are subject to mechanical damage and age during their service life. Due to such degrading processes, such steel pipes tend to produce bubbles and/or have warping defects on the coatings.
In the past, electrochemical protection techniques have been employed to address this challenge: however, it proved to be technically difficult for the cathode current to reach the bottom of the peeled coating, as the metal surface was not benefiting from the effective cathodic protection. As such, steel pipeline protection has remained a daunting task and also a great obstacle in the practical application particularly in the area under the disbonded coating. To address this, researchers from the Institute of Chemical Analysis in China: Dr. No Zhao, Dr. Yunyan Peng, Professor Jun Li, Dr. Binan Shou, in collaboration with Dr. Hongxia Wan at the China University of petroleum and Dr. Chao Liu at the University of Science and Technology Beijing, proposed a novel approach on the influence of crevice thickness on corrosion behavior of API X80 steel under disbonded coating in acid soil environment. Their work is currently published in the research journal Electrochemical Science.
In their approach, the researchers focused on various factors influencing the corrosion under disbonded coating. In particular, the authors studied the corrosion behavior of X80 steel by anodic polarization, electrochemical impedance spectroscopy (EIS) and scanning electron microscope (SEM). In essence, the crack model with different thicknesses was designed to simulate the corrosion behavior. The researchers then used polarization curves and electrochemical impedance spectroscopy to analyze the samples having different crevice thicknesses. The corrosion morphology was observed by SEM. Finally, the corrosion theory was analyzed with respect to the crevice thickness.
The authors reported that the open–circuit potential moved negatively under the coating, and the potential in the bottom region was the lowest. Moreover, their analysis pointed out that the oxygen concentration at the bottom of the disbonded coating decreases, whereas the accumulation of Clˉ and H⁺ at the bottom of the gap enhanced the sensitivity towards stress corrosion. Finally, researchers established that the corrosion effects at a thickness of 150μm were more serious and the pitting appeared due to the enhanced amounts of O₂ and H⁺ migrating in the disbonded area.
In summary, the study presented a novel approach developed with the aim of revealing X80 steel with the passivation of the anodic polarization in the soil simulated solution for the crevice thickness. Based on the presented approach, the researchers proved that passivation decreased on increasing thickness, and the reaction was transformed into anodic dissolution. In a statement to Advances in Engineering, the authors highlighted the importance of their corrosion theory focusing on the area under the disbonded coating was been developed with considerations of the corrosion medium, cathodic protection potential and microorganisms, thereby expanding their applications in different fields, especially in soil environments.
Bo Zhao, Hongxia Wan, Yunyan Peng, Chao Liu, Jun Li, Binan Shou. Influence of Crevice Thickness on Corrosion Behavior of API X80 Steel under Disbonded Coating in Acid Soil Environment. International Journal of Electrochemical Science, volume 16 (2021) 150808.