Recent Progresses in Understanding Pipeline Coating Degradation

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The durability and performance of organic coatings, which are widely used for mitigating corrosion of high pressure oil, gas and water pipelines, are known to be affected by cathodic protection (CP), mechanical stress and the corrosive environment. This paper presents our research findings on the impact of CP and the environment on organic coatings under simulated underground pipeline conditions. Coating’s corrosion resistance, disbondment and electrochemical properties have been assessed using electrochemical, surface analytical and computational modelling techniques such as the electrochemical impedance spectroscopy (EIS), optical microscopy and scanning electron microscopy, X-ray computed tomography and finite element analysis. In situ and ex situ scanning electron microscopy was employed to trace and characterise coating defects from the very early stages of micro shear bands initiation to the formation of full coating cracks. Finite element analysis has been employed to understand the influence of strain on the initiation and propagation of coating defects by modelling stress distributions. A correlation has been found between the applied strain levels and the corresponding coating resistance and coating capacitance values. The strain distribution and shear stress distribution patterns obtained using the finite element analysis were used to explain typical features observable in EIS data based on the formation of electrolytic pathways through unstrained and strained coatings. This approach has been consolidated into a two and a three dimensional model proposed to explain the electrolyte movement in a coating impacted by applied mechanical strain and environmental exposure. On the other hand, the effect of CP potential on cathodic disbondment of a defective coating has been investigated by monitoring the coating disbondment processes during the exposure of coated electrodes to a corrosive solution. The monitoring of coating disbondment was achieved by in situ measurement of local electrochemical impedance and direct current distributions over a multi-electrode array surface under various levels of CP potentials as well as under open circuit potential (OCP). The effects of CP potential and the environmental conditions on the initiation and propagation of coating disbondment have been quantified based on changes in local electrochemical impedance and current distribution maps. The rates of cathodic disbondment have been found to be highly dependent upon the applied CP potential. This result indicates that the evolution of hydrogen likely played an important role in accelerating cathodic disbondment of the coating, and that the ‘critical potential’ at which the hydrogen evolution becomes a significant and dominant electrochemical reaction is important for cathodic disbondment of coatings. It was also found that once the cathodic disbondment initiated under very negative CP potentials, the disbonded distance would spread at an almost constant rate. These results suggest that a practical method of controlling coating disbondment is to ensure that the applied CP potential is below the ‘critical potential’


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