An Electrochemical Measurement for Evaluating the Cathodic Disbondment of Buried Pipeline Coatings under Cathodic Protection

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Steel pipelines are susceptible to corrosion by the action of corrosive substance in the environment and one of the most common failure modes in buried pipeline coatingis cathodic disbondment. Consequently, Electrochemical Impedance Spectroscopy (EIS) was used to assess the effect of three important parameters, the thickness of coating, the artificial defect and electrolyte type, on cathodic disbondment, according to ASTM G8. Experiments were done in 3.5 % wt NaCl, KCl and CaCl2 solutions at room temperature with different coating thicknesses (354, 483 and 1014μm) and several artificial defects diameters (3, 6 and 9 mm). Immersion time was 28 days and during this period, a -1.5 V cathodic potential (vs. SCE) was applied. Further, the EIS measurements were done in Open Circuit Potential (OCP). Investigations showed that a thick polyurethane coating was highly resistant to cathodic disbondment in intact areas and that coating thickness increases caused decreases in the rate of cathodic disbondment in areas with defects. On the other hand, the evaluation on artificial defects showed disputable results, though with no logical correlations between artificial defects’ diameters and any disbonded area individually. In addition, assessment on electrolyte type showed the rate of cathodic disbonding might be classified after two parameters: solubility and mobility of ions. So, the disbonded area was dependent on the cation type in the electrolyte, and the anion diffusion had less effect in the cathodic disbondment process. Furthermore, the comparison between the cations and the anions in solution showed that the most important coating disbondments occurred when cations / anions are in a 1:1 ratio.