Objective The disbondment of high-insulation 3PE anti-corrosion coatings on buried steel pipelines leads to “cathodic protection shielding”, a phenomenon that results in metal corrosion of pipelines in local environments, posing a challenge to the safe operation of oil and gas pipelines. A comprehensive evaluation of corrosion development patterns for pipelines within disbondment zones, along with an investigation into the potential distribution characteristics and corrosion behavior of pipelines covered by disbonded anti-corrosion coatings in various soil environments, is conducive to formulating effective strategies for pipeline corrosion protection.

Methods A self-made experimental device with adjustable disbondment thickness for anti-corrosion coatings based on X80 steel was used to perform cathodic protection potential tests on metal at various positions within disbondment zones. Utilizing scanning results from electron microscopes and laser scanning confocal microscopes, the cathodic protection potential distribution along the pipeline metal in the direction of disbondment extension, as well as the localized micro-area corrosion characteristics at different positions within the disbondment zones, were examined across four soil solution environments: coastal saline soil, inland saline-alkali soil, red clay, and meadow soil.

Results The cathodic protection potential of the pipeline metal was observed to shift to be more positive with increasing depth within the disbondment zones. Effective cathodic protection was primarily concentrated near the damage opening of the anti-corrosion coatings. The cathodic protection potential declined rapidly as the depth of the disbondment zones increased, eventually reaching deeper positions where the pipeline approached a natural corrosion state and cathodic protection shielding occurred. The cathodic protection potential distribution and the effective protection range within the disbondment zones were influenced by the thickness of the liquid film present and the salt content (soil resistivity) of the soil water. Coastal saline soil, characterized by high salt content and low resistivity, was found to facilitate greater penetration of the cathodic protection current. Additionally, the space of the disbondment zones exerted a limiting effect on the ion diffusion and migration processes, significantly impacting the distribution of current and the effective protection range.

Conclusion Comparing the corrosion behavior of pipelines in different soils reveals a descending order of effective cathodic protection range as follows: coastal saline soil, red clay, inland saline-alkali soil, and meadow soil. Buried pipelines are particularly vulnerable to localized severe corrosion in coastal saline soil and red clay, whereas they predominantly experience uniform corrosion in inland saline-alkali soil and meadow soil. Moreover, the corrosion rate of pipelines within disbondment zones is significantly higher than that observed at exposed positions where the anti-corrosive coatings are damaged.