Understanding factors affecting corrosion under disbonded coatings

This paper present an overview of findings over the past several years on corrosion under disbonded coatings (CUD) and major influencing factors affecting this behaviour. The effect of cathodic protection (CP) current shielding by disbonded coating film on CUD has also been assessed and quantified using a method developed for precisely measuring CP current penetration. The main results indicate that all tested commercial coatings, with thicknesses typically used in the field, would shield CP currents in the intact condition. Although it has been found that some thin coatings allow some CP current penetration and that a slight increment of CP current through the coating was observed with the ageing of coatings, unfortunately for the types of coating assessed none was able to create a high pH condition sufficient to protect the pipeline steel surface under disbonded coatings. Understanding of major factors affecting CUD was facilitated by an electrochemically integrated multi-electrode array that was used as a unique tool for probing localised electrode processes evolving and propagating dynamically on pipeline steel surfaces under the effect of cathodic shielding and coating disbondment. Taking the advantage of the high temporal and spatial resolution of the electrode array method, various laboratory devices and probes have been designed to simulate a disbonded coating in order to assess the penetration and contribution of CP currents through coating films. Disbondment geometry, pH and soil saturation status have been shown to be key parameters affecting CUD. For instance, it was found that in non-saturated soil CUD behaviour changed significantly with coating disbondment gap size. These findings have been compared with those reported in the literature and in industry reports.