Studying the Electrochemical Corrosion Behaviour Inside Coatings in Cathodically Polarised Pipelines – DAC and CP Shielding
Authors: Mishra Rituraj 
Published: November 2023

Every coating system has finite life and eventually degrades, allowing oxygen, water, and chemicals to reach the substrate. The deterioration of the coating is attributed to cathodic disbonding whose origins are related to one or a combination of the following three mechanisms: (i) oxide dissolution at the metal/coating interface, (ii) mechanical interfacial failure and (iii) disruption of the polymer metal interface by hydroxide. Excessive heat can cause pipe coatings to soften, flow, or become cracked and brittle. Soil stresses due to backfill weight, soil-induced shear stress applied to the coating due to thermal expansion, pipe settlement or soil settlement also add to it.

Over the past decade, the shielding of cathodic protection (CP) produced by disbonded coatings, commonly referred to as cathodic shielding, has gained significant attention in the pipeline industry. Problems such as stress corrosion cracking (SCC), pitting or crevice corrosion, as well as microbial induced corrosion (MIC) have been reported to be associated with the coatings’ shielding effect.

This paper describes outcomes of brain storming session, why-how analysis on the critical parameters impacting corrosion inside coatings and necessary mitigation and control measures. This work provides an overview and brief discussion on the current understanding of cathodic shielding and non-shielding coatings, and the possible contributions of disbonded coatings to corrosion. This paper also presents a critical analysis and clear interpretation of site based case studies of the key factors viz applied true potential, solution resistivity, holiday size and the direction of flow of electrolyte governing the alkaline generation and the oxygen consumption rate at the holiday leading to Differential Aeration Corrosion (DAC). Major factors that have been reported in the literature affecting the cathodic shielding and corrosion under disbonded coating include: (i) the pH increment at the interface of the coating/steel under disbondment (ii) the electrolytic permeability of the coating; (ii) the true polarisation potential achieved by the pipe.

Results demonstrated that, in the early stage of corrosion of steel, CP cannot reach the crevice bottom to protect steel from corrosion due to the geometrical limitation. Corrosion of steel occurs preferentially inside crevice due to a separation of anodic and cathodic reaction with the depletion of dissolved oxygen in the crevice solution leading Differential Aeration Cell (DAC) formation. The main role of CP in mitigation of sequential corrosion of steel in crevice under disbonded coating is to enhance the local solution alkalinity. With the increase of distance from the open holiday, a high cathodic polarization is required to achieve appropriate CP level at crevice bottom. A potential difference always exists between the open holiday area and inside crevice, reducing the CP effectiveness.

Time is now we approach the problem the right way and extract information from data to take decision on the effective approach towards the challenge being faced by the Industry.