CP permeable coating and corrosion under disbonded coatings

Corrosion under disbonded coatings is generally considered a ‘worst case scenario’ corrosion condition in the pipeline industry. Severe cases of localized corrosion issues including pitting, microbiological induced corrosion and stress corrosion cracking have often been linked to environments that are typically developed under disbonded coatings that shield cathodic protection (CP). It is believed in the industry that some coatings are partially permeable to CP currents, usually referred to as non-CP-shielding coatings, offering protection in case of coating disbondment allegedly by increasing the alkalinity of the local solution in contact with steel. However, this effect is not well understood and evidence on the matter is limited. In the present work, we aim to link the CP currents passing through coatings to the understand their actual effect on the polarisation of pipeline steel and the pH developed under disbonded coatings. Tests on selected coating and CP levels show that the magnitude of currents able to pass through coatings was insufficient to both polarise significantly the steel and generate a protective elevated pH environment. Therefore, we have not be able to identify an obvious advantage in corrosion protection provided by a coating permeable to CP.