CP currents passing through coatings and their effect on corrosion under disbonded coatings

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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. In order to confirm the non-CP-shielding behaviour of some selected pipeline coatings. We recently developed an accuracy-enhanced methodology to measure small currents through coating films. With this method we were able to measure the conduction of ionic CP currents through several coatings and quantify the associated changes in the local pH under simulated disbonded coating. In the present work, we further enhance the methodology and simulation conditions to better link the CP currents passing through coatings to the actual effect on the polarisation of steel pipeline and the pH developed under disbonded coatings. Tests on selected coatings combining several disbondment geometries 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 any coating that could be called non-shielding coating in conventional industry terms.

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