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Overview of latest advances in measuring and understanding cathodic protection current permeability by organic coatings

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Shielding of cathodic protection (CP) by disbonded coatings is considered a “worst case scenario” for external corrosion in the pipeline industry. It has been hypothesized that if coatings were partially permeable to CP, the imbalance between cathodic and anodic reactions would induce a high pH environment under disbonded coatings and that would impede corrosion. However, direct experimental evidence of this process has been limited. Moreover, this line of reasoning is often inversed incorrectly assuming that the only reason for the presence of a high pH environment under disbonded coatings is their partial permeability to CP current. 

The recent development of an electrochemical self-validating technique to measure the relatively small ionic currents that could permeate through defect free coatings has shed a new light on the topic. It has been found that most of the commercial pipeline coatings tested in unaged conditions present a CP shielding behavior. Accelerated hydrothermal ageing of fusion-bonded epoxy (FBE) coatings was performed to study the effect of water absorption. Ageing resulted in major plasticization, which increased the conduction of CP current through FBE coatings. However, FBE seems to be partially permeable to H+ and/or OH-, which reduces its ability to form and retain a high pH environment under disbonded coatings. In conjunction, these results thus indicate the extremely limited capability of commercial pipeline coatings to prevent corrosion in case of disbondment. 

Product Number: 51320-14685-SG
Author: Facundo Varela, Mauricio Latino, Yongjun Tan, Maria Forsyth
Publication Date: 2020
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An experimental setup was designed to investigate the cathodic protection (CP) penetration into coating disbondment. Embedded detection probes underneath the disbondment enabled pH and steel potential measurements without disturbing the environment inside the disbondment.
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Here we would like to elaborate on corrosion risk associated with coatings that shield cathodic protection.
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