Investigating the separation of anodic and cathodic defects in organic coatings applied on metal substrates. An experimental contribution

When artificial defects are produced on organic coatings applied to metals it is common to observe a separation of anodic and cathodic defects. An investigation is presented in which real defects are imitated by two disk electrodes of the same metal, embedded in insulating matrix and electrically connected. A variety of measurements was performed to characterise the corrosion of this twin electrodes set-up: SVET, micro-distributions of pH and dissolved O2, polarisation curves, galvanic current and video monitoring. The tendency for anodic and cathodic processes to proceed in separated electrodes was observed. It became clear that a strong increase in local pH on the cathode renders it passive to metal oxidation while the other electrode corrodes. It is possible to explain the maintenance of the separation in time but what triggers the separation at the beginning of immersion remains unidentified.