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a b s t r a c tHerein, the redox competition mode of scanning electrochemical microscopy (RC-SECM) has been applied to in situ monitor the local reactivity arising from a circular holiday operated in a painted metal. The metal-coating system consisted of a carbon steel substrate coated with an epoxy-polyamine polymeric film containing glass flakes as pigment. The present work demonstrates the possibility to use RC-SECM for investigation of the corrosion reactions occurring when protective coatings are applied on a metal surface without the addition of a redox mediator to the experimental system. Oxygen reduction was employed to monitor the reactive metal-polymer system, though the onset of a redox competition for this redox species between the SECM tip and the bare metal inside the holiday could be found in certain conditions. Whether attack predominated inside the holiday or the system became non-reactive depended on the composition of the test electrolyte, with borate ions acting as corrosion inhibitor.

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Operation of the SECM in feedback mode over a coated metal allows changes in the state of the surface to be monitored using dissolved oxygen in the test environment as redox mediator. The system investigated consisted in mild steel samples coated with a polyurethane film in which a defect was deliberately introduced into it. Accurate approach curves under negative-feedback condition were obtained using a platinum microelectrode over regions of the intact coating. Imaging experiments were also carried out over the artificial defect on the coating. Under these conditions, corrosion of the exposed metal substrate at the defects could be monitored.

No abstract