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.