The effect of oxygen content on the corrosion reactions inside a holiday in a polymer-coated metal substrate was studied by SECM without adding a redox mediator. The system was mild steel coated by polyurethane in KCl. By selecting different values for the potential applied to the ultramicroelectrode tip, local concentrations of species involved in the degradation process are monitored, namely Fe(II) ions, hydrogen peroxide and oxygen. The results show a variation in both the shape and the magnitude of the scan lines measured over the holiday. A critical oxygen concentration was found below which the corrosion reaction is not observed
