| Organic coatings have been widely applied to corrosion protection in everyday life for many years. Both Electrochemical impedance spectroscopy (EIS) and scanning Kelvin probe (SKP) are used extensively to study corrosion protection of metal by organic coatings and to characterize the deterioration of coatings. EIS can only give an average response for the entire surface while we can obtain localized electrochemical information beneath organic coatings with SKP. They can bring out the best in each other.The aim of this paper is to combine SKP with EIS to study the progress of breakdown of organic coatings on a steel substrate both under full immersion and cyclic wet-dry conditions. At the same time open potential technique and observing images of the samples are used.The corresponding experimental results are listed as follows.(1) Generally, with immersion time increasing, the protection ability of the organic coating reduces, the impedance of the coatings drops gradually, the Volta potential obtained by SKP becomes less flat, the open potential becomes lower. The whole deterioration process can be divided into three main stages: water penetrating period, the corrosion occurring period, the corrosion developing period.(2) The characteristic parameters Vmin and Vmax- Vmin obtained by SKP are used successfully to assess the degradation of organic coatings.(3) Under the cyclic wet-dry conditions, as the cyclic wet-dry going, the impedance of the coatings drops gradually and the Volta potential becomes less flat. Prolonging with wet-dry cycles, both the impedance and the Volta potential of the samples show periodic undulation. But after the degradation both of them fall first and then hoist in the dry course.(4) The responses of EIS and the scanning Kelvin probe are consistent with the progress of the degradation of organic coatings. Combing SKP with EIS are better than using only one technique to study the progress of breakdown of organic coatings on a steel substrate.
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