Abstract:

Deterioration process of the organic coatings on carbon steel substrate during exposure to 3.5% NaCl solution was investigated using the electrochemical impedance spectroscopy(EIS) and scanning Kelvin probe(SKP). According to the EIS and SKP responses, the entire deterioration process can be divided into three main stages shown as follows. Stage Ⅰ was the water penetration into coatings. In this stage the EIS plots showed single semi-circel with the impedence of the coatings drops gradually and the Volta potential obtained by SKP keeps decreasing but the potential distribution remaines flat. Stage Ⅱ was the corrosion initiation under coatings. In this stage the EIS plots changed from a single loop to double capacitive loops with the resistance of the coatings decreased sharply, at the same time the surface Volta potential difference of the metal surface increased rapidly. Stage Ⅲ was the corrosion development and extension.The Nyquist plots of the electrochemical impedence showed a capacitive loop in the HF and a oblique line in the LF, with the potential difference remained in the high level. It was confirmed that the novel combination of EIS and SKP can complement and perfect each other to get more exact and credible details in studying the deterioration process of the organic coatings.

Key words: Organic coating, Deterioration, Electrochemical impedance spectroscopy(EIS), Scanning Kelvin prove(SKP)