Characteristics Of Electrochemical Impedance Spectroscopy In Deterioration Process Of Organic Coating

Organic coating is one kind of the most effective and most economic materials to prevent the metal from damage. The metallic corrosion is closely relative with the ability of organic coating. The anticorrosion ability of coating is determined by the bond strength and anti-penetration ability to water and other eroding ions. This dissertation chiefly adopts electrochemical impedance spectroscopy (EIS) method combine with artificial neural network(ANN) to study the deterioration rule of coating under wet-dry conditions and immersion state. The main work of this paper including:(1) Corrosion behaviour of iron oxide red alkyd (IORA) primer is researched at the immersion state by EIS method.The changing rule of OCP on time is researched. OCP moves to negative value at the early immersion stage which indicates that water molecule has reached metal surface.Evolution of impedance model as immersing time is studied. At the early stage of immersion, pure capacitance characteristic is not presented in EIS plot of 40μm coating/metal system. Time constant of EIS increases as time prolonging and diffusion impedance appears at late stage of immersion. But for 60μm coating system, EIS can show pure capacitance characteristic at the early stage.Four kinds of impedance models are adopted to fit EIS plot. The same rule of corrosion model exists in the similar corrosion stage of different thickness coating. And the corrosion behaviour of coating/metal system is characterized by the changing trend of Cc, Rc, Cdl and Rct on time.A new characteristic parameter is selected from logf- |Z| plot which shows a good correspondence with EIS and coating picture for two different thickness coatings. This method will not be limited by other conditions provided logf-|Z| plot is made. According to changing rules of all parameters above, a comprehensive map is drawn to signify coating deterioration process.(2) Electrochemical method is adopted to research the corrosion behaviour of coating under wet-dry conditions.Under wet-dry conditions, the changing rule of EIS for 40μm coating is researched. According to characteristic of EIS under wet-dry conditions, coating state can be divided into three states which are metastable state, transition state and stable state. The characteristics of EIS under these three states are studied, and then equivalent circuit model is established, the parameters- Cc, Rc, Cdl and Rct are analyzed comparing with those at the immersion state.(3) Self-organization map (SOM) artificial neural-network is adopted. The parameter of changing rate of impedance gained from bode plot is regarded as the import sample for training and studying. Coating deterioration process of wet-dry conditions and immersion state are researched by this method. The same results can be gained by EIS method and SOM network, which proves that SOM method can be used as a good assisted tool to research the deterioration process of organic coatings.