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Corrosion Special Topical Papers |
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Protective Coatings: Performance Evaluation and Life Prediction*Dr. Jianhai Qiu
Electrochemical Impedance Spectroscopy (EIS)It has been recognized for many decades that the corrosion protection by organic polymeric coatings is related to the changes in the dielectric properties of the polymer paint film. The response of a coated metal to a small AC signal at certain frequency can be described by an equivalent circuit model as shown in Fig.3 below:
where Rs is the electrolyte resistance, Rpore is the pore resistance in a coating system, Rt is the charge transfer resistance, Cdl is the double layer capacitance at the metal/coating interface and Cc is the coating capacitance. Corrosion underneath a coating system can be described by the charge transfer resistance of Rt and the double layer capacitance Cdl, while the coating performance can be described by the coating capacitance and the pore resistance. With an increase in the permeability of a coating, the conductive paths in the coating will increase and this will lead to the reduction in the pore resistance. By measuring the changes in coating's capacitance, one can calculate the water-uptake (by volume) in a coating system [2]:
where Ct is the coating capacitance at time t and Co is the initial coating capacitance. |
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