DEFECT AREA DETERMINATION OF ORGANIC COATED STEELS IN SEAWATER USING THE BREAKPOINT FREQUENCY METHOD

The breakpoint frequency method, which allows determination of the electrochemically active area of a coated metal in seawater, is described. A computer model is used to explain the basis of the breakpoint method, and the model is compared to impedance and visual data from epoxy-coated steel panels in ASTM artifical seawater with and without an intentional defect of known area. The breakpoint frequency method was found to be extremely useful in determining the electrochemically active area of coated steel in seawater. The equivalent circuit model used in this analysis was found capable of fitting actual data on coated steel panels with and without an intentional defect. A correlation was obtained between the breakpoint frequency and visually estimated electrochemically active area on epoxy coatings of a variety of thicknesses. This method offers a simple alternative to determination of defect areas via the use of the pseudocapacitance from difficult-to-analyze low-frequency impedance data. This approach also can detect the beginnings of coating breakdown long before visual indications are present.