PROBLEM TO BE SOLVED: To grasp fluctuations in coupon AC current density within a measurement time and perform precise AC corrosion risk measurement and evaluation.SOLUTION: Coupon AC current density Iis obtained from the measured value of a coupon current measured at each set sampling interval in a unit measurement time that is equal to one cycle of a commercial frequency. Measurement divisions in which a measurement time Tis divided into a plurality of equal divisions are set. An intra-division maximum value Iof the coupon AC current density is extracted for each measurement division. A combination of the intra-division maximum value Iwith coupon DC current density Iobtained from the waveform of measured values constituting the intra-division maximum value Iis obtained for each measurement division. Evaluation is made as to whether the combination (I,I) meets the coupon DC current density and a cathode corrosion protection standard indexed by the coupon AC current density.SELECTED DRAWING: Figure 1
