交流インピーダンス法を用いた屋外環境における鋼の腐食速度の連続測定

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A new type of corrosion sensor was successfully applied for the monitoring of the atmospheric corrosion rate of steels. The sensor consisted of a steel ring and a pin located concentrically with a narrow gap of plastic insulator. Both high frequency (10 kHz) and low frequency (10 mHz) AC impedance data were retrieved to obtain solution resistance and polarization resistance, respectively. Corrosion rate was estimated from the polarization resistance data. The monitored data indicated that the corrosion rate of steel was higher at night and rainfall than in the sunny condition of daytime. The data suggested the corrosion rate of steel was strongly affected by dewing and relative humidity (RH). As for the effect of sea salt deposition, somewhat complicated corrosion tendency was obtained. In the case when the salt deposition was small, the corrosion rate increased with increasing amount of deposited salt. At higher deposition levels of the salt of 0.1 and 0.01 kg/m2, however, different trend in corrosion was observed. At low RH a steel sample with higher salt deposition (0.1 kg/m2) showed higher corrosion rate, while under higher RH conditions the sample with lower deposition (0.01 kg/m2) gave higher value. The contradictory phenomena can be interpreted on the bases that the water film thickness changes with the salt deposition density and that the corrosion rate is influenced by the water film thickness. By thermodynamical calculation, it is estimated that the film thickness having maximum corrosion rate ranges from several tens to one hundred μm.The corrosion loss estimated by the surface roughness using a laser microscope was compared with the cumulative values of impedance measured on the sensors. A linear relationship was observed between two values. It indicates that the monitoring system by means of the concentric-ring sensor is effective in the estimation of atmospheric corrosion rate of steels.