An electrochemical sensor for monitoring the atmospheric-corrosion of iron and steel

IT IS WELL KNOWN that atmospheric corrosion is an electrochemical process. From this point it is reasonable to predict corrosion behaviour, and to monitor the atmospheric-corrosion rate by electrochemical techniques. In this paper, two types of electrochemical probes were used: galvanic cells consisting of steel and copper electrodes (Fe/Cu), and galvanic cells consisting steel electrodes alone (Fe/Fe). The galvanic currents of the former were monitored by a zero-resistance ammeter. However, the AC impedance measurements we re on ly applied to the latter. The results of 110 days' field testing of the Fe/Cu couple indicate that a linear quantitative relationship was observed between the probe current and the weight-loss data of the atmospheric-corrosion coupons. The optimum thickness of steel and copper sheets was found to be 0.5mm, and 0.05mm for the spacing between the Fe/Cu plate arranged in the probe. In this case, a correlation based on the ratio of the weight loss obtained from the probe and the coupon in the field test was determined as 0.25. AC impedance measurements were applied only to the study of the corrosion of mild steel in a thin-film electrolyte under the cycling of processes of wetting and drying. The results showed that the polarization resistance (R-p) of mild steel varies significantly with the relative humidity (RH) of the atmosphere, provided it lies above a limit of 80%; below this, the corrosion rate was negligible. The method used in this study proved to be an effective means of monitoring the atmospheric-corrosion rate.