Atmospheric corrosion monitoring of steels in marine environments

Corrosion monitoring of Ni-containing steels has been carried out in laboratory and in natural marine atmosphere by Ac impedance method using two-electrode probes. In the laboratory investigation the probes were exposed to an accelerated test condition simulated the marine atmosphere. The impedance characteristics greatly change as the corrosion progresses. The steels covered with thick rusts showed impedance with an abnormally targe time constant in the low frequency and considerably large impedance in the range from the intermediate to high frequency. This can be interpreted by an equivalent circuit model in which a capacitance and a pore resistance of the thick rusts and diffusion impedance through the pore are considered. In the corrosion monitoring under the natural marine environment, the time of wetness (TOW) due to rainfall and the corrosion rate can be estimated at the initial stage of atmospheric corrosion from the values of high frequency (10 kHz) and low frequency (10 mHz) impedance, respectively. Meanwhile, after the probe surface is covered with thick rusts, the TOW by daily wet-dry cycles due to dew condensation can be also monitored. The low frequency impedance at 10 mHz was found to provide the pore resistance of the rusts, namely information on the protectiveness of the rusts.