With epoxy coal tar as the
coating material, the electrochemical
corrosion behavior of Q235 with different kinds of
bulk coating holidays has been investigated with EIS (Electrochemical Impedance Spectroscopy) in a 3.5vol％ NaCl aqueous solution. The area ratio of bulk coating holiday to total coating area of steel is 4.91％. The experimental results showed that at free corrosion potential, the corrosion of carbon steel with
disbonded coating holiday is heavier than that with broken holiday and disbonded & broken holiday with time; Moreover, the effectiveness of Cathodic Protection (CP) of carbon steel with broken holiday is better than that with disbonded holiday and disbonded & broken holiday on CP potential-850 mV (vs CSE). Further analysis indicated that the two main reasons for corrosion are electrolyte solution slowly penetrating the coating, and
crevice corrosion at steel/coating interface near holidays. The ratio of impedance amplitude (Z) of different
frequency to minimum frequency is defined as K value. The change rate of K with frequency is related to the type of coating holiday.
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