Analysis of the electrochemical noise for localized corrosion of type A516-70 carbon steel

Fluctuations of the corrosion potential (E-corr) and the corrosion current were analyzed in the time domain for type A516-70 carbon steel (UNS K02700) exposed to 0.5 M sodium bicarbonate (NaHCO3) solutions containing sodium chloride (NaCl) of different concentrations. Fluctuations of E-corr and current observed for pitting-type corrosion had a distinctly different pattern from those associated with crevice corrosion for this type of carbon steel exposed to these solutions. Sharp potential and current fluctuations with amplitudes approaching several millivolts and several tens of microamperes, respectively, were detected during the pitting process. The fluctuation of potential produced during pitting occurred around E-corr, which was approximate to -250 mV(SCE) to similar to -280 mV(SCE). As crevice corrosion proceeded, E-corr slowly decreased from -250 mV to similar to -280 mV to similar to -480 mV. The corrosion current increased simultaneously. Then, E-corr dropped sharply to values similar to -760 mV and remained at this value, while the coupling current from Working Electrode 1 to Working Electrode 2 rapidly decreased. Then, the current flowing direction reversed, and the current finally decayed to zero. The time dependence E-corr and current was explained by the crevice corrosion mechanism.