Fluctuations of the corrosion potential and current were analyzed in the time domain for type A516-70 carbon steel immersed in 0.5 M NaHCO3 or 0.1% NaNO2 solutions containing NaCl at various concentrations. The fluctuations of corrosion potential and current observed for pitting corrosion showed a pattern distinctly different from that for crevice corrosion of this type of carbon steel immersed in the solutions described. Sharp potential and current fluctuations with amplitudes approaching several millivolts and several lens of microamperes, respectively, were detected during the pitting process. The fluctuations of the potential produced during pitting occurred around the corrosion potential (-250 to -280 mV vs. SCE. As crevice corrosion proceeded in the solution containing 0.5M bicarbonate as inhibitor and chloride ions as aggressive ions, the corrosion potential slowly decreased from -250 similar to -280 mV to about -480 mV. The corrosion current increased simultaneously. Then the corrosion potential dropped sharply to around -760 mV and remained at this value, while the coupling current from working electrode 1 to working electrode 2 through the solution rapidly decreased. In the case of crevice corrosion on the carbon steel in the nitrite base solution. the chloride ions caused a drop of potential into the potential range of -370 to -470 mV. The potential then returned to the original corrosion potential for the dilute chloride solution. and remained in this potential range for the chloride solution of moderate concentration, or declined to the range of -530 to -570 mV due to the active crevice corrosion induced by the chloride ions at high concentration. The magnitude and number of the current fluctuations increased with the concentration of chloride ions. The time dependence of the corrosion potential and current provided information on the mechanisms of crevice corrosion and inhibitors.