This research explores electrochemical correlations between heat-affected zones (HAZs), produced by Gleeble© thermal simulation cycles, and their passivation behavior in bicarbonate–carbonate solutions. The investigations were carried out in comparison with the base API-X100 steel in naturally aerated 0.5 g/L chloride solutions, containing a matrix of bicarbonate and carbonate concentrations at 298 K. The slow 0.05 mV/s potentiodynamic scans revealed that the passivation onsets earlier in proportion with the bicarbonate concentrations, possessing accordingly greater immunity against chloride ions on HAZs cooled at 10 and 60 K/s from 1223 K peak temperature. The HAZs cooled at 10 K/s, particularly in concentrated carbonate solutions, showed the most stable passivation, and the HAZs cooled at 60 K/s showed the lowest cathodic activity regardless of the chemical conditions. −0.1 V, versus saturated calomel electrode, potentiostatic currents and the open-circuit potentials decreased and increased, respectively, with the carbonate concentration, confirming with the main potentiodynamic polarization results, and a corresponding passivation behavior of the HAZs, regardless of a bicarbonate–carbonate condition, was relatively similar. In contrary to the 60 K/s HAZs, the 10 K/s HAZs showed evidence of growing passive films with time and high charge-transfer resistance, as measured by the EIS tests.