The corrosion stability of pipeline steel 20 in the as received state (r) and also after quenching (q), tempering (t), and normalization (n) is studied in acetate buffers saturated with CO2 and containing 0–0.85 M NaCl. The polarization measurements and open-circuit tests showed that the corrosion resistance decreases in the following series: t ~ n > q > r. The lower corrosion rates for t and n samples are associated with the lower content of pearlite and its uniform distribution throughout the microstructure. It is shown that the preferential dissolution of ferrite during corrosion is accompanied by the pearlite component exposure and the increase in cathodic currents, which favors acceleration of corrosion.