Abstract: In this work, pitting corrosion of carbon steel in sodium chloride solution induced by a precipitation inhibitor, cerium(III) ions, was investigated. Upon in situ 3D microscope observation, pitting corrosion was immediately seen on carbon steel in sodium chloride solution containing cerium (III) ions; however, no local corrosion occurred in solutions containing zinc ions. The mechanism of precipitation film formation was investigated using electrochemical impedance spectroscopy and potentiodynamic polarization curves. Results showed that the inhibition efficiency increased after applying a weak cathodic polarization on the electrode in the sodium chloride solution containing cerium(III) or zinc ions. Simultaneously, anodic polarization was provided as a comparison, demonstrating the unprofitability of the accelerated anode process to the precipitation film formation. A minimal change of film resistance after anodic or cathodic polarization in sodium chloride solution was also shown. By applying cathodic polarization, the oxygen reduction reaction was accelerated in the cathode areas on the metal surface, increasing the local OH^- concentration that promoted the precipitation reaction. This indicates that the corrosion resistance of the precipitation film is related to the local concentration of hydroxyl ions. Additionally, in the polarization curve measurements, the E_corr of the electrode in Zn²⁺-containing sodium chloride solution is more negative than that in sodium chloride solution; however, the E_corr of the electrode in Ce³⁺-containing solution is more positive than that in sodium chloride solution. A galvanic mechanism of local corrosion was explored to explain the pitting corrosion on carbon steel induced by cerium(III) ions. Before the measurement, one electrode was immersed in inhibitor-containing sodium chloride solution for a certain time to simulate the places covered with precipitation film, and another electrode was immersed in sodium chloride solution and acted as the breakage. The different changes of galvanic current show that the corrosion was severely suppressed after adding zinc ions in the sodium chloride solution; however, cerium(III) ions failed to suppress the corrosion and the defects gradually evolved into pits. In the zinc ion-containing sodium chloride solution, the open circuit potential of the bare place without precipitation is more positive than the place covered with precipitation film, which resulted in the bare places becoming small cathodes. Under the cathodic polarization, oxygen receives electrons on the cathode and generates hydroxyl ions. The precipitation film is then fixed by Zn(OH)₂ precipitation immediately. However, the open circuit potential of the electrode in cerium(III) ion-containing sodium chloride solution shifted toward a more positive value than that in the sodium chloride solution. The bare places with negative potentials act as small anodes in the cerium(III) ion-containing sodium chloride solution. Moreover, Ce(III) is a precipitation inhibitor and has a similar inhibition mechanism with zinc ions. Combine the results of EIS and galvanic current measurements, the defects, where under the anodic polarization and no hydroxyl ions is generated to react with Ce(III), undergo serious corrosion and develop into pits. In general, the different potentials between the bare and covered places led to the galvanic effect that is attributed to the precipitation film repair and pitting corrosion. X-ray photoelectron spectroscopy and scanning electron microscopy were applied to confirm the composition and morphology of the precipitation films.

Keywords: precipitation inhibitor; EIS; Polarization; XPS; pitting corrosion