Online image monitoring and kinetics study on photocathodic protection of carbon steel using α-Fe2O3 photoanode

Although the photocathodic protection technique is a green and effective metal protection method, the corrosion images monitoring of protected metals and the carrier dynamics of photoanode materials are rarely studied. In this work, we successfully collected the real-time images of carbon steel (CS) by using a particle video microscope probe and verified that the use of a hematite (α-Fe2O3) photoanode could realize the long-term protection of CS under visible-light illumination in a Na2S-containning alkaline solution. In the absence of Na2S, the irradiated α-Fe2O3 electrode could delay but not prevent CS corrosion. Time-resolved spectroscopy investigation revealed that rapid electron/hole recombination via a trapping-detrapping model and slow water oxidation kinetics with a rate constant of 10 s−1 were the main factors limiting the photocathodic protection efficiency of α-Fe2O3. Trap-state-mediated recombination could be reduced by accelerating interfacial hole transfer with Na2S as the hole scavenger. This effect reduced charge transfer resistance by three orders of magnitude relative to water oxidation. Our work showed that photocathodic protection behavior was determined not only by the relative position between the conduction band potential of α-Fe2O3 and the corrosion potential of CS but also by the reaction dynamics on the electrode surface. The acceleration of the interfacial charge transfer of α-Fe2O3 was proposed to be the key to effective photocathodic protection.