An in-depth study of photocathodic protection of SS304 steel by electrodeposited layers of ZnO nanoparticles

Thin layers of ZnO nanoparticles were deposited potentiostaticly on the surface of stainless steel 304 (SS304) at various potentials (−0.7 V, −1.0 V, −1.3 V, −1.6 V) to study the effect of photo-generated cathodic protection on the substrate. Electrochemical methods of open circuit potential (OCP), potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) were employed to investigate the electrochemical behaviors of ZnO-covered SS304 system (ZnO/SS304) in 3.5 wt% NaCl solution under the conditions with and without UV illumination. It is found that the ZnO nanoparticles deposited at −1.0 V was in the shape of hexagonal nanorods, which caused a negative shift to lowest OCP upon UV illumination. Polarization curves were also performed on ZnO/ITO glass system to reveal the photoelectrochemical behavior of ZnO nanoparticles only. Possible anodic and cathodic reactions on the ZnO/SS304 and ZnO/ITO systems at the absence and presence of light illumination have been analysed and discussed in detail. It is found that traditional Tafel extrapolation of the polarization curves cannot be used to get the free corrosion current density of ZnO/SS304 system under light illumination due to the superposition of photo-generated current density. The charge transfer resistance obtained from EIS also includes the influence from the photo-generated reactions on the ZnO/SS304 system. Long-term immersion of the ZnO/SS304 in NaCl solution indicates that the ZnO nanorods underwent morphological transitions to nanotubes at 5 days and finally to needles and thin plates at 10 days. Correspondingly, the photocathodic protection of the ZnO layer decreases with the extension of immersion time. The transformation in morphology is postulated to be caused by the attack of Cl− ions on the surface of ZnO nanoparticles.