Photocathodic Protection Performance of ZnS-CdS-Ag@TiO2
for 403SS Under Visible Light
Authors: Zhu Yanfeng 
Published: February 2020

Up to date, TiO2 nanotubes have attracted extensive interest in photocatalysts [1], dye-sensitized solar cells [2], photocatalytic decomposition of water into hydrogen [3], and sensors [4]. In addition, some researchers have reported that the photoelectrochemical properties of TiO2 could be applied to the cathodic protection of metals [5-7]. Compared with conventional cathodic protection, the photocathodic protection technology has no need to supply current or sacrificial anodes, and can achieve green and nonpolluting protection of metals [5]. However, there are two key drawbacks to the practical application of photocathodic protection of bare TiO2 films.
In this work, an all-solid-state Z-scheme type CdS-Ag@TiO2 nanocomposite was fabricated by pulse electrodeposition and the successive ion layer adsorption and reaction (SILAR) technique. To suppress CdS photocorrosion, a ZnS shell was provided by the SILAR method. The CdS was chosen to sensitize the TiO2, and Ag acted as a transporter of electrons. This nanocomposite shows an impressive photocathodic protection performance for the 403 stainless steel (SS).
The morphologies were observed using field-emission scanning electron microscope Hitachi FE-SEM S4800. The results showed that the film possessed a porous network morphology and the network consisted of scrolled-up nanowires. It can be observed that CdS and ZnS particles were uniformly distributed on the nanowire surface. The EDX spectrum and XPS measurement revealed that the ZnS-CdS-Ag@TiO2 films were composed of Ti, O, Ag, Cd, Zn and S. The UV-vis absorption spectra showed that the photoabsorption spectrum of the composite film was expanded into the visible light region.
The corrosion performances of 403 stainless steel in a 0.5 M NaCl solution coupled to a titanate film photoanode in 1.0 M NaOH solutions were evaluated by photogenerated potential and electrochemical impedance spectroscopy (EIS). The steel was coupled to the composite photoanode under illumination and dark conditions. Under irradiation, the stabilized potential of 403SS coupled to the composite photoanode was recorded at approximately -1174 mV (vs.SCE). Under dark condition, the composite photoanode still provides the certain cathodic protection for the steel.
To further study the corrosion process of 403SS under photocathodic protection, EIS was performed under illumination. It was found that the impedance arc of the 403SS coupled to the composite photoanode was markedly smaller. This may be due to a larger number of photogenerated electrons being transferred from the photoanode to the the surface of 403SS, leading to the cathodic current density increase for the steel. The EIS result revealed composite film could provide a good photocathodic protection effect for 403SS.
Acknowledgements
This work was supported by the the National Natural Science Foundation of China (Nos. 51801013) and the Natural Science Foundation of Shandong Province of China (ZR2016BP11) and A Project of Shandong Province Higher Educational Science (J16LC18).
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