Effect of Co3O4/TiO2 heterojunction photoanode with enhanced photocathodic protection on 304 stainless steel under visible light

The Co3O4/TiO2 compound photoanodes were prepared by anodization and low-temperature hydrothermal method for the photocathodic protection of 304 stainless steel (304SS). The effect of Co3O4 loading on corrosion resistance of 304SS in 3.5 wt% NaCl was studied by changing the cobalt nitrate concentration of hydrothermal solutions, where the photoanodes were immersed in 0.1 M Na2S+ 0.2 M NaOH solution during tests. Compared with pure TiO2, Co3O4/TiO2 shows stronger visible-light absorption, higher separation efficiency of photocarriers, and better corrosion protection in dark. When cobalt nitrate concentration is 1 mM in the reaction solution, the compound exhibits the best protection performance with a coupled potential under illumination/dark of –0.69/–0.45 VSCE. This enhanced corrosion resistance is not only due to the staggered p-n heterojunction at Co3O4/TiO2 interface but also due to the excellent optical absorption activity and electronic storage capacity of Co3O4. Specifically, the p-n internal electric field drives the photoelectrons to flow to the 304SS under illumination. Meanwhile, it triggers the reverse flow once the light is removed. The latter electrons are stored by Co3O4 and then slowly released in the dark, thus realizing the delayed protection. This work provides theoretical support and possible guidance for the design and application of p-n junction photoanode in the photocathodic protection field.