Study On The Photocathodic Protection Of Narrow Band-gap Semiconductor Sensitized Titanium Dioxide For 304 Stainless Steel

304 stainless steel?304SS?has good corrosion resistance.However,when the metal is used in marine environment,its passive film could be damaged by chloride ion,thus inducing pitting corrosion.Photocathodic protection technology is a new type of cathodic protection technology,which is a truly green anticorrosion method for metal.Photogenerated cathodic protection is a new green cathodic protection anticorrosive method,which often uses TiO₂ as photoelectric conversion center.Nevertheless,TiO₂shows poor photoelectric activity due to the wide band gap and easy recombination of photogenerated carriers.Aiming at overcome the shortcomings of TiO₂,in this paper,TiO₂ nanotubes were prepared by anodic oxidation method and then three narrow bandgap semiconductor sensitized TiO₂-based p-n heterojunction materials of Sb₂S₃/Sb₂O₃/TiO₂,Co₃O₄/TiO₂ and BiOI/TiO₂ were designed.Furthermore,the material characterizations and photoelectric properties of the samples were tested,and the protection mechanism was analyzed.Sb₂S₃ and Sb₂O₃ were simultaneously deposited on titanium dioxide tubes by one-pot hydrothermal method,and the effect of reaction source concentration on photo-induced cathodic protection properties was investigated.The results show that nano-reticulated Sb₂S₃ promotes the visible light absorption of TiO₂,while micron rod-shaped Sb₂O₃ makes carriers move along their axes,providing a direct channel for carrier transport;Sb₂S₃ with negative conduction band pulls the Fermi level of TiO₂ to a more negative potential,therefore,facilitating electron transport to 304SS;the built-in electric fields of p-type Sb₂S₃ and n-type Sb₂O₃/TiO₂ promotes the directional transport of carriers.When antimony source concentration is 48 mmol/L,the ternary composite has the best protective effect on 304SS with a photoinduced coupling potential of-0.76 V.Co₃O₄/TiO₂ composites were prepared by low-temperature hydrothermal method,and the effect of Co₃O₄ loading amount on protection properties were studied.The results show that after introducing Co₃O₄,the absorption wavelength of TiO₂ shifts to visible light and the band gap reduces to 2.2 eV.When the concentration of cobalt nitrate is 1 mmol/L,the composite film has the best cathodic protection performance for 304SS.In this case,the coupling photovoltaic potential reaches-0.69 V,additionally,the coupling photocurrent increases by three times,and charge transfer resistance decreases by an order of magnitude.After the contact of Co₃O₄ and TiO₂,the previous straddling band structure was transformed into a staggered p-n heterojunction.Under the action of built-in electric field,the timely separation and directional transmission of carriers are driven,which shows good cathodic protection effect for 304SS.BiOI was grown in situ on the orifice of TiO₂ by successive ionic layer adsorption and reaction method,moreover,the effect of cycle times on the photocathode cathodic protection performance of composite was studied.The results show that with the presence of BiOI,the absorption band edge of TiO₂ is red-shifted and the absorption intensity is significantly increased.The composite film exhibits the best protection performance with five cycle of BiOI.At this situation,compared with titanium dioxide,the photoinduced current density between composite film and 304SS increases more than 9 times,and the charge transfer resistance reduces by two orders of magnitude,which greatly promotes the separation rate of photogenerated carriers.The composite photocathode can reduce the potential of 304SS to-0.66 V,and cathodic protection of304SS can be carried out for more than 10 hours in dark.Its excellent performance comes from the synergistic effect of BiOI thin layer structure and staggered p-n built-in electric field.