Study On Preparation And Protection Mechanism Of TiO₂-based Photocathodic Protection Coating

The marine environment is harsh,and marine facilities and equipment are facing problems of high temperature,high salinity,and strong light.This corrosive environment is bound to accelerate the corrosion of materials,thus shortening the service time of equipment.At present,coating and cathodic protection are commonly used anti-corrosion methods,but these methods consume energy or are harmful to the environment.The photocathodic protection coating is a new anti-corrosion technology with the characteristics of both coating protection and cathodic protection.The principle is that under light,the electrons excited by the semiconductor migrate from the valence band to the conduction band,and finally to the connected material,resulting in cathodic polarization of the steel,thus achieving cathodic protection.TiO₂ has the characteristics of stable light response,stable performance,and no consumption.It is the first choice for photocathodic protection technology.However,it has problems such as a large band gap,low photoelectric conversion efficiency,and inability to protect metals in the dark state,which greatly hinders its utilization of sunlight and cathodic protection of metals.The construction of heterojunction is the main way to effectively solve the above problems.The band structure of TiO₂ can be adjusted by combining semiconductors,thus enhancing the absorption of sunlight.At the same time,it can promote the separation of photogenerated electrons and holes,and enhance the mobility of photogenerated electrons,thereby achieving effective cathodic protection of metals.The related research results are as follows:（1）In view of the large band gap of TiO₂,which affects the absorption of light.A series of Zn₃In₂S₆/TiO₂ nanocomposite films have been prepared.The absorption edge of the films has a red shift,and shows obvious absorption in the visible light range.The band gap of the composite films is smaller than that of TiO₂.This heterojunction expands the absorption range of sunlight,thus achieving efficient cathodic protection for carbon steel.（2）In view of the problem of low reducibility of photogenerated electrons,Z-scheme TiO₂/Au/Cd S nanostructure coating has been constructed.The TiO₂/Au10/Cd S nanostructured coating coupled with carbon steel has a maximum OCP drop of-0.591 V,and the photocurrent density is 1.253 m A·cm^-2.The unique photogenerated electron-hole transfer path of the Z-scheme heterojunction enhances the reducibility of photogenerated electrons,thereby achieving good photocathodic protection performance for carbon steel.（3）In view of the problem of poor cathodic protection performance of the composite system,TiO₂/Cd S@Zn S nanocomposite films have been prepared.Zn S can be easily deposited on the surface of Cd S to enhance the stability of the composite films.And the three semiconductors have a hierarchical structure due to the different Fermi levels,the Fermi level is rearranged,which achieves effective cathodic protection for carbon steel.（4）In view of the low stability of the films due to photocorrosion,TiO₂/Zn Cd S heterojunctions have been prepared.The most negative OCP of the TiO₂/ZCS-1composite films is-0.98 V（vs.SCE）,the photocurrent density is 0.54 m A·cm^-2,and the charge transfer resistance is about 20 times smaller than that of TiO₂.Zn Cd S has an appropriate visible light capture band gap,and can improve the photocorrosion resistance of the films and enhance the cathodic protection performance of carbon steel.（5）To solve the problem that metal cannot be protected in the dark state,TiO₂/ZIF-67composite films have been prepared.The most negative OCP of the TiO₂/ZIF-67-20sample coupled with 304 stainless steel is-0.675 V（vs.SCE）,and the OCP is still lower than that of 304 stainless steel after turning off the light for 10 h.This is due to the electronic storage and release of Co²⁺in TiO₂/ZIF-67,so stainless steel can be cathodically protected under light and dark conditions.