| Metal and alloys are facing serious problems about corrosion,especially for offshore metal equipment（The harbour installation,ships,vessels,marine drilling platform,offshore wind power,etc.）in the harsh marine environment.Because of no stable and effectual power supply,the installation and maintenance for the most commonly used cathodic protection devices are very inconvenient,and also there are problems in marine transportation,the corrosion rate of the offshore metal equipment are very large.Corrosion caused a great many of accidents,such as safety problems,economic loss,and so on.Photocathodic protection is a new type of corrosion protection technology which have been studied in recent years.Its main theory originated from the photoelectric effect of semiconductors and cathodic protection,and corrosion protection are realized by means of sunlight.Photocathodic protection technology is a new type of corrosion protection technology,which is green,environmental and energy conservation,and it has great application potential.In this paper,we established the photocathodic protection research model,through the analysis of materials selection,we chose Ti O2,ZnO and g-C3N4with low cost and excellent performance to be the base materials.Through doping,construction of heterojunction and homojunction structure,we successfully improved the separation rate of the photogenerated carriers,and promoted the photocurrent density.The photocathodic protection performance of 316L stainless steel and Q235 carbon steel have been effectively improved.In addition,by coupling WO3and Sn O2with electronic storage properties,the continuous cathodic protection of metal materials under dark conditions is realized.The main contents are as follows:（1）Model selection and the design of material systemIn this section,the advantages and disadvantages of the two models（the direct coating model and the photoanode model）are analyzed and evaluated in depth.According to the actual condition,both of the two models have been adopted in our research.Taking the three key factors（energy band structure,system stability and economic cost）into consideration,we chose Ti O2,ZnO and g-C3N4as the base materials,and through doping,heterojunction and homojunction construction,the photocathodic protection performance have been improved effectively.Through coupling with WO3and Sn O2which have the electronic storage ability,the continuous protection of the metal have been realized under dark condition.（2）Photocathodic protection of cobalt doped ZnO nanorod arraysCobalt doped ZnO nanorod array films were prepared on FTO substrate by a simple aqueous solution method.The results of XRD and EDX showed that the structure of cobalt doped ZnO rod array is single crystal hexagonal wurtzite structure,and Co2+has successfully doped into ZnO lattice.When the ratio of Co/Zn was 15%,the actual doping amount of cobalt（C3 sample）reached the maximum.With the increase of the actual doping amount of cobalt,the bandgap of ZnO has slightly narrowed,the separation ability of photogenerated carriers were enhanced,and the photocathodic protection performance of 316L stainless steel and Q235 carbon steel have been greatly improved.The photocurrent densities of the C3 samples coupled with 316L stainless steel and Q235 carbon steel were 136.27μA/cm2and 133.02μA/cm2,respectively.The corresponding open circuit potentials（OCP）drops in 3.5wt.%Na Cl solution were 791 m V and 440 m V compared with the corrosion potentials of the metals in 3.5 wt.%Na Cl solution.The principle of this process can be attributed to the narrowed band gap,the lower recombination rate of the photogenerated electron-hole pairs,the intermediate impurity level,and the split of the hypo-outer shell of cobalt ions.（3）Photocathodic protection properties of ZnO/Ti O2heterojunction filmTwo layers complex materials composed of Ti O2nanorod array film and ZnO nanorod array film were prepared on FTO substrate by hydrothermal method and simple aqueous solution method.The ZnO nanoparticles layer on the Ti O2nanorod array film is helpful for the uniform growth and good crystalline of the ZnO nanorod arrays,and the efficiency of photogenerated carrier separation has been improved.Compared with the pure ZnO and the pure Ti O2,the construction of ZnO/Ti O2heterojunction construction greatly promoted the separation of photogenerated carriers,and greatly improved the photocathodic protection performance.The photocurrent densities of the coupling 316 L stainless steel and Q235 carbon steel were 126μA/cm2and 238μA/cm2,respectively.The corresponding open circuit potentials（OCP）drops in 3.5 wt.%Na Cl solution were 827 m V and 466 m V compared with the corrosion potentials of the metals in 3.5 wt.%Na Cl solution.（4）Photocathodic protection properties of graphitic carbon nitride（g-C3N4）homojunction filmsThe g-C3N4 homogeneous junction films were prepared on ITO substrate by cocalcining two mixtures of melamine and thiourea with different ratio in a crucible.The photoluminescence spectroscopy and Bode-phase curves show that the construction of g-C3N4homojunction is helpful to promote the separation of photogenerated carriers and increase the lifetime of photogenerated electrons.Through the construction of g-C3N4homojunction,the photocathodic protection properties of 316 stainless steel and Q235 carbon steel is greatly improved.The photocurrent densities of the coupling 316 L stainless steel and Q235 carbon steel were 143.44μA/cm2and 111.48μA/cm2,respectively.The OCP drops of the coupling316 L stainless steel and Q235 carbon steel in 3.5 wt.%Na Cl solution were 818 m V and 401 m V compared with the corrosion potentials of the metals in 3.5 wt.%Na Cl solution.The mechanism of enhancement may be result from the narrowed band gap and the small difference potentials between n-n homojunction which promote the separation of photogenerated electron-holes,and improve the photon-to-electron conversion efficiency.（5）Study on the photocathodic protection under dark condition.By adopting the direct coating model,the WO3/ZnO/g-C3N4composite photoanode film with dark protection function was prepared on 316 stainless steel substrate by hydrothermal method and aqueous solution method,which accelerated the photogenerated carriers separation efficiency,and the potential drops of WO3/ZnO/g-C3N4composite photoanode film coupled with 316 stainless steel was479 m V,which proved that the composite has greatly improved the photocathodic protection performance.Due to the storage electronic properties of WO3,316 stainless steel can also be protected under dark.Based on the photoanode model,Sn O2/ZnO/NiO p-n junction photoanode film was prepared on FTO substrate by gas-phase synthesis,hydrothermal and co-precipitation methods.Because ZnO is n type semiconductor,and NiO is p type semiconductor,the constructed p-n junction structure has improved the separation efficiency of photogenerated carries.The potential drops of Sn O2/ZnO/NiO photoanode film coupled with 316 stainless steel was 538 m V,which also proved the composite has greatly improved the photocathodic protection performance.（6）The analysis of experimental factors on photocathodic protectionThe different experimental conditions have a great influence on the photocathodic protection performance.The influence on the photocathodic protection performance is tested by changing several factors.When 0.1 M Na2S+0.1 M Na2SO3is used as sacrificial agent,the photoelectric conversion performance is much better than that of 3.5 wt.%Na Cl solution with H2O as the sacrificial agent,and photogenerated holes can be separated quickly and effectively.When the light intensity is 10 m V/cm2,30 m V/cm2and 50 m V/cm2,there is no significant difference in the photocathodic protection performance.The main reason may be that the current density under different light conditions is not large enough,and the polarization effect on metal is not strong.In different corrosion environments,the potential of the metal in the simulated seawater is the most negative,the simulated carbonate is the largest,and the simulated of groundwater is in the middle.This may be attributed to the fact that the simulated carbonate and the simulated groundwater are both alkaline,and Fe OOH,Fe CO3and other protective films are formed on the surface of stainless steel in alkaline environment.These protective films have excellent corrosion resistance.Our subject confirmed that photocathodic protection can make effectively protection for metal materials,and can be used as an application strategy for offshore metal facilities corrosion protection in marine environment.However,due to the limitation by the current material research and the complexity of the marine environment,the photoelectric conversion efficiency is still low,there is still a long way to go in the future.Because of its energy conservation and environmentally friendly,the photocathodic protection technology is a promising protection technology which can substitute for the traditional cathodic protection technology in the future.These prepared materials in our experiment can be used for reference to the preparation of materials and the promoting of photocathodic protection performance. |
PDF Full Text Request