## 3. Sacrificial Anode CP
### Photoanodes
The formation of the pn junction electric field enhanced the migration of electrons and holes in opposite directions, resulting in a better separation of holes from electrons. Due to the pn junction electric field formation, the \(i_{\rm corr}\) of 304 stainless steel coupled with Co(OH)\({}_{2}\)/TiO\({}_{2}\) was the highest under light conditions compared to pure TiO\({}_{2}\), indicating that the Co(OH)\({}_{2}\)/TiO\({}_{2}\) photoanode produces the highest number of photogenerated electrons and the highest hole-electron separation efficiency. [PERSON] et al. [41] modified ZnO films with polyaniline (PAN), and the currents of both ZnO films and ZnO/PANI composite films were very low (around 0.5 \(\mu\)A) under dark-state conditions. When simulating sunlight irradiation, the current of ZnO film increased by 7 \(\mu\)A, and that of ZnO/PANI composite film increased by 22 \(\mu\)A. The current increase in the ZnO/PANI composite film was significantly higher than that in the ZnO film, indicating that the ZnO film modified by PAN absorbed light more strongly. The CaIn\({}_{2}\)S\({}_{4}\)/TiO\({}_{2}\) nanotube array heterojunction (NTA) photoanode prepared by [PERSON] et al. [42] provided over 12 h of CP for 316 stainless steel in 3.5 wt.% NaCl solution, even in the dark state. Graphitic phase carbon nitride (g-C\({}_{3}\)N\({}_{4}\)) is an emerging, inexpensive, non-metallic polymeric photoanode material that has broken the monopoly of traditional metal-semiconductor  
Figure 1: (**a**) CP schematic diagram of steel rebar; (**b**) CP schematic diagram of the current generated by the photogenerated system [33]. materials in the field of photoanodes. g-C\({}_{3}\)N\({}_{4}\) has excellent wear, high temperature, and corrosion resistance properties and can be used as a coating to provide good physical protection for metals in the dark state [43]. However, the urgent problem that needs to be solved when using g-C\({}_{3}\)N\({}_{4}\) for photochemical CP in the marine environment is the insufficient oxidation capacity of g-C\({}_{3}\)N\({}_{4}\) photogenerated holes. g-C\({}_{3}\)N\({}_{4}\) valence band potential is around 1.27 V (v.s.SHE), while the oxidation potential of water is 1.23 V (v.s.SHE); therefore, in the electrolyte system, where the photogenerated hole trapping agent is water (e.g., 3.5 wt.% NaCl solution), the anodic depolarization of photogenerated holes is difficult. Thus, in the photoelectrochemical CP process, it is necessary to add an appropriate amount of hole-trapping agents, such as S\({}_{2}\)- or some organic substances that can be easily oxidized in the electrolyte [44]. In addition, the g-C\({}_{3}\)N\({}_{4}\) photoelectrochemical reaction has fewer active sites and lower electron mobility. The sacrificial anode CP system does not require a power supply, has low driving voltage, and does not interfere with neighboring facilities. It also has good current dispersion ability, requires simple equipment, and generally does not require great technical investment. After the installation of the protection system, its operation requires almost no cost, only the occasional check of the potential or protection current. The system also has low protection voltage and no safety concerns. Therefore, sacrificial anode CP is widely used for the protection of ships, offshore equipment, underwater equipment, underground oil and gas pipelines, underground cables, seawater cooling systems, etc. It is also very suitable for the protection of concrete structures in the wave splash zone, tidal difference zone, and total immersion zone, which remain in wet conditions for a long time. However, the sacrificial anode protection method has some drawbacks. (1) The protection current cannot be adjusted and may be under-protected or over-protected. Materials such as high-strength steel used in large quantities in marine environments have hydrogen embrittlement sensitivity [45], and the over-negative protection potential will easily lead to hydrogen precipitation and diffusion into the internal steel.