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All commonly used metal materials and structural steels are likely to corrode in seawater, so the corresponding technical specifications must be followed when selecting materials. In addition, due to the good conductivity of seawater, the corrosion risk of mixed structure with different metal materials is higher. When evaluating the possibility of bimetallic corrosion, the potential sequence, surface area rule, geometry and structural composition of metals in seawater should be considered.
The corrosion unit can be formed in different parts of the material and its composition is uniform. Cathodic protection can effectively control the corrosion unit. But in practice, we should be more careful to make the surface area of cathodic protection as small as possible, so as to avoid the electric shielding effect on the cathode surface with high power consumption.
Generally speaking, for mixing devices composed of different metals, the protection potential and protection range must be determined according to different materials, which may limit the use of cathodic protection or take necessary special potential control methods.
Because aluminum alloy ships cannot avoid contact with copper or steel, and the whole range of aluminum alloy materials is not suitable for cathodic protection, special attention should be paid to the protection of aluminum alloy ships.
Therefore, even if good anticorrosive coating and cathodic protection are combined in construction, they are often not enough to protect these gaps and openings. Therefore, follow-up protection measures must be taken. The anticorrosive coating plays an important role in passivation and protection of ships. It is also an indispensable carrier of chemical substances to prevent marine organisms from adhering.
The purpose of the combination of anticorrosive coating and cathodic protection is to reduce the demand of protection current, increase the polarization parameters and expand the protection range. In addition to chemical and mechanical durability, the quality of the coating depends on its resistivity, porosity and damage.