The unprotected metal thermal spray film is easy to be damaged and fail. After all, there are still many pinholes on the metal thermal spray film, so it must be covered with a paint shielding layer to allow the paint to penetrate into the pinhole of the metal film and strengthen the external protection . However, this form of joint protection actually belongs to the scope of paint protection, only the metal film replaces the primer. When the outer coating is partially damaged and the inner layer forms local electrochemical corrosion, the metal film may also partially produce cathodic protection for the steel pipe, but this protection is still incomplete. In important projects, metal thermal spraying is often used in combination with cathodic protection with applied current, because cathodic protection can cause cathodic polarization on the surface of the steel pipe at the defective part of the coating to avoid local corrosion of the steel pipe; The current consumption is reduced, thereby saving protection costs.
The method of using active metals with negative potential to corrode and dissolve in the electrolyte to generate current to protect the steel pipe is the cathodic protection method of sacrificial anode. As the metal of the sacrificial anode, magnesium alloy, aluminum alloy, zinc alloy, etc. are commonly used. Aluminum alloy and zinc alloy sacrificial anodes are mainly used in medium with low resistivity such as seawater, and magnesium alloy sacrificial anodes are mainly used in medium with high resistivity such as fresh water and soil. The magnesium alloy anode has low efficiency, short life, and high price, which is not suitable for protecting long-term projects. The zinc alloy anode has high efficiency, long life, and low polarization, but its driving voltage is low and the output current is small, so that the protection project The cost is expensive. However, aluminum alloy anodes have become the most important anode materials due to their advantages of long life, high capacitance, light weight, and low cost. The effectiveness of sacrificial anode protection is directly related to the electrochemical performance of the product. The general method is to add a small amount of Sn, Ga, Bi, Mn and other elements to the aluminum anode or magnesium anode to prevent the aluminum anode from being passivated and the service life of the magnesium anode extend.
In terms of the shape of the sacrificial anode, there are usually strips, blocks and bands, which are not suitable for subsea operations. A bracelet aluminum alloy anode has been successfully developed in China, which can be tightly clamped on the steel pipe wall to facilitate the use of submarine pipelines. In summary,
(1) Long-distance water transmission steel pipelines will be corroded by various environmental media, and electrochemical corrosion and biological corrosion are the main ones among various corrosion factors.
(2) Anti-corrosion coating is used to shield the inner and outer walls of the steel pipe and its accessories on the pipeline, which is the most convenient and commonly used protection method. Because there are many kinds of anticorrosive coatings on the market and different performances, careful analysis and screening should be carried out during the construction. It is best to carry out comparison tests of various brands on the spot to find an optimal matching plan.
(3) The anti-corrosion period of steel pipes and fittings should be synchronized with the service life of the project. Special attention must also be paid: the quality of the pretreatment of the surface of the steel parts will greatly affect the effective life of the anti-corrosion coating, so it should not be taken lightly and relax the supervision.
(4) Since it is difficult to maintain and repair the cross-ocean steel pipe after it is put into production, it is necessary to consider the use of a composite protection scheme compatible with cathodic protection, and actively consider the use of new materials and technologies to extend the duration of cathodic protection.
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