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    Coating technology for ship propeller

    Propeller is a key component of the ship’s propulsion system. In the course of use, it is not only faced with cavitation corrosion caused by high-speed operation, but also by sea water corrosion and sea biological fouling. The working condition is bad and the corrosion protection of the propeller is key points and difficulties of ship protection.

    The damage of cavitation corrosion mainly has three aspects:

    (1).The formation of low pressure zone bubble along with the water flow to the high pressure area of the rudder blade and propeller leaves will be closed burst, resulting in a strong impact, resulting in hull vibration and huge noise;

    (2).The cavities generated on the propeller blades will change the flow pattern and reduce the propulsion efficiency of the propeller, thus reducing the speed.

    (3).The long-term effect of the bubble will make the rudder blade and propeller surface pits, leaves more easily eroded and defaced.

    The hazards of marine pollution are shown in the following four aspects:

    (1). Reduce the propeller propulsion efficiency, increase energy consumption. Although the propeller surface area is small, but its corrosion and fouling caused by the energy loss is great. Studies have shown that only 3 months of defacement can reduce the propeller propulsion efficiency of more than 10%, so that the speed down 20%, and increase energy consumption by 40%.

    (2).Increase noise. Sea biological attachment changes the shape and surface of the blade. The noise of the propeller increases, which is detrimental to the sound of the underwater vehicle.

    (3). Accelerate the corrosion of metal substrates barrageenan, algae, oysters and other macro-sea biological fouling and micro-marine corrosion will reduce the mechanical properties of the propeller body, shorten its service life.

    (4).Increase the maintenance of the ship time, reduce the rate of flight. Fouling increased the number of ship propeller docking and maintenance, is not conducive to the ship to maintain combat readiness.

    Coating protection is the most effective, economical and convenient way to slow down propeller corrosion and marine organisms. For the propeller, a variety of materials and processes have been carried out, some of the more mature coating has been tested on the propeller.

    1. Metal coating

    The metal coating for propeller protection includes copper alloy coating, cobalt-based cermet coating, nickel coating and stainless steel coating. The main coating method is thermal spraying, and some coating has obvious effect on cavitation erosion.But a single metal coating is difficult to meet the propeller anti-fouling and anti-cavitation requirements. Although the corrosion resistance of the cobalt-based cermet coating is strong, but in the sea and the propeller surface with the dissimilar metal contact corrosion, and do not have the anti-fouling ability; other metal coatings such as nickel layer, stainless steel layer of the overall performance and propeller requirements greater difference.

    2.Ceramic coating

    Ceramic coating with high hardness, chemical stability and electrical insulation, as a coating can effectively improve the propeller wear resistance, impact resistance and corrosion resistance, and can play a role in isolation current. Ceramics have sufficient hardness to prevent the barnacles, pipe worms and other marine fouling coverage; while maintaining a sufficient toughness, easy to crack, with sufficient impact resistance. Anti-cavitation corrosion is the advantage of ceramic coating, and anti-fouling ability is its weakness, in addition, the application of ceramic coating to face the complex surface of the propeller spray and subsequent grinding and polishing, the process is more difficult. In general, the cavitation corrosion resistance is the advantage of ceramic coating, and anti-fouling ability is its weakness, in addition, the application of ceramic coating to face complex curved surface of the propeller of lapping of uniform coating and subsequent problems, its craft is difficult

    3. Organic coatings

    As early as 1987, Japan had been using self-polishing anti-fouling paint and organic silicon synthetic resin paint as the protective layer of propellers, with different coatings on the four blades of the propeller. The test ship has a running time of 500 h, and the tip of the propeller blade is about 30 m/s, and the rest of the time is moored. Results show that the coating of organic silicon is a special synthetic resin coating on the surface of propeller, almost did not see the sea biological adhesion and film stripping, still maintained a very clean condition, host, a 6% reduction in fuel consumption, energy saving effect is obvious.

    4. Complex coating

    The joint application of metal, ceramics and organic coatings as composite coatings can overcome the shortcomings of single coating and realize the coordinated protection of propellers.

    5. Conclusion

    (1). The corrosion protection of propeller is the key and difficult point of warship protection due to its high speed erosion, defacing and cavitation. Coating corrosion is an important means of propeller protection.

    (2). In the aspect of organic coating, the foreign propeller protection coatings are mainly elastic coatings, which have been commercialized, with the actual application cases of military ships and civilian ships. Domestic research is in its initial stage of development.

    (3). A single metal coating is difficult to meet the anti-fouling and cavitation requirements of propeller. Some of the metal coating’s anti-cavitation ability is strong, although it is not resistant to Marine pollution, but it is applicable in sea and inland water where the damage is not outstanding.

    (4). Ceramic coating is resistant to cavitation, but the antifouling effect of ceramics lacks reliable experimental data. In addition, it is difficult to apply ceramic coating to uniform spraying and subsequent grinding and polishing on the complex surface of propeller.

    (5). Composite coating can overcome the deficiency of single coating, which is beneficial to realize the coordinated protection of propeller.

     

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