ã€China Aluminum Network】
The corrosion protection of aluminum alloys refers to the corrosion protection of the hull, which is essentially related to the service life of the ship. The corrosion resistance of the hull depends on the corrosion resistance of the raw materials and the anti-corrosion technological measures for the repair of the hull, especially after the completion of the hull. The anti-corrosion measures taken.
Coating protection layer China's shipbuilding sector tried to test the corrosion resistance of the aluminum alloy hull with oxyacetylene flame sprayed polychloroether plastic and perchloroethylene paint in the mid-1960s. It was found that the former adhered to the surface of the anodized aluminum alloy plate. It is not easy to peel off; it is easy to peel off; the latter's adhesion is also very poor, when the ship is sailing, due to the impact of water flow, it quickly peeled off the paint on the bottom of the boat. As a result, they are not suitable for corrosion protection of aluminum alloy hulls. Later, they tried to pack glass steel, but due to the complicated process of packaging fiberglass on the boat's hull, the high cost, complicated operation technology, and difficulty in repair after bumping, it is not applicable to fiberglass.
Later, GNA-type neoprene was used as a coating material to achieve good results. During the hot summer period, the hull stayed in the sea for 30 days and was put on a shelf inspection. Although there was a small spot to touch the broken part, the paint was used as a repair point and it was intact. No need for scraping and maintenance. The combination of neoprene and aluminum alloy is very strong. It can be used continuously for more than 10 years after coating, which can significantly reduce the usual maintenance costs. Technical and economic analysis is cost-effective. Therefore, neoprene is a good material for coating aluminum alloy hulls to prevent corrosion. However, the quality of raw materials and the brushing process have yet to be further improved and continuously improved, and better coating materials have yet to be developed.
Paint coating The paint coating has been widely used in the corrosion protection of aluminum alloy hulls and superstructures. The commonly used paints for aluminum structures in Chinese ships are shown in the table below. Anti-aluminum alloy ship structure corrosion paint effect and the quality of raw materials, brush coating is very close, usually in the paint brushing must be done sample cavitation test, qualified above can be painted, epoxy zinc yellow coating process It is simple, and the phosphating primer process is relatively complicated, requiring high cleanliness, temperature and humidity, poor control, and easy rework.
Sacrificial anode protection The vast ocean is an endless "electrolyzer", seawater is a good electrolyte, aluminum alloy ships in the electrolyte, copper alloy thruster, steel shaft bracket, stern shaft, rudder blade have Higher potential, while aluminum alloy has lower potential, is the anode. The 5XXX series alloy welded hull has virtually no visibly noticeable corrosion in seawater because of the oxide film protection, but it is subject to corrosion in flowing seawater, resulting in ulcerative corrosion. To effectively prevent the corrosion of aluminum alloy ships, it is obviously not enough to paint the protective layer. The most effective protective measures at present are comprehensive measures that combine sacrificial anode electrochemical protection and paint coating. The so-called sacrificial anode protection method is to install a metal and alloy below the hull waterline that is more negative than the aluminum electrode potential of the hull. It becomes an anode in the seawater and is corroded, while the hull aluminum alloy material is protected as a cathode, usually Low-potential zinc or zinc alloy strips and magnesium strips serve as sacrifices to protect the aluminum alloy hull from corrosion or to reduce corrosion.
Zinc plate as a sacrificial anode zinc plate purity should be ≥ 99.99%, strictly limit the impurities, especially Fe content should be ≤ 0.01%. The requirement for the zinc plate is: the composite plywood; the surface can not be painted with paint; the device is correct, and the bracket and the fixing screw should be in close contact with the hull aluminum alloy plate. Otherwise, no protective effect will be achieved, the zinc plate will be intact, and the aluminum alloy on the ship plate will instead corrode.
Zinc alloy with ternary zinc-aluminum-zirconium alloy ZAC as sacrificial anode can achieve better anti-corrosion effect, because the potential in the process is very stable, its composition (mass%): Al0.4 ~ 0.6, Cd0.05 ~0.1Fe0.005, Pb0.005, Cu0.005, remaining Zn.
ZAC features: potential stability, just -0.13V potential when immersed in seawater, -1.09V or so after long-term use, ie good resistance to anode passivation; high anode current efficiency, generally 90% to 95% The service life is longer; the anode has a stable current, a certain degree of self-control, and a good protection effect. The installation of the ternary zinc alloy sacrificial anode is very important and the installation is not properly protected. The precautions are as follows:
The front, the installation of zinc alloy sacrificial anode can be welded or screwed. The bracket can be welded to the hull when welding, suitable for welding the hull. It is also possible to use a countersunk screw to attach the sacrificial anode to the crotch extension or to the bung plate. For sharp-tapped gliding boats and hydrofoil boats, the alloy sacrificial zinc anodes can also be fixed on the stern foam spray baffles to reduce the hull resistance. Never paint on the outer surface of the ternary alloy sacrificial anode, otherwise it will lose the anti-corrosion effect.
Second, a paint can be applied to the back of the sacrificial anode prior to installation to prevent the anode from eroding off. At the same time, it also eliminates the effect of the backside of the anode, which ensures the stable area of ​​the anode and prevents over-protection.
Third, the zinc alloy block should be close to the back of the aluminum alloy hull to ensure good contact with the hull and conductive.
Fourth, the surface of the sacrificial anode to be installed must be clean, free of paint and dirt.
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