[China Aluminum Network] Dissimilar metal contact corrosion requires the use of multiple metals in shipbuilding, so dissimilar metal contact is inevitable. When the aluminum alloy is directly connected to copper, brass and bronze, its electrochemical corrosion may be stronger than that of the aluminum alloy and steel, lead and nickel, and the aluminum alloy in the bimetal (dissimilar metal) joint soaked in seawater Chemical corrosion is much more intense than in other media. For example, the corrosion of the copper alloy submarine valve housing on the aluminum alloy hull is particularly serious, and some watertight fillers contain lead, mercury, and other corrosion materials or the joints are not closely attached, which is one of the causes of corrosion. Although some dissimilar metals are not immersed in seawater, they can also corrode. If the aluminum alloy plates under the steel brackets of the board mounted parts are often corroded very seriously. The reason is that the steel bracket is not galvanized and other necessary protection is taken. The same steel bracket is used. If the zinc plating is applied or the rubber gasket is placed between the fittings, the corrosion of the aluminum alloy is extremely slow.
The contact between the steel flange installed in the non-flammable tube outlet of the bulge and the hull aluminum alloy plate is relatively serious due to dissimilar metal contact semi-submerged in seawater and the temperature of the exhaust pipe is relatively high. The edge of the opening is often cracked and perforated due to corrosion.
Corrosion of aluminum alloy and wood Contact corrosion between aluminum alloy and wood is often seen during ship repairs. For example, aluminum alloy angles that come into contact with the fenders are very corrosive due to direct invasion of the wood, and some angle aluminum. Broken off, perforated aluminum plate, angle aluminum corrosion as loose as bagasse.

Unreasonable structural corrosion of the aluminum alloy hull structure must not have water “dead ends”. For example, in China, there was a type of aluminum alloy hydrofoil. The hydrofoil support box installed on the side of the vessel was in the shape of a “dish”. After the outside was covered with a protective cover, both ends became “dead ends”. It is not easy to maintain at ordinary times, and the “dead corner” part is generally heavily corroded by the outer shell of the aluminum alloy on the side. If the structure is improved to eliminate dead angle, the outer aluminum plate will not be corroded.
Improper arrangement of rivet holes can also cause corrosion. If the distance between the rivet holes is too large, it will produce an edge which will allow the seawater to invade the interlayer and begin to erode from the inside. Figure 2 shows that at the bottom of a watertight bulkhead (commonly known as an iron bucket) of a fast-billing bulkhead in China, due to the large spacing between rivets, water accumulated in the bottom of the tank is immersed into the gap and the bottom aluminum alloy plate is corroded.
When the ship is docked with scattered current, the steel wire and the steel wharf are used to conduct electricity. When stray current flows through the water to the steel wharf, the hull becomes an anode and suffers severe corrosion. Preventive measures: Wires are directly connected between the welder and the parts to be welded.
About 25% of the outer plates are replaced by irreconstructed hulls, and about 15% are replaced by dissimilar metals. Corrosion damage is caused by dissimilar metal contacts. About 45% of them belong to electrochemical corrosion. This means that as long as effective protection measures are taken for weak links, corrosion can be significantly reduced. The replacement of aluminum alloy plates in hulls is reduced by 50%. This is what is said for 2024 aluminum alloy plates and replacement of 5XXX series aluminum alloy hull plates. Then almost no.

Corrosion of superstructures Before 2000, ship superstructures, deckhouses, light enclosures, etc. were made of 2XXX series alloys. Although not as immersed in seawater as the hull would undergo electrochemical corrosion, the superstructure was impacted by splashing seawater. In a humid ocean atmosphere, this atmosphere carries salt and corrosion damage can also occur.
Now, the superstructure of the ship and the facilities of the cruise ship are made of 5XXX series, 6XXX series, and 3XXX series alloy materials with strong corrosion resistance by anodizing or electrophoretic painting. Corrosion will hardly occur, even if they occur occasionally. There will be no destructive corrosion.

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