Aluminum and aluminum alloys are very reactive, but because they can form dense and passivated oxide films with oxygen, corrosion resistance is much better than steel. When the aluminum alloy is used on a ship, it is more or less in contact with seawater, or is attacked by seawater droplets and the sea atmosphere, and is subject to certain corrosion. Corrosion of aluminum alloys is a complex process that is both environmentally affected and related to the nature of the alloy. The common types of corrosion of marine aluminum alloys in the marine environment are: pitting corrosion, crevice corrosion, intergranular corrosion, spalling corrosion and stress corrosion cracking.
Current Status of Corrosion Protection Technology for Aluminum Alloys for Ships
The marine environment is a harsh environment, and the anti-corrosion requirements for aluminum alloys in this working environment are higher. For ships, the corrosive environment of different parts is different. The bottom of the ship is mainly affected by the penetration and erosion of natural seawater and the attachment of aquatic organisms. The above waterline is mainly affected by salt spray corrosion and atmospheric aging.
Sacrificial anode cathodic protection
For small high-speed ships, impressed current cathodic protection measures are generally not used. At present, it is commonly used abroad to set a sacrificial anode on a hull plate (with an anti-corrosion coating). Aluminum alloy itself is a good sacrificial anode material for steel, copper and other materials; for aluminum hull, aluminum alloy sacrificial anode with sufficient electrode potential must be used for cathodic protection, and the anode should be considered good. The solubility characteristics and the potential cannot be excessively negative, resulting in alkali corrosion of aluminum. At present, the main component of the aluminum alloy sacrificial anode material is Al-Zn-In-Mg-Ti; the sacrificial anode is fixed by bolts, the iron core in the anode must use aluminum core, and the bolt must use aluminum bolt.
Aluminum alloy marine anticorrosive and antifouling coating
The corrosion environment of the aluminum alloy ship bottom and the waterline is different. The bottom of the ship is mainly the osmotic erosion of natural seawater and the attachment of aquatic organisms. The above waterline is mainly caused by salt spray corrosion and atmospheric aging. Therefore, the bottom of the ship and The requirements for anticorrosive paint on the waterline above are not exactly the same.
For parts above the waterline, the topcoat should have good weathering resistance, good gloss retention and decorative properties, and have good compatibility with the primer. Polyurethane topcoat and alkyd topcoat can be used. Acrylic topcoats, etc., are now commonly used in polyurethane topcoats. With the continuous improvement of coating performance requirements, fluorocarbon coatings with superior performance or epoxy and acrylic modified fluorocarbon coatings have also begun to be applied to the matching coating system of aluminum alloys.
Micro-arc oxidation treatment of aluminum alloy
Micro-arc oxidation treatment is a new surface treatment technology that has emerged in recent years. It uses a higher energy density to introduce the anodizing working zone from the Faraday zone to the high-pressure discharge zone, and through the interaction of electrochemistry, thermochemistry and plasma chemistry, on the surface of non-ferrous metals such as Al, Mg, Ti, Nd. A ceramic oxide film is formed at the bit.
Prospect of corrosion protection technology for aluminum alloy for ships
Aluminum alloys have been widely used due to their various advantages. As the use of aluminum alloys on ships increases, the requirements for corrosion protection of aluminum alloys are also increasing.
When the aluminum alloy hull sacrificial anode is arranged, it should take into account two different states when the ship is docked and at high speed. It should be simulated and optimized. According to the static natural corrosion stable potential value and dynamic open circuit potential value of aluminum alloy, based on the experimental data of the sacrificial anode material, the number, arrangement position and protection period of the sacrificial anode are quantitatively determined. At the same time, in different sea areas, sacrificial anode materials that can effectively protect the water should be used for the corresponding sea features.
Aluminum alloy surfaces have special requirements for coatings. Choosing the right coating and setting the right matching are essential for the anti-corrosion effect of aluminum alloy ships. With the introduction of environmental protection regulations, anti-corrosion coatings for aluminum alloys will adopt new technologies in the future, and will develop toward non-toxic, general-purpose and high-performance. The development of new environmentally-friendly and excellent anti-rust pigments is the focus of future development.
Micro-arc oxidation technology has not entered the stage of large-scale industrial application at home and abroad, but the characteristics of this technology determine that it is more suitable for the protection of the cabin corrosion of aluminum alloy ships. Therefore, in the field of anti-corrosion of aluminum alloy ships, micro-arc oxidation technology has broad application prospects.