How About Boatbuilding with Aluminum
Marine aluminum sheet is a main material for boatbuilding. For ships, the corrosion resistance is very important besides the lightweight. How about the aluminum and aluminum alloy’s corrosion resistance?
Aluminum and aluminum alloys exhibit excellent corrosion resistance in the atmosphere and fresh water, but corrosion often occurs in actual use, and localized corrosion such as pitting corrosion, intergranular corrosion, exfoliation and stress corrosion also occurs in some environmental media. The marine aluminum sheet 5754, 5083 and 5059 has better corrosion resistance than other aluminum alloys.
The electrochemical behavior of aluminum in corrosive media can be analyzed and discussed through the potential-pH of aluminum. In aqueous solution, when the electrode potential is below -1.66V, aluminum is located in the corrosion-free zone and does not corrode. Above the anti-corrosion area, there are corrosion area and passivation area, which is related to the pH of the medium related.
When the pH is between 4.5 and 8.5, aluminum is in the passivation zone, and a passivation film can be formed on the surface of aluminum, which makes aluminum have good corrosion resistance. And when the pH < 4.5, it is an acidic corrosion zone; and when the pH > 8.5, it is an alkaline corrosion zone.
In the atmosphere and fresh water, aluminum is generally located in the passivation zone. Oxygen in the air or dissolved in water and water itself are good oxidants for aluminum. Due to the action of oxidants, a dense oxide film is formed on the surface of aluminum, which hinders the contact between the active aluminum surface and the surrounding medium, and prevents the corrosion of aluminum, so the corrosion resistance of aluminum in the atmosphere and fresh water is very good.
Due to the protective effect of the passivation film on the aluminum surface, aluminum is also resistant to corrosion by many organic acids and solvents. It is stable in nitric acid and oxidizing salt solutions such as sulfate, aluminate, and nitrate with a concentration of less than 10% or greater than 80%. Aluminium 5083 factory has good corrosion resistance in sea water, so it is widely used in making marine yachts.
The oxide film formed by aluminum and aluminum alloy in the atmosphere and fresh water is extremely thin, dense and uniform. The natural oxide film is generally only 5 ~ 15nm. With the prolongation of the parking time in the atmosphere or the increase of humidity in the atmosphere, the oxide film thickens. The oxide film thickness of aluminum and aluminum alloys after 5 years in the atmosphere is about 5-200nm (due to different alloys and humidity).
A thicker oxide film can be artificially generated by chemical oxidation, anodic oxidation and other methods. The thickness of the chemical oxide film is about 2. 5 ~ 5μm, the anodized film can even reach 250μm.
The oxide film on the surface of aluminum and its alloys has a good protective effect on the body. If the oxide film has sufficient stability in the medium, aluminum and aluminum alloys will have good corrosion resistance in the medium.
Therefore, the corrosion resistance of aluminum and aluminum alloys basically depends on the stability of the oxide film on the surface of aluminum and aluminum alloys in a given environmental medium.
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