Alloy and state selection
When selecting an alloy, the first consideration is material mechanical properties, weldability and resources, followed by corrosion resistance. In the marine environment, the 5XXX series alloy has the highest corrosion resistance, followed by the 1XXX series alloy. The following order is: 3XXX series alloy, 6XXX series alloy, 2XXX series and 7XXX alloy, in the 8XXX series alloy, except for the AL-Li alloy. In addition, other alloys are similar in corrosion resistance to the 1XXX series alloy. The corrosion resistance of the 6XXX series alloy is slightly worse than that of the 3XXX series alloy. In the use, the anti-corrosion treatment is not carried out, and the painting and anodizing of the building material are mainly for the appearance, decoration and hygiene.
The importance of material state to intergranular corrosion and stress corrosion cracking to marine materials has been as described above. The aluminum-clad 2XXX alloy and the 7XXX alloy have good corrosion resistance, and the coating potential is at least 100 mV lower than that of the coated core layer for effective cathodic protection. After 30 years of use of the coated 3004 alloy roofing panel in the marine atmosphere, the corrosion depth of the coating is only 76 μm.
Insufficient design considerations can cause severe corrosion. Special attention should be paid to three points during design: the correct selection of the alloy and state of the material; the correct determination of other metal or non-metallic materials that are etched with the aluminum component; and the correct selection of the packing material. The following guidelines should be considered when designing.
1. Avoid contact with dissimilar metals and avoid contact between aluminum parts and water absorption >20%. If the design requires it, strict corrosion protection measures should be taken. According to the statistics of the Chinese ship department, in the outer panel repaired in the hull, the corrosion caused by unreasonable structure accounts for 25%, the corrosion caused by the dissimilar metal corrosion accounts for 15%, and the corrosion caused by contact with wood accounts for 15%. 45% of electrochemical corrosion. Obviously, careful design can reduce the corrosion of the workpiece and extend the ship repair period.
2. Avoid gaps. Seal the inevitable gap tightly to prevent seawater and moisture from entering.
3. Try to use continuous welding, no need or less use of spot welding and riveting. The best welding method is friction stir welding (FSW).
4. It is easy to drain and easy to clean.
5. The surface of the aluminum is not easy to absorb water and contact with moisture-absorbing materials. Inevitably, effective moisture-proof measures should be taken.
6. Try to avoid sharp turns outside the piping system.
7. Avoid heat transfer to hot spots.
8. Avoid direct impact of fluids.
9. Avoid large mechanical stress concentrations.
10. The equipment is placed in a minimum of corrosion.
11. If equipment or facilities are to be painted, sharp edges should be eliminated.
In order to avoid contact corrosion, the design should be:
1. The potential of the dissimilar metal should be as close as possible to the aluminum potential.
2. When fastening different metal workpieces, the fastener should be a cathode for aluminum. For example, aluminum-steel parts can be fixed with stainless steel screws, aluminum screws should not be used, and aluminized stainless steel screws are preferred.
3. When the two metals must be completely electrically insulated, a non-conductive insulating mat is used. At the same time, the two metals must not be connected to external circuits.
4. If the paint must be painted on the two metal joints, it should be applied to the cathode metal and should not be applied to the aluminum, otherwise it will cause pinhole corrosion.
5. When the aluminum tube is connected with other metals, replace the thick-walled aluminum joint or flange, and do not use the screw.
6. Keep the dissimilar metal joints away from corrosion as much as possible.
7. Add a corrosion inhibitor to a liquid circulation system made of a dissimilar metal.
8. Adopt cathodic protection.
9. In the conductive environment, the cathode-anode area ratio. For example, in seawater, this ratio should be at least 1:5.
Avoid sedimentation corrosion criteria:
1. Do not use heavy metal parts.
2. Paint on metal parts that can cause corrosion.
3. Aluminum coated aluminum.
4. Add a corrosion inhibitor.
5. Frequently clean heavy metals and dirt deposited on aluminum components.
Prevention of crevice corrosion: First of all, the design should be as seamless as possible. If it is unavoidable, it should be sealed with non-hardened elastic material to prevent liquid and moisture from entering. Commonly used sealing materials are butyl rubber, silicone rubber, and epoxy resin. The sealing material must not crack due to aging. The aluminum surface shall not be in contact with moisture-absorbing materials (paper, cloth, wood, asbestos, etc.).
The application of organic coatings on aluminum equipment and components is mostly for aesthetics, such as building facades and car bodies, but in special cases there are also anti-corrosion. Regardless of the purpose, the coating should be carefully selected and rigorously pre-treated before painting. The coating is a physical barrier between the aluminum surface and the environment. Some coatings contain corrosion inhibitors such as chromate. In order to reveal the true color of aluminum, a clear lacquer can be applied. In storage and transportation, in order to prevent corrosion, it is possible to apply an easy-to-clean coating, and to apply coal tar to aluminum members embedded in soil and concrete. Organic anti-corrosion coatings should be regularly maintained in severely corrosive environments.
There are two types of corrosion inhibitors: anodic corrosion inhibitors, such as chromate, which reduce the anodic reaction of aluminum; and cathodic corrosion inhibitors, such as polyphosphates, which slow down the cathodic reaction. When the amount of anodic corrosion inhibitor is not enough, it will accelerate the pitting corrosion of aluminum. Therefore, the cathode corrosion inhibitor is safer and more reliable, and the two corrosion inhibitors will be better at the same time.
Corrosion inhibitors for aluminum and aluminum alloys include phosphates, silicates, fluorides, triethanolamine benzoate, lactic acid gels, etc., which may be used singly or in combination. In a closed-loop system, if a copper or copper alloy part is present, the use of sodium mercaptan benzothiazole as a corrosion inhibitor can prevent copper corrosion and subsequent deposition corrosion of aluminum. The addition of sodium silicate to a weakly alkaline solution inhibits the corrosion of aluminum.
The current used for cathodic protection of the aluminum component can be supplied by a sacrificial anode or by an external power source. The sacrificial anode can be made of Zn, Mg and Zn alloys, and some aluminum alloys can also be used in seawater. The Zn and Zn alloy sacrificial anodes are safe and can effectively protect aluminum components.