[China Aluminum Industry Network] The corrosion types of aluminum and aluminum alloys and their protective measures have been mentioned before, and now we will talk about anti-corrosion measures for aluminum components and structures of ships and marine facilities: alloys and state selection, design, organic coatings, and Etchant, cathodic protection, thickened surface oxide film, improved environment, etc.
Alloy and State Selection When selecting an alloy, the first consideration is the mechanical properties of the material, weldability and resources, followed by corrosion resistance. In the marine environment, the corrosion resistance of 5XXX series alloys is higher, followed by the 1XXX series alloys. The following order is: 3XXX series alloys, 6XXX series alloys, 2XXX series and 7XXX alloys, in 8XXX series alloys, except AL-Li Outside the alloy, the corrosion resistance of other alloys and 1XXX series alloys is similar. The corrosion resistance of the 6XXX series alloys is slightly worse than that of the 3XXX series alloys. The anti-corrosion treatment may not be performed during use. The painting and anodization of building materials are mainly for aesthetics, decoration and hygiene.
The importance of the material state to intergranular corrosion and stress corrosion cracking for marine materials has been as described above. Aluminized 2XXX series alloys and 7XXX series alloys have good corrosion resistance, and the coating potential is at least 100 mV lower than that of the coated core for effective cathodic protection. After 30 years of cladding of 3004 alloy roofing panels in the marine atmosphere, the depth of corrosion of the cladding is only 76 μm.
Inadequate design design can cause serious corrosion. Three points should be taken into account when designing: properly select the alloy and the state of the material; correctly determine other metal or non-metal materials that are to be eroded by the aluminum component; properly select the packing material. . The following guidelines should be fully considered when designing.
1. Avoid dissimilar metal contact and avoid contact between aluminum parts and water absorption> 20% wood. If the design requires, strict anti-corrosion measures should be taken. According to statistics from China's shipping department, in the outer plates of the hulls, the corrosion caused by irrational structures accounted for 25%, the corrosion caused by dissimilar metal joints accounted for 15%, and the contact caused by contact with wood accounted for 15%. 45% of electrochemical corrosion. Obviously, careful design can reduce workpiece corrosion and extend the duration of ship repairs.
2. Avoid gaps. Seal tightly against inevitable gaps to prevent seawater and moisture from entering.
3. Use continuous welding as much as possible, without or with less spot welding and riveting. The preferred welding method is friction stir welding (FSW).
4. Easy to drain, easy to clean structure.
5. Aluminum surface is not easy to absorb water, moisture absorption material contact, when inevitable, effective measures should be taken to prevent moisture.
6. Try to avoid sharp turns inside the piping system.
7. Avoid heat transfer to hot spots.
8. Avoid direct fluid impact.
9. Avoid large mechanical stress concentrations.
10. The equipment is located in a less corrosive place.
11. If the equipment or facility is to be painted, sharp edges should be eliminated.
To avoid contact corrosion, design should be:
1. The dissimilar metal potential should be as close to the aluminum potential as possible.
2. When fastening different metal workpieces, the fastener shall be cathode to aluminum. For example, aluminum-steel parts may be fixed with stainless steel screws, aluminum screws may not be used, and aluminum-plated stainless steel screws are preferably used.
3. When the two metals are to be fully electrically insulated, non-conductive insulating mats are used. At the same time, the two metals must not be connected to external circuits.
4. If the two metal surfaces must be painted with paint, they should be applied to the cathode metal and must not be applied to aluminum, otherwise it will cause pinhole corrosion.
5. When connecting aluminum tubes to other metals, replaceable thick-walled aluminum fittings or flanges should be used.
6. Keep dissimilar metal joints as far away as possible from corrosion.
7. Add a corrosion inhibitor to a dissimilar metal liquid circulation system.
8. Use cathodic protection.
9. In the conductive environment, the cathode-anode area ratio. In seawater, for example, this ratio should be at least 1:5.
Avoid deposit corrosion criteria:
1. Do not use heavy metal parts.
2. Apply paint on metal parts that can cause corrosion.
3. Aluminum coated aluminum.
4. Add corrosion inhibitors.
5. Frequent cleaning of heavy metals and dirt deposited on aluminum components.
Crevice corrosion prevention: First, try to be as seamless as possible in the design. If it is unavoidable, seal it 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 surface of aluminum must not come into contact with moisture-absorbing materials (paper, cloth, wood, asbestos, etc.).
Organic Coatings Organic coatings on aluminum equipment and components are mostly for aesthetic purposes, such as building facades and car bodies, but in special cases there are also anti-corrosion treatments. Regardless of the purpose, paints should be carefully selected and strictly pre-ordered before painting. The coating is a physical barrier between the aluminum surface and the environment. Some paints contain corrosion inhibitors such as chromates. In order to reveal the true color of aluminum, it can be coated with clear paint. In storage and transportation, in order to prevent corrosion, paint can be coated easily, and aluminum components embedded in soil and concrete can be coated with coal tar. Organic corrosion protection coatings should be regularly maintained in severely corrosive environments.
Corrosion Inhibitors There are two types of corrosion inhibitors: Anodic corrosion inhibitors, such as chromate, which can reduce the anode reaction of aluminum; Cathode corrosion inhibitors, such as polyphosphate, which can slow the cathode reaction. When the amount of anode corrosion inhibitor is not enough, it will accelerate the point corrosion of aluminum. Therefore, the cathode corrosion inhibitor is safer and more reliable, and the two corrosion inhibitors will have better effect at the same time.
Corrosion inhibitors for aluminum and aluminum alloys include phosphates, silicates, fluorides, triethanolamine benzoate, and lactic acid gels, which may be used alone or in combination. In closed-loop systems, the presence of copper or copper alloy parts with sodium thiol and thiazole as corrosion inhibitors can prevent copper corrosion and subsequent aluminum deposition corrosion. Adding sodium silicate to the weak alkaline solution can inhibit the corrosion of aluminum.
The cathodic protection of the cathodic protection of the aluminum component can be supplied either by the sacrificial anode or by an external power source. Sacrificial anodes are available with Zn, Mg, and Zn alloys. Some aluminum alloys can also be used in seawater. The sacrificial anodes of Zn and Zn alloys are safe and can effectively protect the aluminum components.
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