Tips to avoid corrosion in marine environments
Aluminium has excellent corrosion resistance because it spontaneously forms a thin but effective oxide layer that prevents further oxidation. And aluminium oxide is impermeable.
However, aluminium alloys perform differently in saltwater environments or in a saline atmosphere. In such cases, the best alloys regarding corrosion resistance belong to the 5000-series (5052, 5083, 5383, 5059) and 6000-series (6082, 6005A, 6063).
Intergranular and exfoliation corrosion performance of aluminium alloys
The strength of the above alloys increases with the magnesium (Mg) content. However, normally with Mg exceeding 3 percent, the risk for intergranular and exfoliation types of corrosion increases. This is caused by the tendency of Mg atoms to precipitate as anodic β-phase (Mg5Al8) particles, on grain boundaries.
This process occurs during the production of the material, as well as after. It is called sensitization. Sensitization is a very slow process occurring even at room temperature, but significantly faster at higher temperatures (>65°C). However, with use of the right alloying elements and production recipe, this increased risk can be eliminated.
Testing the intergranular and exfoliation corrosion performance of an aluminium alloy is done by the NAMLT and ASSET tests, specified in the ASTM B928 standard for "High Magnesium Aluminum Alloy Products for Marine Service and Similar Environments."
Corrosion-resistant aluminium alloys in marine and offshore
In addition to choosing the right aluminium alloy, here are some tips to ensure a long-lasting aluminium structure in marine and offshore environments:
- Where different metals are used in combination, galvanic corrosion can be prevented by electrically insulating them from each other. The insulation must break all contact between the metals. This established method is also used to connect pipe flanges offshore. Another insulation alternative is to use sealing wax or caps on the bolt head, nuts and washers.
- Use A4-quality (316l or 1.4571) stainless steel fasteners or aluminium rivets.
- On aluminium boats, prevent additional electrical currents on the metal parts of a boat, do not ground to the boat structure, and use an isolation transformer for shore connection.
- Avoid galvanic contact with carbon steel.
- Painting used as an insulator should only be applied to the cathodic surface.
- Natural cleaning of the surface by rain is advantageous.
- Permanent moisture in crevices is to be avoided.
- In a submerged zone, about 10 percent of the sacrificial anodes (zinc, magnesium) necessary for protecting carbon steel, provide effective protection for aluminium.
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