Corrosion of aluminium

Aluminium is very durable – it is known for its protective oxide layer. But like all metals, aluminium also has weaknesses that you need to account for.

This page provides an overview of how to avoid aluminium corrosion. If you are working on a specific project, feel free to contact us for advice and guidance.

Aluminium in marine environments

Aluminium can be used in marine environments. Although moisture and salt can degrade aluminium, this is much less of a problem with marine grade alloys.

See examples of aluminium in offshore environments here.

The marine grade alloys are, according to EN13195:2009, a large part of the 5xxx and 6xxx series. 5083, 5754, 6060 and 6082 are typical examples of alloys that can be marine grade.

In addition, EN1999-1-1 (Eurocode 9) recommends using screws, bolts and other fasteners in stainless steel A4 1.4404/316L to avoid galvanic corrosion.

The oxide layer protects against corrosion

Aluminium is highly resistant to corrosion compared to a lot of other metals.

This is due to the thin oxide layer that aluminium forms when it comes into contact with oxygen. If the oxide layer is damaged, it restores itself within milliseconds – if oxygen is present.

The oxide layer is very hard and difficult to penetrate, providing the metal with protection from external threats.

In a dry environment with fairly stable temperatures, the oxide layer alone is enough to protect aluminium from corrosion. No additional surface treatment is required.

See an example of untreated aluminium as facade cladding here.

To anodise aluminium is to stimulate the growth of the natural oxide layer to make it thicker. This improves the corrosion resistance and can also have an aesthetic purpose.

Untreated aluminium will not retain its shiny surface over time because the oxide layer causes a natural patination of the surface. If you have specific surface requirements, you may need to treat the surface by anodising or painting it.

Acids and bases

The optimal pH value for the oxide layer is between 4 and 9. This means that acidic and alkaline environments outside this range degrade the oxide layer and cause pitting in the exposed aluminium surface.

Bases break down aluminium faster than acids. For example, concentrated sodium hydroxide reacts so violently with aluminium that it can start to boil.

Wet concrete in a construction site is a typical risk factor to be aware of. Concrete has a pH value of 12.5-13.5, far above the tolerance of the oxide layer. To avoid corrosion, the aluminium should be kept away from wet concrete or covered with a protective material while the concrete is wet. Anodising cannot prevent this type of corrosion.

Types of corrosion in aluminium

The most common types of aluminium corrosion are galvanic corrosion and pitting. If your material is corroding, you have probably encountered one of the two.

Galvanic corrosion

Galvanic corrosion occurs when two metals degrade each other. If aluminium comes into contact with a more noble metal (e.g. copper, zinc and certain types of steel) through an electrolyte (e.g. salt water), the aluminium will degrade.

The electrolyte is necessary for galvanic corrosion to happen. This makes marine environments the main risk factor, and you should take precautions in these areas.

In a dry indoor environment or a non-marine outdoor environment, combining aluminium with a more noble metal is generally not a problem.

Prevention of galvanic corrosion
There are various methods to prevent galvanic corrosion. One simple method is to insulate the different metals with electrical insulation. This breaks the electrical contact created by the electrolyte.

You can also use a surface treatment, e.g. paint, on the most noble metal to protect against galvanic corrosion. Make sure that the coating is tightly sealed - even small scratches can be enough to ignite the corrosion reaction. Read more about surface treatment as a protective measure here.

Another method is to use sacrificial anodes. This involves "sacrificing" another metal that is less noble than aluminium. The metal is placed on the aluminium, which is now no longer the less noble metal, and the corrosion will attack the sacrificial anode.

Mechanical surface treatment can be a source of galvanic corrosion if you are not careful. It is very important that the tools do not contain copper or steel, which can react with aluminium.

It is also important that the tools are not used on other metals. If the tool transfers particles or chips from other metals to the aluminium surface, there is a high risk of galvanic corrosion. For the same reason, make sure to keep a physical distance from aluminium parts when treating or machining other metals so chips from the other metal does not land on the aluminium.

Pitting

Pitting is less severe than galvanic corrosion. Pitting is usually a local issue that is primarily aesthetic.

Like galvanic corrosion, pitting is caused by an electrolyte - i.e. salty water or moisture.

Aluminium in humid, salty environments is at risk of pitting. Dirt on the aluminium surface can also be a risk factor if it traps moisture on the surface and creates a breeding ground for pitting.

Prevention of pitting
Pitting is usually superficial and does not affect the strength of the material. However, to prevent pitting, it's important to keep the aluminium surface dry and reasonably clean.

We recommend rinsing the surface with water regularly (1-2 times a year) and designing the surface in a way that leads water and moisture away.

A simple surface treatment such as anodising or painting also offers good protection against pitting.

Filiform corrosion

With painted items there is a risk of filiform corrosion. Filiform corrosion occurs when liquids get under the paint and react with the exposed aluminium surface.

Risk factors for untreated aluminium

Due to the oxide layer, aluminium can often be used as is without surface treatment, even outdoors. However, as described above, there are limits to what the oxide layer can withstand.

By choosing the right alloy for the specific environment and avoiding risk factors, such as strong acids and bases, you increase the lifetime of the metal.

Risk factors

As a rule of thumb, you should avoid exposing untreated aluminium to direct contact with the following substances and materials:

• Other metals, e.g. copper, lead and iron. This is especially important in humid conditions.
• Inorganic acids, e.g. hydrochloric acid and sulphuric acid.
• Formic acid, oxalic acid and chlorinated solvents.
• Bases
• Mercury and mercury salts.
• Chloride solutions.
• Water containing heavy metals.
• Acidic wood, damp wood and wood impregnated with copper-containing salts.
• Alkaline building materials, e.g. fresh concrete.

However, with the right alloy or surface treatment, you can minimise the risk from most of the above factors. You might not avoid corrosion altogether, but you can minimise the damage.

How to clean aluminium

Accumulated dirt on an aluminium surface can retain moisture and cause corrosion. To avoid this, we recommend cleaning aluminium surfaces 1-2 times a year to prevent corrosion.

Read more about methods and recommended cleaning agents here.

Corrosion classes

Corrosion classes for aluminium are not defined by any standard. The corrosion class must be calculated for each individual situation.

Note that a given environment may have different corrosion classes for different metals. For example, a marine environment will generally be C5 for steel but only C3 for aluminium.

The corrosion class for a given environment is determined in three steps:

1. Define the following four parameters for the given climate:
   
   
   1. T = average temperature
   2. RH = average humidity
   3. Pd = average SO2 content in the air (typically higher in industrial areas)
   4. Sd = average chloride (Cl-) content in the air (typically higher near salt water)

2. insert the parameters into the formula and calculate the corrosion rate (r_corr)
3. Find the corrosion class in the table: