Aluminium corrosion resistance

Aluminium corrosion resistance is very good in untreated aluminium. Untreated aluminium has very good corrosion resistance in most environments. This is primarily because aluminium spontaneously forms a thin but effective oxide layer that prevents further oxidation.

Aluminium oxide is impermeable and, unlike the oxide layers on many other metals, it adheres strongly to the parent metal. If damaged mechanically, aluminium’s oxide layer repairs itself immediately.

This oxide layer is one of the main reasons for aluminium’s good corrosion properties. The layer is stable in the general pH range 4 – 9.

Jump to:

  1. Galvanic corrosion
  2. Pitting
  3. Crevice corrosion
  4. Aluminium in the open air
  5. Aluminium in soil
  6. Aluminium in water
  7. Aluminium and alkaline building materials
  8. Aluminium and chemicals
  9. Aluminium and dirt
  10. Aluminium and fasteners
  11. Corrosion checklist

The most common types of aluminium corrosion are:

  • galvanic corrosion
  • pitting
  • crevice corrosion

Stress corrosion, which leads to crack formation, is a more special type of corrosion. It occurs primarily in high-strength alloys (e.g. AlZnMg alloys) where these are subjected to prolonged tensile stress in the presence of a corrosive medium.
This type of corrosion does not normally occur in common AlMgSi alloys.

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Galvanic corrosion

Galvanic corrosion may occur where there is both metallic contact and an electrolytic bridge between different metals. The least noble metal in the combination becomes the anode and corrodes. The most noble of the metals becomes the cathode and is protected against corrosion. In most combinations with other metals, aluminium is the least noble metal. Thus, aluminium presents a greater risk of galvanic corrosion than most other structural
materials. However, the risk is less than is generally supposed.

Close-up of galvanic corrosion in an aluminium rail post (25 year’s use). The rectangular hollow profile was held in place by a carbon steel bolt. The contact surfaces between the steel and the aluminium were often wet and attack was aggravated by wintertime salting.

Galvanic corrosion of aluminium occurs:

  • Only where there is contact with a more noble metal (or other electron conductor with a higher chemical potential than aluminium, e.g. graphite).
  • While, at the same time, there is an electrolyte (with good conductivity) between the metals.

Galvanic corrosion does not occur in dry, indoor atmospheres. Nor is the risk great in rural atmospheres. However, the risk of galvanic corrosion must always be taken into account in environments with high chloride levels, e.g. areas bordering the sea. Copper, carbon steel and even stainless steel can here initiate galvanic corrosion.

Problems can also occur where the metallic combination is galvanised steel and aluminium. The zinc coating of the galvanised steel will, at first, prevent the aluminium being attacked. However, this protection disappears when the steel surface is exposed after the consumption of the zinc.

As it has a thicker zinc coating than electroplated material, hot dip galvanised material gives longer protection. Thus, in combination with aluminium in aggressive environments, hot dip galvanised material should be used.

Preventing galvanic corrosion

The risk of galvanic corrosion should not be exaggerated – corrosion does not occur in dry, indoor atmospheres and the risk is not great in rural atmospheres.

Electrical insulation

Where different metals are used in combination, galvanic corrosion can be prevented by electrically insulating them from each other. The insulation has to break all contact between the metals.

The illustration shows a solution for bolt joints.

Breaking the electrolytic bridge

In large constructions, where insulation is difficult, an alternative solution is to prevent an electrolytic bridge forming between the metals. Painting is one way of doing this. Here, it is often best to coat the cathode surface (i.e. the most noble metal).

A further solution is to use an insulating layer between the metals.

Cathodic protection

Cathodic protection can be gained in two ways. The most common is to mount an anode of a less noble material in direct metallic contact with the aluminium object to be protected. The less noble material “sacrifices” itself (i.e. corrodes) for the aluminium. It is thus referred to as a sacrificial anode.

For the above to work, there also has to be liquid contact between the surface to be protected and the sacrificial anode.

Zinc or magnesium anodes are often used for aluminium.

Another way of obtaining cathodic protection is to connect the aluminium object to the negative pole of an exterior DC voltage source.

The illustration below shows the cathodic protection of an outboard motor.

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Pitting

For aluminium, pitting is by far the most common type of corrosion. It occurs only in the presence of an electrolyte (either water or moisture) containing dissolved salts, usually chlorides.

The corrosion generally shows itself as extremely small pits that, in the open air, reach a maximum penetration of a minor fraction of the metal’s thickness. Penetration may be greater in water and soil.

As the products of corrosion often cover the points of attack, visible pits are rarely evident on aluminium surfaces.

Preventing pitting

Pitting is primarily an aesthetic problem that, practically speaking, never affects strength.

Attack is, of course, more severe on untreated aluminium. Surface treatment (anodising, painting or other coating methods) counteracts pitting.

Cleaning is necessary to maintain the treated surface’s attractive appearance and its corrosion protection. Rinsing with water is often sufficient. Alkaline detergents should be used with care. Mild alkaline detergents are now available. These are used in, amongst other areas, the industrial cleaning of aluminium.

Pitting can be prevented by cathodic protection (above). It is also important to design profiles so that they dry easily.

Avoid angles and pockets in which water can collect. Instead, use a shape that promotes draining.

Stagnant water is avoided by suitably inclining the profile and/or providing drain holes. The ventilation of "closed" constructions reduces the risk of condensation.

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Crevice corrosion

Crevice corrosion can occur in narrow, liquid-filled crevices. The likelihood of this type of corrosion occurring in extruded profiles is small. However, significant crevice corrosion can occur in marine atmospheres, or on the exteriors of vehicles. During transport and storage, water sometimes collects in the crevices between superjacent aluminium surfaces and leads to superficial corrosion (“water staining”).

The source of this water is rain or condensation that, through capillary action, is sucked in between the metal surfaces. Condensation can form when cold material is taken into warm premises. The difference between night and day temperatures can also create condensation where aluminium is stored outdoors under tarpaulins that provide a tight seal.

Preventing crevice corrosion and aluminium corrosion resistance

Using sealing compounds or double-sided tapes before joining two components prevents water from penetrating into the gaps.

In some cases, rivets or screws can be replaced by, or combined with, adhesive bonding. This counteracts the formation of crevices.

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Aluminium in the open air

The corrosion of metals in the open air depends on the so-called time of wetness and the composition of the surface electrolytes. The time of wetness refers to the period during which a metal’s surface is sufficiently wet for corrosion to occur. The time of wetness is normally considered to be when relative humidity exceeds 80% and, at the same time, the temperature is above 0°C (e.g. when condensation forms).

In normal rural atmospheres, and in moderately sulphurous atmospheres, aluminium’s durability is excellent. In highly sulphurous atmospheres, minor pitting may occur. However, generally speaking, the durability of aluminium is superior to that of carbon steel or galvanised steel.

The presence of salts (particularly chlorides) in the air reduces aluminium’s durability, but less than is the case for most other construction materials. Maximum pit depth is generally only a fraction of the thickness of the material. Thus, in marked contrast to carbon steel, strength properties remain practically unchanged.

The picture shows an untreated sample after 20 years off the south-west coast of Sweden. UV radiation, sulphuric acid and nitric acid in combination with chlorides have not left any deep marks. After 22 years in a marine atmosphere, examination of an untreated aluminium sample (alloy AA 6063) showed that corrosion attack was so limited (max. depth approx. 0.15 mm) that strength was not affected.

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Aluminium in soil

Soil is not a uniform material. Mineral composition, moisture content, pH, presence of organic materials and electrical conductivity can all vary widely from site to site. These differences make it difficult to predict a metal’s durability in soil. Furthermore, other factors (e.g. stray currents from DC voltage sources) can also affect durability.

Aluminium’s corrosion properties in soil very much depend on the soil’s moisture, resistivity and pH value. Unfortunately, present knowledge about the corrosiveness of different types of soils is not comprehensive.

When using aluminium in soil, some form of protective treatment, e.g. a bitumen coating, is recommended. Corrosion can also be prevented by cathodic protection.

Bitumen coating prevents corrosion

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Aluminium in water

A metal’s corrosion in water is largely dependent on the composition of the water. For aluminium, it is the presence of chlorides and heavy metals that has the greatest effect on durability.

In natural fresh water and drinking water, aluminium may be subject to pitting. However, with regular drying and cleaning, the risk of harmful attack is small. Pots, pans and other household equipment can be used for decades without there being any pitting.

The likelihood of harmful attack increases where water is stagnant and the material is wet for long periods.

In sea water, AlMg alloys with over 2.5% Mg (and AlMgSi alloys) show particularly good durability.

Copper containing alloys should be avoided. Where they are used, they must be given effective corrosion protection.

When correct attention has been paid to design, especially as regards use with other materials (and the risk of galvanic corrosion), aluminium is an excellent material in a marine context. One example of this is the extensive use of aluminium in many types of ships and boats. Cathodic protection against corrosion is widely used here.

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Aluminium and alkaline building materials

Splashes of damp alkaline building materials, e.g. mortar and concrete, leave superficial but visible stains on aluminium surfaces. As these stains are difficult to remove, visible aluminium surfaces should be protected on, for example, building sites. Other materials also require the same sort of protection.

Aluminium cast into concrete is similarly attacked. This increases the adhesion between the materials. Once the concrete has set (dried), there is normally no corrosion. However, where moisture persists, corrosion may develop. The volume of the products generated by corrosion can give rise to cracks in the concrete.

This type of corrosion can be effectively prevented by coating the aluminium with bitumen or a paint that tolerates alkaline environments. As the oxide layer is not stable in strongly alkaline environments, anodising does not improve durability here.

Provided that the concrete has set, aluminium does not need to be protected in dry, indoor atmospheres.

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Aluminium and chemicals

Thanks to the protective properties of the natural oxide layer, aluminium shows good resistance to many chemicals. However, low or high pH values (less than 4 and more than 9) lead to the oxide layer dissolving and, consequently, rapid corrosion of the aluminium. Inorganic acids and strong alkaline solutions are thus very corrosive for aluminium.

Exceptions to the above are concentrated nitric acid and solutions of ammonia. These do not attack aluminium.

In moderately alkaline water solutions, corrosion can be hindered by using silicates as inhibitors. Such kinds of inhibitors are normally included in dishwasher detergents.

Most inorganic salts are not markedly corrosive for aluminium. Heavy metal salts form an exception here. These can give rise to serious galvanic corrosion due to the reduction of heavy metals (e.g. copper and mercury) on aluminium surfaces.

Aluminium has very good resistance to many organic compounds. Aluminium equipment is used in the production and storage of many chemicals.

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Aluminium and dirt

Coatings or build-ups of dirt on the metal’s surface can reduce durability to a certain extent. Very often, this is attributable to the surface now being exposed to moisture for considerable periods. Thus, depending on the degree of contamination, dirty surfaces should be cleaned once or twice a year.

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Aluminium and fasteners

When choosing fasteners for use with aluminium, special attention should be paid to avoiding galvanic corrosion and crevice corrosion (above). Galvanic corrosion of aluminium occurs where there is metallic contact with a more noble metal. It should be pointed out that, indoors and in other dry atmospheres, aluminium can be in permanent contact with brass and carbon steel with no risk of galvanic corrosion.

The pictures below show the results of an accelerated corrosion test, the Volvo indoor Corrosion Test (VICT). The test cycle is 12 weeks and corresponds to five year’s use of a car in a moderately large town.

Left – Zinc/iron-coated steel nut and bolt. The fastener is completely rusted. In the aluminium, 0.43 mm deep pits have formed.

Right – Dacrolit-coated steel nut and bolt. The fastener has not been attacked. No pits have formed in the aluminium.

At-a-glance guide for choosing fasteners

The table below lists some of the most common materials and coatings for fasteners used with aluminium. It also gives an evaluation of corrosion resistance in different environments.

Substrate material Surface treatment
Atmospheres
Comments
Marine
Industrial
Rural
Carbon steel Electroplated (Zn/Ni)approx. 7 – 10 μm+ yellow chromating.
++
+++
+++
Used in the automotive industry.Good protection against galvaniccorrosion.
Carbon steel Electroplated (Zn/Fe)approx. 7 – 10 μm+ yellow chromating.
-
+
+++
Negative results on vehicles.The Zn layer disappears relatively quicklyand galvanic corrosion then sets in.
Stainless steel, 18/8 Electroplated,approx. 7 – 10 μmZn + yellow or brightchromating.
++
+++
+++
Used primarily in the building industry.The Zn coating is principally to reducefriction (bolt threads).
Stainless steel, 18/8 Dacrolit – Zn and Al flakesin an organic bindercontaining, amongstother things, chromate.
+++
+++
+++
Used primarily in the building industry.The Dacrolit coating is used to reducefriction (bolt threads) and the risk ofgalvanic corrosion.
Stainless steel, 2302 Electroplated (Zn/Fe)7 – 10 μm + yellow orbright chromating.
+
+++
+++
Used primarily in the building industry.
Carbon steel Dacrolit – Zn and Al flakesin an organic bindercontaining, amongstother things, chromate.
++
+++
+++
Used primarily on vehicles and, in somecases, buildings. Withstands 12 weeksVICT (Volvo Indoor Corrosion Test)
Carbon steel Geomet – Zn and Al flakesin a matrix of Si, Zn andAl oxides. Chrome-free.
++
+++
+++
Very good corrosion resistance shown intests in the automotive industry. Suppliersstate that it withstands 1,000 hours in aneutral salt spray (ISO 9227).
Carbon steel Polyseal – Zn phosphatingapprox. 3 μm + organicprotection layer (seal)+ organic top coat.
-
++
+++
Used in the automotive industry,Good results in acetic acid and neutralsalt spray (ISO 9227).
Aluminiumrivet withelectroplatedsteel mandrel. No coating.
++
+++
+++
Used in the building industry.
Stainless steel(18/8) rivetwith stainlesssteel mandrel. No coating.
+
++
+++
Galvanic corrosion in marineatmospheres.

Evaluations: +++ = very good; ++ = good; + = acceptable with moderate demands as regards lifetime (up to 10 years) and surface finish.

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Corrosion checklist

The summary below is intended to give a picture, from the perspective of durability, of aluminium as a construction material. Used correctly, aluminium has a very long life.

Environments
Rural atmosphere Aluminium has excellent durability.
Moderately sulphurous atmosphere Aluminium has excellent durability.
Highly sulphurous and marine atmosphere Superficial pitting can occur. Nonetheless, durability is generally superior to that of carbon steel and galvanised steel.
Corrosion problems can be overcome
Profile design The design should promote drying, e.g. good drainage.Avoid having unprotected aluminium in protracted contactwith stagnant water.Avoid pockets where dirt can collect and keep the material

wet for protracted periods.

pH values Low (under 4) and high (over 9) values should, in principle,be avoided.
Galvanic corrosion In severe environments, especially those with a high chloridecontent, attention must be paid to the risk of galvanic corrosion. Some form of insulation between aluminium andmore noble metals (e.g. carbon steel, stainless steel, copper)is recommended.
Closed system (liquid) In closed, liquid containing systems, inhibitors can oftenbe used to provide corrosion protection.
Severe, wet environments In difficult, wet environments, the use of cathodic protectionshould be considered.

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