Corrosion

Aluminium's
corrosion resistance
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. In strongly acid or alkaline environments, aluminium normally corrodes relatively rapidly.

Corrosion resistance in common profile alloys
Between Sapa's most widely used alloys, there is little variation in corrosion resistance. However, alloys containing more than 0.5% copper generally have poorer resistance. Therefore, they should not be used unprotected in environments with a high chloride content (e. g. where there is road salt or near sea water).

The most common kinds of corrosion
The most common types of 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.

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.

A small cathode surface and a large anode surface results in negligible corrosion.

Galvanic corrosion and aluminium
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 is often attributable to unsuitable structural design. 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 Ð 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 electrolyte 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 sacrifi cial 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.

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 and coating with HM-white) 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 (see previous page). 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.

The risk of dirt build-up is reduced with radiused corners.

Stagnant water is avoided by suitably inclining the profile and/ or providing drain holes (min. Ø 8 mm, or 6 x 20 mm, so that capillary forces do not prevent the water running off). The ventilation of "closed" constructions reduces the risk of condensation.

Crevice corrosion
Crevice corrosion can occur in narrow, liquid-filled crevices. The likelihood of this type of corrosion occurring in extruded profiles is small.

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

Aluminium in 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 suffi ciently 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.

Field exposure tests by the Swedish Corrosion Institute
in a range of outdoor atmospheres, the Swedish Corrosion Institute has carried out field exposure tests on untreated metals. For plates that had received no surface treatment, the weight losses after eight year's exposure are given here. After the eight years, the average pit depth in the aluminium plates was 70 µm (0.07 mm). The bar chart shows that aluminium's weight loss near the sea was: Ð approx. 1/ 100 th that of carbon steel (Fe). Ð approx. 1/ 10 th that of galvanised steel (see Zn in the bar chart). The rate of corrosion decreases rapidly with distance from the sea. Approximately 1 km from the sea, aluminium behaves more or less the same as it does in a rural atmosphere. The corrosion rate of the pits decreases with time.

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.

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.

Pitting can however be prevented by:

• design solutions that reduce the risk of water being trapped
• cathodic protection
• corrosion inhibitors, e. g. used in car radiators.

Corrosion at the water line
Aluminium that is only partly submerged in water can corrode directly under the water line (so-called waterline corrosion). This type of corrosion, which only occurs in stagnant water, can be prevented by coating the area around the water line.

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

Aluminium and chemicals
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.

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.

Aluminium and fasteners
When choosing fasteners for use with aluminium, special attention should be paid to avoiding galvanic corrosion and crevice corrosion (see sections 16.2.1, 16. 2.2 and 16.2.5).

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 table on page 129 shows some of the most common surface coatings for fasteners. The evaluation of the surface coatings is based on the findings of fastener and coating suppliers, as well as the experience of Sapa and its customers (primarily in the building and automotive industries).

In deciding which fasteners to use, the table should be regarded as an introductory guideline. As development is rapid, Sapa also recommends that fastener and coating suppliers be contacted.

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

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

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.