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Aluminium and Corrosion

Aluminium is a highly reactive material. In most surroundings, inside or in the open air, Aluminium has a very high resistance to corrosion. It rapidly forms a thin and dense oxide layer that protects the underlying material very well. The presence of the oxide layer makes aluminium very suitable for many applications. But some environments aluminium may show corrosion behaviour. This section will explain in detail the principles of aluminium corrosion and surface engineering that will enable the right use of aluminium in the right place.

Image of exfoliation of aluminum Image of hot-rolled AA8006 aluminum alloy
The left picture shows exfoliation of aluminum, right picture shows Surface layer on hot-rolled AA8006 aluminum alloy.

Aluminium is covered spontaneously by a thin but effective coating of oxide which protects the Aluminium from further oxidation. The Aluminium oxide is impermeable and is integral with the base metal. If the coating of oxide is damaged mechanically this coating will be renewed immediately. The coating of oxide is a main reason for the good corrosion characteristics of Aluminium. The coating stays stable for pH-values between 4 and 9. Generally alloys with more than 0.5% copper have a worse resistance to corrosion and they should not be used in unprotected, strong chlorine surroundings.

The most frequent types of corrosion of Aluminium are Galvanic Corrosion, Pitting and Crevice Corrosion.

Galvanic Corrosion

Galvanic corrosion takes place when two different metals contact with each other in the prescence of an electrolyte. While the noble (cathode) will be protectedThe, less noble metal (anode) of the combination corrodes . A small surface of the cathode and a large surface of the anode lead to a low corrosion. In reverse case the Aluminium will be attacked quickly. In most combinations with other metals, Aluminium is the less noble. Aluminium is therefore of greater risk of galvanic corrosion than that of the other constructive materials.

These dangers of galvanic corrosion only exist in metallic contact with more noble metals (or other conductors of electrons, for example graphite) and then when an electrolyte with good conductivity exists between the metals. The occurrence of galvanic corrosion will be encouraged because of a disadvantageous construction of the extrusion.

Common forms of metal joining, e.g. brazing, welding etc. provide junctions at which galvanic corrosion can develop. Galvanic corrosion on a microscopic scale can also occur between constituents of multi-phase alloys and of impure metals which contain foreign particles and intermetallic compounds.

Galvanic corrosion does not happen in dry surroundings, however contrary to this, in surroundings with chloride the risk of galvanic corrosion always exists, for example near the seaside. In these surroundings it could be that copper, carbon steel and stainless steel suffer galvanic corrosion.

It should be noted that problems can occur in the combination of galvanised steel and Aluminium. This is due to the fact that the Aluminium will be protected by the coating of zinc but when the zinc is used up the bare surface of steel can attack the Aluminium. For this reason the Aluminium should be combined with warm zinced material in an aggressive surrounding because warm galvanising, gives a harder coating of zinc, than hot-dipped.

Galvanic corrosion can be prevented if certain steps are taken. One being by electric insulation between the metals, where the insulation has to interrupt the metallic contact completely.

In large constructions where electric insulation is difficult it is possible to use an electrolyte insulator between the two metals for example by painting to interrupt the connection. Often it is beneficial to conceal the surface of the cathodes (that of the nobler metal), however another possibility is the installation of an intermediate layer

Another method of protection is Cathodic protection. Cathodic protection can be achieved in two ways. Often anodes, consisting of less noble metal can be in contact with the surface of Aluminium that has to be protected. Within this process the less noble metal was sacrificed (it is corroding ) and is therefore called a sacrificial anode. The existence of a contact of liquid between the coating which has to be protected and the anode is another condition for prevention of corrosion. Often Z- or Mg-anodes are used for Aluminium. Cathodic protection can be reached by an exterior constant-potential supply and by connecting an Aluminium object to the negative pole.

Pitting corrosion

Corrosion of aluminum in the passive range is localized, usually manifested by random formation of pits. The pitting potential principle establishes the conditions under which metals in the passive state are subject to corrosion by pitting.

Pitting is a frequent type of corrosion of Aluminium and occurs due to the presence of an eletrolyte. The corrosion is seen as very small pits and only penetrates a small way into the surface when induced in air however, in water and earth larger pits can occur. As residues of corrosion often cover the small pits the attacked places in the surface of Aluminium are rarely visible. Small pits are mainly a problem of appearance and have no negative influence on the temper, however treatment of the surface, anodising and painting can prevent this corrosion. The treated surface is cleaned so that the attractive appearance is kept and corrosion is prevented. Occasional washing with water is sufficient to prevent pitting on untreated metal. Alkaline cleaning materials must be avoided.

Pitting corrosion is most commonly produced by halide ions for aluminium, of which chloride (Cl -) is the most frequently encountered in service. Pitting of aluminum in halide solutions open to the air occurs because, in the presence of oxygen, the metal is readily polarized to its pitting potential.

Generally, aluminum does not develop pitting in aerated solutions of most nonhalide salts because its pitting potential in these solutions is considerably more noble (cathodic) than in halide solutions and it is not polarized to these potentials in normal service.

Pitting can be prevented by cathodic protection. It is important that the extrusions are shaped in a way that can dry easily and edges and hollows in which dirt and moisture can accumulate should be avoided. Stagnant water can be prevented by inclination of the extrusions or drain holes (minimum 8 mm respectively 6x20 mm because otherwise the water cannot run off due to the capillary force). Also airing of "closed" constructions reduces the risk of condensation.

Crevice corrosion

Crevice corrosion can arise in small crevices filled with fluid however this rarely happens in extruded constructions and this risk is increased in marine atmospheres. It can happen during transport and storage that water gathers in the crevices between Aluminium surfaces opposite to each other and this can lead to corrosion on the surface (water spots). The rain water is sucked up between the surfaces of the metal or condensation water occurs when cold material is brought into warm surroundings. Condensation water can also arise when Aluminium is stored carefully covered in the open air because of the varying temperatures.

The use of sealing compound or tape that is adhesive on both sides, before joining the pieces prevents water from entering into crevices. Sometimes rivets or screws can be substituted by glue or they can be combined. By this formation crevices corrosion can be prevented.

Other types of Aluminum Corrosion are Filiform, Intergranular and Exfoliation, Erosion, Cavitation, Impingement, Fretting, Environmentally Assisted Cracking, etc. 
 

Corrosion Fatigue

Corrosion fatigue is a special case of stress corrosion caused by the combined effects of cyclic stress and corrosion. No metal is immune from some reduction of its resistance to cyclic stressing if the metal is in a corrosive environment. Damage from corrosion fatigue is greater than the sum of the damage from both cyclic stresses and corrosion.  Control of corrosion fatigue can be accomplished by either lowering the cyclic stresses or by corrosion control.

Under conditions of simultaneous cyclic stressing and corrosion, the reduction in strength is greater than the additive effects of the separate processes. Although it is often possible to provide adequate protection for metallic parts which are stressed under static conditions, most surface films, including naturally protective oxides, can be more easily broken or disrupted under cyclic loading.

The fatigue strength of a material in a particular corrosive medium can be related to the corrosion resistance of the materials in that medium. In this corrosion, all types of aluminium alloys exhibit about the same percentage reduction in strength when compared with their fatigue strength in air. When tested in sodium chloride solutions, the fatigue strength at 108 cycles is normally between 25% and 35% of that in air. Also the corrosion-fatigue strength of a particular aluminium alloy can appear to be virtually independent of its metallurgical condition.

Filiform Corrosion

This type of corrosion occurs under painted or plated surfaces when moisture permeates the coating. Lacquers and "quick-dry" paints are most susceptible to the problem. Their use should be avoided unless absence of an adverse effect has been proven by field experience. Where a coating is required, it should exhibit low water vapor transmission characteristics and excellent adhesion.

This appears as a random non-branching white tunnel of corrosion product, either on the surface of non-protected metal or beneath surface coatings. It is a structurally insensitive form of corrosion which is often more detrimental to appearance than strength, although thin foil may be perforated, and attack of thin clad sheet, as used in aircraft construction, may expose the less corrosion-resistant aluminium alloy core.

Aluminium in the Atmosphere

The corrosion of metal in the atmosphere depends on the time of exposure and the composition of the electrolyte on the surface. The time of moisture is the time in which the surface of metal is so wet that corrosion can happen and the time of moisture is when the relative moisture exceeds 80% and the temperature is over 0oC at the same time. In the mainland atmosphere and in a moderate sulphurous atmosphere the durability of Aluminium is excellent. In strong sulphurous atmospheres it is possible that small corrosion may appear on the surface, but generally the durability is better than the durability of carbon steel and galvanised steel. The occurrence of salts, especially chlorides, in the atmosphere reduce the durability of Aluminium slightly compared with other materials. Mostly the maximum depth of small pits is only a fraction of the thickness of the material, so the characteristics of durability are nearly constant. However those of carbon steel are totally different.

Aluminium in the Ground

The ground is not a uniform material and it varies in the composition of minerals, the moisture content, degree of acidity, existence of organic materials and electric conductivity. For these differences, it is problematic to predict the durability of metals in the ground. In addition to these, other factors like the leakage current of constant -potential supplies can influence their durability. The characteristics of corrosion of Aluminium in the ground depends on the moisture, the specific resistance and the pH-value of the earth. The present knowledge about the characteristics of corrosion in different types of ground is not sufficient. When using Aluminium in the ground a protective treatment e.g. A coating of tar, is recommended. The corrosion can be interrupted by a cathodic protection.

Aluminium in Water

In water the corrosion of metal  mainly depends on the composition of water. Chlorides and heavy metals influence the durability of Aluminium. In fresh water and drinking water small pits in the Aluminium can occur. By regular cleaning the risk of corrosion is very low. Pots and other household objects can be used for decades without damage from corrosion. Stagnant water and long exposure increases the risk.

The Aluminium can be protected from small pits by designs which prevent the risk of banked-up water level, by using cathodic protection and chemical additives to delay the corrosion, for example, car radiators. The rate of pitting in fresh water decreases strongly over time. The corrosion present after three years will only have doubled over 24 years. In sea water A1Mg-alloys with more than 2.5% but also A1MgSi-alloys show a good stability. Cupriferous alloys should be avoided, however when still using them they have to be effectively protected from corrosion. When using the correct construction in combination with other metals (please note the risk of galvanic corrosion) Aluminium is an excellent material in the surroundings of the sea. An example is useful, frequent use of Aluminium in boats. Very often the protection from corrosion of this material is a cathodic protection.

Aluminium, which is only partly in the water, can corrode directly under the water surface. This type of corrosion, which only appears in stagnant water, can be protected by varnishing.

Aluminium and Alkaline Building Materials

Splashes of alkaline building materials like mortar and concrete cause visible spots on the surface of the Aluminium. These are difficult to remove. For that reason Aluminium should be protected on building sites. Aluminium cast in concrete will be attacked similarly and the adherence between the materials increases. After the cementation of the concrete corrosion cannot happen. Longer moisture can continue the process of corrosion so that the corrosion produces splits in the concrete. This type of corrosion can be stopped by coating the surface of Aluminium with tar or by coating with alkaline varnish. Anodising does not improve the durability because the coating of oxide is not stable in an alkaline surrounding. Indoors in a dry environment, the surface of Aluminium does not have to be protected.

Aluminium and Chemicals

Aluminium has a good durability against many chemicals. Low or high pH-values (less than 4 and more than 9) lead to a dissolution of the coating of the oxide and to a rapid corrosion of the Aluminium. Inorganic acids and strong alkaline solutions can then easily attack Aluminium. In moderate alkaline solutions of water the corrosion can be restricted by insertion of silicates as an inhibitor of corrosion. Normally such kinds of inhibitors are included in dishwasher detergents. Most inorganic salts have no marked effect of corrosion of Aluminium. Exceptions are heavy metal salts, which can start a strong galvanic corrosion on the surface of Aluminium because of the reduction of the heavy metals ( for example copper and mercury).

Aluminium has a strong durability against many organic combinations. Often objects of equipment for the production and the storage of chemicals are made of Aluminium.

Aluminium and Dirt

Coatings and accumulation of dirt on the intermediate coating can cause a reduced durability. Very often this is a consequence of long-term moisture. Dirty surfaces should be cleaned once or twice a year depending on the grade of contamination.