Aluminum and aluminum alloys have seen widespread use in almost all industrial applications over the past 100 years, with its usage second only to that of steel. The main reason is the relative abundance of the material along with its great mechanical and metallurgical properties.
These same properties can be precisely engineered to fit a wide range of demands, by using different alloying, tempering and fabrication processes. The flexibility of aluminum alloys, along with its inherent corrosion resistance allowed for its use in industries ranging from transportation, packaging and general construction to the aerospace industry.
Key Properties of Aluminum
Aluminum is a metallic chemical element. Its atoms form a crystalline lattice similar to that of austenitic steels, which means that it doesn’t have a ductile-brittle transition temperature (unlike non-austenitic steels).
It is also lighter than steel, with only one-third the density. However, aluminum alloys have a high strength-to-weight ratio. Pure aluminum is a fairly ductile material with low tensile strength, but high-strength alloys can have tensile strengths of up to 690 MPa (100 ksi).
Furthermore, aluminum retains its strength at extremely low temperatures, which makes it a good material for use in cryogenic applications. It is also an excellent conductor of heat and electricity, and it is nonferromagnetic, which is an important characteristic for use in the electronics and electrical industries (Learn more about this subject in Controlling Corrosion in Electronic Devices).
Aluminum is highly reflective, with an attractive natural finish. Most of the alloys are easily recyclable and it is nontoxic, which makes it an excellent choice for food and beverage packages.
In addition, it can be easily fabricated by most common metalworking methods, and its weldability ranges from pretty good to decent, depending on the alloy.
Natural Corrosion Resistance of Aluminum
One of the most important properties of aluminum is its corrosion resistance, which allows for structures to have a fairly long service life even in cases where the said structure is exposed to extreme weather changes, seawater and freshwater environments, or various soils and chemicals.
Combined with its great mechanical properties, this natural corrosion resistance contributed to aluminum's use in almost everything, from building ships, pipelines, powerlines, various chemical tanks and piping, to the beverage and food industry and high-performance aerospace applications (for more about aerospace applications please see Aviation Coatings for Corrosion Prevention).
In a bit of a twist, the reason for this corrosion resistance is not due to low reactivity of aluminum, but quite the opposite. Aluminum is a highly reactive element, with only beryllium and magnesium being more reactive of all the structural metals.
However, this same reactivity causes pure aluminum to spontaneously bond with oxygen into a highly inert oxide layer. This oxide barrier is only 5 nm (50 Å) thick in a normal atmosphere, but that is more than enough to protect the core as the layer reforms instantly if damaged (assuming oxygen is present).
In addition, while the melting temperature of pure aluminum is fairly low at 660°C (1,220°F), this oxide film has a melting temperature of over 2,000°C (3,632°F), making aluminum welding fairly tricky. AC current is used, as it is able to break the film instead of melting it.
In essence, the only situations where aluminum is susceptible to corrosion is when there is no oxygen to reform the oxide layer if damaged, or if the layer is damaged more rapidly than the self-repair can occur (e.g., in extremely acidic environments).
Corrosion of aluminum is in general associated with the flow of electrons between the anodic and cathodic regions of the material in an electrolyte, as it usually is in electrochemical corrosion of other common metals. The intensity of corrosion in this case depends on the difference of potential of the two regions, which is due to microstructural defects caused by fabrication, welding and other joining methods. The effect is further augmented by the difference in the electrical potential of the alloying materials (the alloy is never perfectly homogeneous, so there are microregions where the alloying material can be found in slightly larger quantities).
It goes without saying that this effect is only worsened if the aluminum alloy is in contact with a metal (or another alloy) with a different potential.
Due to the anti-corrosion effect of the oxide film, it is rare for uniform corrosion to form, and it is more likely that some form of localized corrosion event is going to occur, usually due to a combination of electrochemical and mechanical factors.
This most commonly happens in the form of mechanically assisted degradation, in which the corrosion process is augmented by pitting, cavitation, erosion and fretting.
Localized corrosion can also be caused by stress corrosion cracking (SCC), where we have static tension stress in an aggressive environment, or corrosion fatigue, if the loads (and stresses) are dynamic.
On the other hand, while uniform corrosion of aluminum is rare, it can occur in highly acidic or alkaline environments (thus pH values under 4 or over 9 should be avoided).
In environments where the oxide film can be dissolved, such as sodium hydroxide or phosphoric acid, aluminum breaks down at a steady rate, depending on the concentration and temperature of the solution. Depending on these factors, corrosion can range from superficial damage to complete and rapid dissolution. This uniform corrosion can most easily be assessed by measuring the weight or thickness loss.
Uniform corrosion is most common in pure aluminum, dilute alloys and non-heat-treatable alloys. The surface roughness, thickness variations and different concentrations of alloying elements can significantly change the material's surface makeup and create localized regions of positive and negative ions, which lead to more localized forms of corrosion.
In addition, if the surface oxide layer is insoluble in the environment, it leads to localized weak spots in the film where the chances of corrosion are significantly greater than in the other regions. As already stated, these weaknesses are further influenced by mechanical action.
The most common types of localized corrosion in aluminum are:
- Pitting corrosion.
- Crevice corrosion, including staining corrosion and poultice corrosion.
- Filiform corrosion.
- Intergranular corrosion.
- Exfoliation corrosion, which is a more severe form of intergranular corrosion that often occurs in the fuselage empennage and wing skins of aircraft.
- Galvanic corrosion (Although galvanic corrosion typically has a highly-localized nature, in some cases it can appear as a uniform thinning of the material if it occurs in the presence of a highly conductive electrolyte and if the anodic area is large enough).
- Biological corrosion, which usually accompanies and accelerates pitting or crevice corrosion.
Both uniform and localized corrosion are electrochemical in nature, and in the case of localized corrosion (which is more common) it is caused by a difference in the electric potential of the localized region. The most common culprits for this variance in potential are cathodic microconstituents found in the surface layer, such as CuAl2, FeAl3 and Si. There are other mechanisms, mostly caused by impurities, inclusions or differential aeration cells.
The only type of localized corrosion that can occur in aluminum that is not electrochemical in nature is fretting corrosion, which is a form of dry oxidation.
The main product of corrosion is almost exclusively aluminum trihydroxide (bayerite). While most types of corrosion occur in the presence of an electrolyte (mainly water), localized corrosion of aluminum usually does not occur in extremely pure water at ambient temperature.