Aluminum is the second most abundant metal on the planet and, due to its excellent properties, it is one of the most widely used metals today. So, it is helpful to be aware of the conditions that lead to the premature life of these metals.
Corrosion of any metal can affect its effective strength, resulting in structural distress that can lead to cracks, partial fracture, and total material failure in the worst case scenario. In this post, we will look into the corrosion of Aluminum in detail, to get a better understanding of the types of corrosion that can affect the metal.
What is Aluminum Corrosion?
Aluminum corrosion results in aluminum molecules decayed into oxides which impact the physical and chemical properties of aluminum.
Aluminum is a reactive metal by nature; however, it is also a passive metal.
This means, while nascent aluminum will react with oxygen and water in its environment, at this point the compound that is formed is layered on the surface of the aluminum and ambiguous below it is protected from further corrosion.
This non-reactive oxide layer adheres well to the surface, does not flake away easily, like stainless steel.
Unlike other intended processes such as laser etching, anodising Aluminum, or brightening, corrosion is a slow process and will take place over many months or years.
What’s interesting about aluminum corrosion in general is that there are many different types of aluminum corrosion pathways. The first step in applying mitigation controls to reduce or prevent their occurrence is understanding these different corrosion phenomena.
Types of Aluminum Corrosion
#1. Atmospheric corrosion
The most prevalent type of Aluminum corrosion. Atmospheric corrosion of Aluminum is a type of corrosion caused by exposure to environmental conditions and natural elements. Atmospheric corrosion forms the bulk of the total damage caused to aluminum worldwide, though other types of corrosion can be significant too; however atmospheric corrosion can happen anywhere.
Atmospheric corrosion can be broken down into three different types: dry, wet and damp. This separation is based on the level of moisture in the service environment.
Moisture content can vary greatly depending on the location of an organisation, leading to different levels of corrosion.
Wind direction, temperature, and changes in precipitation are also significant factors that affect the rate of atmospheric corrosion. Other environmental factors, including concentration and types of pollutants in the air, the relationship to large bodies of water, etc. can also have significant impact.
Atmospheric corrosion can be aggravated with a design that doesn’t drain moisture. For example, pockets of water in a design by rain and condensation are counterproductive in design.
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#2. Galvanic corrosion
Galvanic corrosion, which is also referred to as dissimilar metal corrosion, can impact aluminum, if the aluminum is attached either physically or through an electrolyte to a noble metal. A noble metal can any metal that is less reactive than aluminum.
The reactivity of the conducting metal depends on its position in the electrochemical series. The galvanic corrosion will be less severe if the other metal is closer to aluminum in the electrochemical series.
The rate of corrosion is most severe at the contact of the metals and decreases as we move away from the interface. For example, aluminum and brass, under the contact of seawater will form a galvanic cell. In that case, with aluminum being the anode (positive terminal), will corrode.
This has been documented in boats whereby brass fittings can be in close proximity or even in contact with the aluminum fittings when submerged in seawater. The galvanic action and electron flow will take place from the aluminum to the brass through the seawater.
A galvanic cell can occur by accident in other service environments which may result in galvanic corrosion. The effect of galvanic corrosion can be much quicker than normal atmospheric caused corrosion.
#3. Pitting corrosion
Pitting corrosion is a surface corrosion phenomenon of aluminum metal which occurs as small holes (pits) on the surface of the material. Pitting often does not change the strength of the product. Normally, this type of corrosion is an aesthetic problem but can cause failure if the surface appearance is critical.
Pitting corrosion generally occurs in locations where atmospheric salt is present, and this salt is important due to the presence of chloride anions. Sulphate salts can contribute to pitting corrosion but not to the same extent as chlorides. The most aggressive pitting corrosion comes from alkaline and acidic salts.
There must be a difference between the alloy potential, which is above the electrolyte (brine) potential, in order for pitting to take place. The existence of surface defects at grain boundaries and second phase particles is a pathway to pitting corrosion.
#4. Crevice corrosion
Crevice corrosion represents a form of localized corrosion in materials. Crevices can create an area with overlapping materials or unintentional design features that protect that area from the external environment.
This crevice can trap seawater (from the marine aggression) into the crevice itself, which facilitates crevice corrosion.
Crevice corrosion can develop in any small opening: even a small space between a bolt and a structure can be enough to initiate this form of corrosion.
Over time aluminum dissolves from the material’s surface and into the seawater, becoming ionic aluminum. The ionic aluminum will extract Oxygen from the surrounding air and Hydroxide ions from the electrolyte and will form aluminum hydroxide.
The consumption of oxygen within the crevice makes the crevice acidic when introduced chlorides (often with seawater), which continues to promote corrosion.
#5. Intergranular corrosion
When considering aluminum for galvanic and corrosion potential, the grain boundary is electrochemically dissimilar as compared to its alloy microstructure. This results in an electron exchange with an electrochemical potential being setup between the grain boundary and the microstructure.
There are different types of intergranular corrosion based on thermochemical treatment as well as metal structure, and is present to varying degrees in different series of aluminum alloys. The 6xxx series alloys for example, are relatively less prone and susceptible to this type of corrosion to aluminum.
The anodic path will be dissimilar between alloy systems. In the 2xxx series it appears as a narrow band on each side of the grain boundary. In the 5xxx series it is sealed by the intermetallic, and appears as a continuous path uninterrupted along the grain boundary.
Similarly to pitting corrosion, intergranular corrosion starts from a pit; but it propagates and progresses far more rapidly along a susceptible grain boundary.
#6. Exfoliation corrosion
Exfoliation corrosion is a unique form of intergranular corrosion of aluminum alloys that are of directional grain structure. It is typically seen in aluminum products that have been heavily processed by cold or hot rolling until they reach directional structures.
It occurs along the long grain boundaries of the microstructure. The term exfoliation comes from the fact that the corrosion product is more voluminous and appears to be lifting from the metallic surface.
Exfoliation corrosion in aluminum expands to the surface and laterally allowing stresses to grow in the product. This creates a wedging action to occur initial at the surface and then subsequently into the product. Severe delamination occurs in the material weakening the product. Surface degradation will also occur like pitting, flaking, and blistering.
Exfoliation corrosion is associated with aluminum alloys are typically the 2xxx, 5xxx, and 7xxx series; exfoliation corrosion is faced with these aluminum alloys due to their highly directional grain structures. This also means that the grain boundaries are even more susceptible to intergranular corrosion.
The susceptibility of exfoliation corrosion can be affected by the heat treatment method which will change the distribution of the precipitates.
#7. General corrosion
When corrosion occurs nearly uniformly across the surface of the aluminum product, it is classified as uniform or general corrosion.
This type of corrosion can occur with products that are subject to constant exposure with a strongly acidic or basic medium. It may occur with high electrochemical potential while the product is in an electrolyte. A common example is rusting of an aluminum plate immersed in an acidic solution.
Uniform corrosion results from the continued migration of anode and cathode regions in contact with the electrolyte. The result is the appearance of uniform corrosive attack on the surface.
Both high and low pH solutions would mean the oxide layer is also unstable and does not protect the metal below. As the thickness of the material decreases, the material will eventually dissolve completely.
The attack is not entirely uniform and therefore there will be peaks and valleys. If the few small deep corroded areas are not concerns enough to consider an example of general corrosion, their existence satisfies the uniformity and hence general corrosion as an example.
#8. Deposition corrosion
Deposition corrosion takes place when a different metal gets deposited on the aluminum surface, resulting in serious localised corrosion.
Think of water flowing, through copper tubing. When the water flows, it picks up copper ions. These copper ions are now in a solution form. When this solution comes into contact with an aluminum surface/vessel, it deposits copper ions on it.
These ions essentially create a galvanic cell and corrosively attack the aluminum via pitting if they are lower than aluminum on the electrochemical or galvanic series. The bigger the difference from aluminum and the deposited ion in the galvanic series, the worse it is.
In fact, even 1 ppm of copper ion solution is capable of performing serious corrosion on the aluminum surface.
The metals known to cause deposition corrosion of aluminum are generally called. ‘heavy metals’. The important heavy metals are copper, mercury, tin nickel, and lead.
This type of corrosion is more severe in acidic solutions than alkaline solutions, because these ions have low solubility in alkaline solutions.
#9. Stress corrosion cracking (SCC)
Stress corrosion cracking (or SCC) is a type of intergranular corrosion that has the potential to cause catastrophic failure of aluminum components.
For this form of corrosion to occur, there are three requirements. The first requirement is a susceptible alloy. Not all aluminum alloys are equally susceptible to SCC, due to the yield stress. High yield strength alloys may be susceptible to stress corrosion cracking.
Second, the service environment needs to be humid/wet. Third, tensile stress must be present in the material. The tensile stress leads to the crack, and enables propagation of the crack through the metal.
There are two types SCC processes: intergranular stress corrosion cracking (IGSCC) where cracking propagates around the grain boundary; transgranular stress corrosion cracking (TGSCC) where cracking continues through the grain bodies as well.
#10. Erosion corrosion
The erosion corrosion of aluminum occurs due to the high-speed water jet’s impingement against the aluminum body.
There are two factors that exacerbate erosion-corrosion, the water velocity and the pH of the water. The corrosion rate can be increased additionally by non-metallic minerals, present in the water, including carbonate and silica content.
In the case of pure water, corrosion of the aluminum proceeds at into a very slow reaction rate. When the pH exceeds 9, the reactions rates increase. However, in acidic water, the corrosion rate increases again.
Controlling the above factors can prevent erosion-corrosion. By either reducing the velocity of the water, or by controlling the water quality, or by conducting a combination of both, erosion-corrosion can be minimized effectively.
With increasing water quality we mean maintaining water quality close to neutral conditions (pH<9) and reducing the silica and carbonate content of the water.
#11. Corrosion fatigue
It is common knowledge that fatigue can bring about the complete failure of a product if allowed to occur. For the case of aluminum, fatigue cracks can contribute as initiation sites for pitting corrosion.
Corrosion fatigue in aluminum occurs when it is subjected to low stress over long periods of time. Because of the corrosive environment, such as seawater and salt solution, cracks can be more easily initiated and propagated.
Corrosion fatigue cannot occur without the presence of water in the atmosphere. It also remains mostly unaffected by direction of stress since crack propagation is mostly transgranular. The stresses do not alter the propagation of the cracks the way they do under SCC.
#12. Filiform corrosion
Filiform or wormtrack corrosion commences as pitting corrosion; it begins at sites where the paint has peeled away from the aluminum surface. The cause may be surface scratches or blemishes exposing the underlying metal surface.
Filiform corrosion occurs and propagates in the presence of chloride anions and high humidity. Although it initiates as saltwater pitting corrosion, the means of propagation is that of crevice corrosion.
The head of the wormtrack is acidic and epitomizes a high chloride content. It draws in oxygen and acts as the anode, while the tail end of the wormtrack acts as the cathode, and the reaction follows.
Filiform corrosion can be avoided by maintaining the surface free of damage and by being exhaustive in covering all of the tiny voids with either a paint or wax. If the environmental relative humidity can be lowered, it should be.
#13. Microbiological induced corrosion
Microbiologically Induced Corrosion (MIC) is corrosion that occurs due to microorganisms and/or fungi. This type of corrosion is found in fuel and lubrication oil tanks.
Microbiological and fungal growth can thrive in the presence of water in the oil. Some of these organisms can accumulate the oil and excrete acid which will initiate corrosion of the aluminum vessel.
The acid creates pitting corrosion in the aluminum vessel, lubricants, and hydrocarbons that would eventually lead to leaks.
If not, the oil must be purified as much as possible to remove the water from it, and any water will need to be drained at regular intervals from the fuel tanks after purification. If sanitizing the fuel is not feasible, then using fungicides will limit germination.