Before the 1900s, commonly used metals were especially vulnerable to rust or corrosion. For example, this was the case with carbon steel, which will inevitably rust when exposed to moisture. This was a particularly problematic situation for applications and structures in wet climates and marine environments, such as shipyards.
Aluminum is an important metal, as well, especially in the aerospace industry.
Aluminum ore is also the most abundant mineral on Earth, which is why it is one of the most widely used metals and valued for its exceptional properties. However, it is typically alloyed with other elements to achieve enhanced properties, such as strength, which are not attainable with aluminum alone.
The process of being alloyed also makes it more susceptible to corrosion than pure aluminum. Consequently, many aluminum alloys must undergo processes to make them less prone to corrosion.
Fortunately, one of the beneficial features of aluminum is its ability to undergo passivation.
Passivation is a process of adding an extra layer of protection to the aluminum surface of material commonly used in aerospace applications, for example. In the chemistry and engineering fields, passivation refers to the process applied to a material such as aluminum that causes it to become less susceptible to environmental corrosion.
Passivation typically results in the formation of an outer protective layer.
Typically, this involves the application of a micro-coating to the surface of the aluminum that results in a chemical reaction with the base metal, or the spontaneous oxidation that occurs once it comes into contact with the oxygen in the air.
Essentially, aluminum passivation is the process of anodizing an aluminum alloy to provide an aluminum oxide coating. This aluminum oxide coating helps prevent further oxidation and corrosion.
The Results of Aluminum Passivation
As we’ve noted here, passivation triggers a chemical reaction with aluminium to create an outer layer of shield material. Passivation is used in several industries and is especially important in the aerospace industry, where aluminum coatings enhance the resistance to corrosion for critical structural components.
Strictly speaking, the result of the passivation process is a type of corrosion, since the aluminum reacts to form a hard, relatively inert surface. However, it is the formation of this layer that acts to reduce or minimize any further corrosion.
Another way to describe the intended outcome of passivation is “controlled oxidation.”
This type of oxidation excludes corrosion accelerators by depositing a thin layer of pure aluminum over the alloy, known as alclading. The resulting inert surface layer on aluminum, which is called the “native oxide layer,” can retain a thickness of 5 nm (50 Å) after several years.
The Benefits of Aluminum Passivation
There are two major benefits of passivation that are also closely linked. The process renders the treated aluminum material as clean as possible, while at the same time protecting it from future corrosion, which often results from impurities that would otherwise exist if the metal were not cleaned.
Because protection against the elements is a primary concern for aerospace applications, passivation is a feature that must be strongly considered, especially if an alloy or grade not naturally anti-corrosive is required.
An additional benefit of the passivation of aluminum is that the process will enhance the appearance of the metal, giving it an extra sheen.
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