What is the state of aluminum?
Aluminum Lexicon - the material from A – Z
Corrosion in metals results from their strong tendency to return to their natural, stable chemical state. This state is the one in which we encounter metals in nature, namely in the form of ores, that is, chemical compounds mainly with non-metallic elements. The metal atoms are freed from these compounds by melting them at high temperature, for example. Therefore, every pure metal - with the exception of the precious metals, which do not corrode - is in an unnatural, unstable state. The first reaction of a just melted pure metal with its environment in which it exists is usually that it surrounds itself with a very thin layer that seals it against this environment and protects it from further corrosion.
Pure aluminum, for example, spontaneously coats itself in the air with a firmly adhering, pore-free layer of aluminum oxides, as atoms on its surface chemically combine with oxygen atoms in the air. After a few days, this oxide layer reaches its greatest thickness, which, depending on the humidity, can be up to a few thousandths of a millimeter. Compared to most common metals, it gives the metal a high level of corrosion resistance, namely against the weather (with not very polluted air, rain and snow), weak alkalis and acids (e.g. citric acid) and many chemicals.
Opportunities for improvement
Even ultra-pure aluminum is not resistant to very strong alkalis and acids (usually with pH values less than five or greater than eight) and aggressive vapors and gases. Such chemicals remove the oxide layer and then loosen aluminum atoms in places or on larger surfaces. However, aluminum products can be specifically protected against this: by strengthening the oxide layer during anodizing, by surface treatment (e.g. coating), painting or a combination of these processes.
Alloying, i.e. adding other metals, offers one way of protecting aluminum from the outset against corrosion caused by certain substances. A few percent magnesium makes it seawater-resistant and suitable for shipbuilding. Copper and zinc, on the other hand, can increase susceptibility to corrosion.
During construction and in various applications, care must be taken to avoid contact corrosion. At contact points with more noble metals such as iron and copper, a contact element can form after the ingress of moisture, i.e. a low electrical voltage, under the chemical effect of which the aluminum is attacked.
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