The subject of aluminum and coating of aluminum came up in a recent post. The subject is an interesting one with many surprises that illustrate processes used to form coatings and the types of coatings that can be formed. Before we get started, there are a couple of things about aluminum that should be noted.
Perhaps the most important thing to know about aluminum is it has a long memory. A very long memory. When we say that a material has memory, we are referring to a tendency of materials to retain the effects of their processing history. The effects of early processing steps tend to fade with additional processing but some effects are lasting and can even change the outcome of later processes. We sometimes use the word memory to describe that tendency. Aluminum remembers everything that ever happened to it from the time is extracted from the parent bauxite ore (using the Bayer process) and reduced (using the Hall-Héroult process) and then through all the handling, shaping, forming, heat treating, tempering, fabrication and use. Most materials possess some degree of memory but aluminum’s memory is exceptional: it is eidetic.
The second thing to know about aluminum is its ability to form strong and stable bonds with oxygen. This ability is so important to the coating developer that it is worth considering why it is so and, along the way, to introduce some important concepts. The first concept we should cover is electronegativity. Electronegativity is the attraction that an atom has for electrons. All atoms are electronegative to some degree. Aluminum atoms are not as strongly electronegative as oxygen atoms with the result that when aluminum and oxygen react to form a compound, the oxygen swipes all three of the aluminum atom’s valence electrons (valence electrons are the electrons in an atom’s outer electron shell) producing the aluminum(III) ion (which is a cation because it has a positive charge. If the charge were negative, it would be an anion).
The aluminum(III) ion does not behave like ordinary “highly ionic” ions such as the sodium ion. Ions are highly ionic when the bonding between the cation and the anion is principally electrostatic due to the ions’ opposite electric charges. The sodium ion is a cation and it is what makes sea water taste salty. The salt in sea water is primarily sodium chloride which is table salt. When you put a spoonful of table salt in a glass of water and stir it, it readily dissolves because the water is a polar liquid and can easily separate the sodium and chlorine ions to form a salt solution.
The aluminum(III) ion is not highly ionic when the anion is oxygen, and it is not soluble in water even though the difference in electronegativity between aluminum and oxygen is about the same as the difference in electronegativity between sodium and chlorine. Unlike the ions of sodium and chlorine, the ions of aluminum and oxygen form a strong and stable inorganic bond which is what makes the aluminum(III) ion so useful to the coating developer. The strong inorganic bond keeps the coating from washing away in the rain and imparts durability and chemical resistance.
Several concepts have been touched on in this post: memory or the lasting imprint of processing on a material, electronegativity, the types of ions and the broad spectrum of inorganic bonding. In future posts, I hope to show how these are very useful tools in the coating developer’s toolbox. In the next post, I plan to discuss the nature of strong inorganic bonds and show why understanding them is so valuable.
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