Why does aluminum turn black or brown?
hmwadeseo > 06-20-2023, 08:46 AM
Why does aluminum turn black or brown?
To answer the question: why does aluminum foil change color to black or brown, understanding what aluminum foil is made up of is of paramount importance. Fortunately, the name gives us a hint of what the main component of aluminum foil is, and that is the aluminum element with the chemical formula Al. That being said, aluminum is not necessarily the only constituent of aluminum foil; however, to answer the question we only need to focus on aluminum.
Aluminum metal has high a affinity for oxygen. Affinity for oxygen is just a measure of how quickly or easily a substance, in this case, aluminum, can be oxidized or react with oxygen in the earth's atmosphere, with a higher affinity for oxygen meaning that a substance oxidizes pretty quickly. The higher concentration of oxygen in the earth's atmosphere makes it easy for substances with a higher affinity for oxygen, such as aluminum, to be oxidized.
The affinity of oxygen of a metal depends on how a metal occurs naturally. Metals that naturally occur as oxide metals, such as iron, magnesium, and aluminum, will have a higher affinity for oxygen because they want to return to their natural state. However, metals that naturally occur as native metals, such as silver and gold, have a lower affinity for oxygen and they are much less reactive.
The oxidation of aluminum foil can be confirmed by its change in color to brown or black, and this process results in what is referred to as corrosion or the breakdown of a metal. Oxidation of aluminum does not continue to occur forever, it will stop when there is no free aluminum available to react with oxygen in the atmosphere. The reaction of aluminum and oxygen or aluminum oxidation results in aluminum oxide(Al2O3).
The thickness of the aluminum oxide layer on the surface of the aluminum foil tells us how long has the aluminum foil been exposed to atmospheric oxygen or how long oxidation has been taking place. The layer will quickly get to 1.5nm, and after 1 month to 1 year, it can get to about 2nm - 4.5nm. And of course, as the layer becomes thicker, less and less aluminum will be available to react with atmospheric oxygen.
In a nutshell, the change of aluminum foil color to black or brown can be attributed to oxidation which is significantly dependent on the affinity of aluminum to oxygen element.