It’s well known that aluminum is highly resistant to rusting and corrosion. But how? Why is this metal so different than others that it can resist the effects of the elements for so much longer? It has to do with the molecular structure of aluminum, and how it reacts with elements like oxygen and water that are known to corrode other metals. In this week’s blog, we’ve got a neat little science lesson on how aluminum is so resistant to rust!
Why Doesn’t Aluminum Rust?
Pure 100% aluminum, like most metals on the periodic table, reacts very strongly with water. If your high school science teacher ever showed you how pure sodium (Na) reacts with water, it’s the same with aluminum. You get a very violent reaction, which makes it even more surprising that things like aluminum fences can do anything other than immediately break down when it rains. So how does aluminum do it? With a little help from its friend, oxygen.
Aluminum Oxide (Al2O3)
When aluminum is exposed to air in the atmosphere, it bonds with oxygen to form a compound known as aluminum oxide. This molecule contains 2 aluminum atoms for every 3 oxygen atoms. The aluminum oxide spreads out over the surface of the aluminum to create a thin barrier that is highly effective against water, or H2O.
When Water Meets Aluminum Oxide
When water meets aluminum oxide, the aluminum and oxygen atoms in the oxide actually move apart, “in some cases separating by more than 50 percent compared to their normal molecular positions.”
“As a result, when the outer layer of aluminum oxide gets hydrated or wet, its structure changes just enough to become chemically inert and thus unable to react rapidly with additional water molecules or atmospheric oxygen. This change in molecular structure is why aluminum oxide metal resists corrosion.”
Anodising
For extra corrosion resistance, the aluminum oxide layer can be anodised, which is a chemical process that makes the layer thicker.
The metal is treated with a sodium hydroxide solution, which actually completely removes the aluminum oxide layer. Then electrolysis of diluted sulfuric acid is performed, with the aluminum is the anode. Electrolysis is when an electric current is passed through a liquid that conducts electricity using a negative and positive electrode. In this case, the positive electrode (also known as the anode) reacts with the aluminum. During the process, the anode attracts non-metal ions, which lose their electrons, which are discharged as non-metal atoms, in this case oxygen gas. This forms a thicker oxide layer around the aluminum, making it stronger and more corrosion resistant.
Hercules Custom Iron has more than 70 years of experience crafting ornamental iron and quality aluminum in its in-house manufacturing facility. Contact Us today to learn more about how we can help you with your next custom iron, specialty metal or aluminum project! Also, be sure to follow Hercules Custom Iron on Facebook, Twitter and LinkedIn!