Why Metal Rusts
Ask Science explains why metal rusts, how rust is formed, and some methods to prevent it.
Lee Falin, PhD
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Why Metal Rusts
Some of my kids love gardening. They love planting seeds, they love harvesting fruits and vegetables, and they love watering the plants (though none of them like 
weeding). However their favorite part of gardening is digging holes, especially after the final harvest when the vegetable patch is just one big slab of mud, just begging for someone to come along and dig holes in it.
Unfortunately, despite all of their expert digging skills, one important gardening skill my kids haven’t yet mastered is putting the tools away when they’re finished with them. So the tools sit there in the mud, silently rusting away, like the tin man in the Wizard of Oz. But instead of lamenting the fate of our garden tools, let’s take this opportunity to learn about the science behind rust.
Holy Rusted Metal, Batman!
Rust is a special type of corrosion. Corrosion is the word we use to describe what happens to something when it is slowly destroyed by some chemical reaction. If our garden tools are rusting, they’re probably made of iron or steel. However, as we learned in our first Science of Minecraft episode, steel is just iron with some carbon mixed in to make it a little stronger.
The series of chemical reactions which result in rust are actually a bit more complicated than you might think. (Unless you already know what they are, then they’re exactly as complicated as you think).
There are two things needed for iron to rust: oxygen and water. When oxygen and water come into contact with iron, a series of chemical reactions occur which convert the iron to an iron(III) oxide, more commonly known as rust.
Preventative Measures
So now that we know what rust is, what can we do to prevent it? First, it’s important to note that both water and oxygen are required to carry out this reaction, so one of the easiest ways to keep your tools from rusting it to keep them dry. Of course there is water vapor in the air, so all iron stored in normal conditions will rust eventually, but a shovel stored in your tool shed will last a lot longer than one left sitting outside in a muddy hole that someone has dug in your vegetable patch.
Earlier I mentioned that steel is just iron with a little carbon mixed in. However if we also mix in a few other metals such as chromium and nickel, we can make stainless steel. Stainless steel is so named because of its ability to resist corrosion. The secret to this super-powered steel is the chromium, which combines with oxygen to form a thin layer of chromium oxide over the surface of the metal. This chromium oxide layer protects the underlying iron from reacting with oxygen, thereby preventing it from rusting.
Another method of preventing rust is through a process called galvanization. Galvanized steel is steel which has been dipped in a layer of molten zinc, much like dipping a vanilla ice cream cone into chocolate (though far less tasty). Aside from providing a physical barrier between the iron and oxygen, when zinc reacts with oxygen, it forms zinc oxide, which further protects the underlying iron. The protective layer eventually wears away, allowing the iron to rust as normal.
Since galvanization is so temporary, you might wonder why people don’t always use stainless steel instead. The answer is simply that stainless steel is a lot more expensive than galvanized steel.
A final method used for protecting iron from rusting is to coat it in oil. The oil forms a physical barrier that keeps out oxygen and water. As you can imagine, this is even more temporary than galvanization.
Conclusion
So now you know a little more about rust and why it forms, as well as some method for preventing it. If you don’t want to replace all of your garden tools with stainless steel, or dip them in molten zinc, the easiest way to protect them is to simply keep them from being left out in the rain. Are you listening kids?
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Shovel image, Kelly Sikkema at Flickr. CC BY 2.0.
