The terms "corrosion" and "rust" are often used interchangeably, though they can be entirely different chemical reactions. Besides the fact they both affect metals, they share few similarities. For example, corrosion is a form of oxidation. Rusting is a type of corrosion.
So what are corrosion and rust? How do they differ depending on the metal? Perhaps most importantly, what can you do to slow down these processes?
This primer will get you started.
Key differences between corrosion and rust
Corrosion can be defined as a loss of structure due to chemical reactions with the outside environment. When something corrodes, it deteriorates over time.
Rust, as a noun, is another term for iron oxide hydrate (iron oxide by itself is not a form of rust). Rust forms when iron or its alloys are exposed to oxygen and water. In verb form, when something is "rusting," it refers to the oxidation reaction that creates iron oxide hydrate.
Let's go over the major differences between the two.
Addition vs subtraction
Rust forms on top of iron and its alloys, such as steel, but doesn't always destroy the underlying material. Corrosion turns metal into a more stable substance, causing it to deteriorate.
Corrosion can happen to many types of materials, and rust only one
Corrosion isn't limited to iron alloys and can affect almost anything, from plastic to ceramics to skin. Rust can only form on top of iron and its alloys. Not all metals rust, but they can all corrode without exception.
Rust is typically orange or reddish in color, though under certain conditions, it can be other colors. The color of corrosion depends on the material it affects. For example, copper corrosion looks green or blue-green.
The three types of rust
There are three types of rust defined by their color: Black, red and white. Each is the result of a different chemical composition.
Also referred to as magnetite, black rust (or black oxide) is thicker and harder than the other types. It also has the unique property of shielding the underlying alloy from more oxidization. Black rust is more likely to form when oxygen content is low.
Red rust is more common when iron or steel is exposed to natural amounts of oxygen and moisture. This type of rust destroys the material it forms on.
Specific to zinc/iron alloys, white rust is actually zinc oxide. It forms just like red (iron/steel) rust. A layer of chrome is often applied to zinc plating to prevent white rust from developing.
How corrosion and rust affect different metals
Adding to the metals we've already discussed, consider these:
Aluminum is resistant to corrosion. Instead of a reddish or orange color, aluminum corrosion has a bright, off-white color. It resembles black rust in that it forms a protective layer on the surface metal, preventing further oxidation.
Unlike other surface metals, magnesium does not have an oxide layer. However, it still rapidly corrodes in the presence of acidic or neutral chemicals.
When dissimilar metals have a common electrolyte, galvanized corrosion can occur. This often means one metal corrodes much faster than the other.
Stainless steel is highly resistant to corrosion due to its high concentration of chromium, which is why it's used so often in materials that are commonly exposed to moisture, such as silverware or car bodies. Still, all stainless steel corrodes eventually. When this happens, it's sometimes called inox corrosion.
Strategies to combat corrosion and rust
You might have already picked up on one common method of preventing corrosion and rust: adding chromium. Manganese and nickel can also be used. Generally, all approaches of preventing corrosion act to block moisture from coming into contact with the underlying metal.
Other ways include:
Applying a special dry coating.
Dowsing it in oil.
Adding a layer of acrylic, vinyl or epoxy.
All metals corrode, but you can make them more resistant
There's no such thing as metal that doesn't corrode, only metals that are more resistant to the process than others. However, there are ways to greatly slow the process, and the first step in helping prevent it is testing the corrosion to find the best possible solution.
At Auto Technology, we specialize in corrosion — especially how to prevent it. Contact us to learn more about what we offer.