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Q & A: Aluminum, Iron, and Brass

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Most recent answer: 10/22/2007
Q:
What chemicals do Aluminum, Iron, and Brass contain? Also, I’m doing a project on "Which metals rust the fastest" and I am trying to figure out how to make rust in less than 3 months. Do you have an idea?
- Kyle
Shorecliffs Middle School, San Clemente, Ca.
A:
Aluminum and Iron are elements, which means that every atom in a block of pure aluminum is an aluminum atom, and similarly for iron (there are no molecules with different kinds of atoms in them). All atoms of a certain element have the same number of protons.

Aluminum has 13 protons in its nucleus and Iron has 26.

Brass is an alloy. Alloys are created when two or more metals (or a metal and a nonmetal) are mixed and stirred together. You might compare it to putting cocoa in milk and stirring it. The milk tastes a little different, looks different, but has some properties of both. The use of alloy lets you choose what properties you want for your material.

Brass is an alloy with mostly copper and zinc (with a few other metals as minor contributors). It is a way of combining copper's lack of reactivity with Zinc's strength. Brass was one of the first alloys used to make weapons and, as example, it is now used in the manufacture of musical instruments, doorknobs, locks, keys, and some plumbing fixtures.

Jason

If a metal is going to rust (technically, we use the word "corrode" for oxidation or other chemical reaction of a metal that's not iron or steel, and even "rust" is just an example of corrosion) it will get started at the first possible moment. Iron and steel will start rusting as long as there's oxygen and water around (I imagine you can get iron to oxidize under normal conditions without the water, but most stuff we call "rust" actually has water molecules incorporated into it).

Some metals don't corrode, or at least do so very slowly. You'll havea tough time detecting any rust or corrosion on stainless steel, for instance, and gold, platinum, and chromium are notably impervious to corrosion. Some steels are coated with other materials ("galvanized") to reduce the rate of corrosion. So if you want to observe rusting sooner, don't pick one of these metals. Go and get some raw iron (unpainted, no oils or other coatings), and leave it in a moist environment with air flow. Most chemical reactions take place faster when the temperature's higher, so it cannot hurt to keep it in a warm place. The exposed surface area of the metal helps enormously -- each bit of surface should rust at the same rate. If you have a solid lump of iron, only the outside will rust. Something that should rust very very quickly is a piece of ungalvanized steel wool (some scouring pads are actually made of stainless steel! Don't use one of these) which is kept wet.

Aluminum forms an oxide coating very quickly too, but it is very thin and tough, and prevents further oxidation of the underlying aluminum.

Other ways to speed corrosion -- adding salt to the water may help. Joining dissimilar metals in something which contacts saltwater may be an invitation for corrosion. This problem comes up in boats all the time. Check our answers for "zinc cathodes" which people attach to metal hulls on boats so that the zinc corrodes but other parts of the hull do not.

Tom

(published on 10/22/2007)

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