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Q & A: Rusty metal and magnetism

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Most recent answer: 09/15/2011
Q:
I am working on a science project on whether or not there is a difference in how magnets attract metal that is and is not rusty or corroded. Do you know anything about that?

Also, what metals are magnetic?

Thank you.
- Nathaniel (age 9)
San Diego, CA US
A:
Hi Nathaniel,

That sounds like a very good project to work on! Just be careful with the kinds of metals you investigate. Do not investigate lead, because it is poisonous. It also only has very weak magnetic properties anyway. Ask your teacher about the safety of other metals.

Rust and corrosion strongly affect the magnetic properties of metals. Ferromagnetic metals like iron in which interactions between the electrons of neighboring atoms tend to make their little bits of magnetism point in the same direction, forming magnetic domains. In a magnetic field, these domains line up with the field making a strong magnet. Other common ferromagnetic metals are nickel, and cobalt. Some of the strongest magnets are made with neodymium. In some materials, the domains can get stuck so the material stays magnetized even when the field is removed, leaving a permanent magnet, but that usually involves introducing some non-magnetic components.

Rusting and corrosion introduce atoms of other elements (typically oxygen), making new chemical forms with different interactions between neighboring atomsí electrons. Usually these end up either non-ferromagnetic or less ferromagnetic than the pure magnetic metal.

There are several different oxides of iron, with different fractions of oxygen. They are Fe0, Fe2O3, and Fe3O4. Rust consists mostly of Fe2O3, with additional water molecules attached. There are several forms of Fe2O3, and a common mineral composed of Fe2O3 is called hematite, which is a shiny-blackish mineral. Hematite is not ferromagnetic, but it does still respond to a magnetic field and will be attracted to the poles of a permanent magnet. Hematite itself has the interesting property of being nearly antiferromagnetic, in which the spinning electrons producing magnetic fields in neighboring atom groups like to align opposite to one another, canceling their fields out. This isnít perfect in hematite, with a small tip, or "canting" of the spins so that they donít cancel exactly, hence the attraction to the poles of a permanent magnet. I suspect that the addition of water molecules in ordinary orangey-yellow rust does not help the material become more magnetic than its hematite cousin, but itís hard to be less magnetic than an antiferromagnet. Why donít you try an experiment? FeO is also not ferromagnetic, but it is pulled about twice as much as Fe2O3 towards the poles of a magnet. Magnetite, Fe3O4, is ferromagnetic, and is about 1/4 as strong as pure iron. One warning is that rust may be a collection of the different oxides quite possibly other contaminants. In particular, there may be bits of unrusted iron left in a sample of rust, and these would be attracted very strongly to a magnet.

Some substances change the signs of their response entirely when they corrode. For instance, aluminum is very weakly attracted to the poles of magnets, while aluminum oxide is very weakly repelled by the poles of magnets. The scientific words for these interactions is:

Materials attracted by magnets: Paramagnetic

Materials repelled by magnets: Diamagnetic

Materials which spontaneously form magnetized domains: Ferromagnetic

Materials in which neighboring spins like to align opposite to each other: Antiferromagnetic

Only ferromagnets are useful for making permanent magnets. Paramagnetic and diamagnetic forces tend to be very weak, except for the diamagnetism of superconductors, which is strong enough to levitate them.


U.S. nickels contain rather little nickel -- they are about 75% copper, and only 25% nickel. This alloy seems to be not very magnetic. Older pure nickel nickels stick to magnets. Pennies are 97.5% zinc with a thin coating of copper on the outside.

Some steels (steel is mostly made of iron) are more magnetic than others. Try comparing stainless steel with other kinds of steel you might find around the house.

Two suggestions of things that can affect your experiments:

1) Rust and corrosion usually only occur in a very thin layer of the material near the surface. The rest of the material will be just as magnetic as it ever was, just minus the rusty layer. You may not notice any effect of the rust if most of the material is intact.

2) The shape and orientation of the metal object is very important in determining how strongly it will be attracted to a magnet. Be sure that the shapes of your uncorroded and corroded objects are the same when you compare their magnetic attraction or you could have other effects confusing the results.

Good luck!

Tom J. and Mike W.

(published on 10/22/2007)

Follow-Up #1: Non-magnetic rust spots

Q:
The body of my car has orange rust spots on it. My magnetic screwdriver is attracted to the car , but not to the rusty spots. why does this happend?
- Anna Beth Sherrod (age 13)
Flint,TX U.S.A
A:
Rust (a collection of some iron oxides: http://en.wikipedia.org/wiki/Rust) is virtually non-magnetic, unlike plain iron or most types of steel.  If the sheet metal on your car has really rusted through, there will be almost no magnetic force between it and the magnetized screwdriver. It sounds like those spots should be patched, since that's not just a thin surface layer of rust.

Mike W.

(published on 09/15/2011)

Follow-up on this answer.