Metals are usually considered to be very strong, but once they have been exposed to air and water, they can turn porous through a process called oxidation, but more commonly referred to as rusting. While you might not think that you have that much metal in your home, there is actually quite a bit that might be hiding just beneath the surface.
Take, for example, your foundation. If it is made from cement, you will find rebar inside of it to provide strength and structure. Usually, you will also find an innumerable amount of nails and screws used to fasten everything together. On top of that, you might have a metal roof or metal siding. Rust in any one of these locations could be damaging, or even disastrous. One of the more troubling places to find rust is in the rebar of your foundation. This could happen if there is a crack in the cement that lets in water. When the metal inside rusts, it will expand, possibly cracking open the cement even further and letting the water get in deeper. It needs to be dealt with as soon as possible.