Corrode
The Death of Metals
Corrosion is the term for the oxidization of iron. Iron becomes oxidized when it is exposed to excess amounts of oxygen and/or water. The presence of salt, such as may be found in sea water, accelerates this decay process.
While scientifically the correct term is "corrosion" or "oxidization", it's commonly referred to as "rust": a flaky or gritty coating of reddish-brown material on the surface of metal. The process continues until either a) the metal is entirely consumed, or b) the oxidizing agent (water, oxygen, and/or salt) is entirely consumed or removed.
Corrosion can be halted with the use of a metal with slow reaction kinetics. For example, steel nails are hot-dip galvanized, meaning they're coated with zinc. When exposed to the air, the zinc reacts with oxygen and creates zinc oxide, which then reacts with carbon dioxide to become zinc carbonate. Zinc carbonate is dull grey in color, and can virtually stop corrosion in its tracks, as zinc has a very slow reaction kinetic. The corrosion still happens, but it happens at such a slow rate that it might not even be noticed for several years - as opposed to regular nails, which might corrode after only a few weeks of heavy exposure to the elements.
Example: Your House Number Sign
Let's suppose you have just purchased a lovely wooden house number sign in an ornate scrollwork iron frame. You hang this house number sign on the porch pillar outside your front door. A month or two later, you notice the metal frame of your house number sign has a discolored spot of orange rust on it. What to do now?
An iron or steel alloy house number plaque frame can be easily and cheaply fixed by rubbing the rusted area with a piece of aluminum foil dipped in water. The aluminum will actually polish the metal house number sign frame while it removes the rust, after which the frame can be painted with a rustproofing layer of paint or varnish.
Likewise, rusted or beginning-to-rust steel nails can benefit from this treatment. Take a good look at the nails first, however - if they're too rusted, don't waste your time trying to salvage them. Replace the nails in your home sign at the first indication of rust, rescue those that you can, and store them for another project.
Natural and Ultimately Harmless
On a grand scale, rust is not going to cause you any major problems. Sure, your house sign might fall or crack, or the house number sign frame might rust to the point of being usless - you might even have to purchase a new house number sign in the near future. But your home is not likely to fall apart due to metal corrosion.
By following the few simple tips listed here, and keeping a careful watch over the metals you have outdoors (especially the house number sign), you'll be ready to meet the rust when it finally does appear.