Iron, used to make steel alloys, is more stable as an oxide than as a metal. In the corrosion process, iron oxidizes in the presence of water, yielding Fe2+ ions. The Fe2+ ions, meanwhile, react with hydroxide ions in the water to make Fe(OH)2, or "green rust," which reacts with oxygen in air to form Fe(OH)3—what we commonly recognize as rust.

It doesn't take a lot of water to begin the corrosion process. Even a moderate level of humidity will cause enough water to condense on a metal to initiate rusting. It could just be a few molecules on the surface.

The sodium and chloride ions in salt water add conductivity that accelerates the reaction of the iron. It is a little like a battery. If you don't connect the battery, you stop the reaction.

Acidity will also accelerate rusting. Moreover, the corrosion process itself acidifies water, a problem that influences rusting in steel crevasses or joints and can lead to serious structural problems. Rust expands when it forms, Roberge points out, much as water does when it freezes. "Pack rust" can build immense pressure if left unchecked. It has been seen that  rivets popping out of very thick plates. The phenomenon can also occur on aluminum structures such as airplanes.

Corrosion can be prevented or at least controlled. On steel structures such as bridges, paint provides a physical barrier between the moisture of the environment and the metal substrate. But Roberge says there are usually some defects and failures in the coating system.

That's why those charged with maintaining such structures tend to use primer coatings rich in zinc. Zinc is a metal lower in the galvanic series than steel, meaning it is more active and has a greater tendency to corrode. Thus, when exposed to the elements, zinc will corrode before the steel does. The same principle is at work in galvanization, whereby steel is plated with zinc.

Stainless steel is a more costly alloy of iron and other metals, most important among them chromium, which forms a stable oxide that halts the corrosion process. This is similar, he notes, to anodization of aluminum, in which a stable oxide layer is formed on the surface to prevent further corrosion. For ordinary steel, there is no such luck; rust is porous and water can always penetrate down to the nonrusted surface. It will go right through the steel if you give it a chance.