But new advances in the chemistry of the surface of materials are helping to hold that off for as long as possible.
"At least 20 percent of bridges in the United States are in a serious structural condition. The cost to the nation of corrosion is something like half the cost of health care, so it's a very big problem," said Peter Sherwood, university distinguished professor and head of the chemistry department at Kansas State University.
For 30 years, he has studied the chemistry of surfaces, which are the top few atoms on a material. The surface is the part of an object that meets the environment.
"Corrosion is the eating away of all the material. If that surface was protected, it would prevent an attack on the interior," Sherwood said.
Iron is a fairly unreactive metal, but it corrodes because oxygen from the air reacts with its surface, causing it to turn into the brittle, red material we call rust. It has no protective barrier. Aluminum is highly reactive, yet it is protected from corrosion by a thin layer of oxidized aluminum, which is impervious to oxygen and water. In other words, oxygen actually helps aluminum and hurts iron.
"If you look around us, every metal has an oxide on it, and that can be good, in case of aluminum, or bad in the case of iron," he said.
Sherwood's investigation of surface oxides on aluminum alloys is being published in the Journal of Vacuum Science and Technology this year.
His newest protective process uses phosphates - not oxides - to bond with the surface of a metal. His group now uses an inexpensive method that reacts metal with phosphoric acid, resulting in a protective barrier of oxide-free, phosphate film.
In March, Sherwood's group received a new $426,000 grant from the National Science Foundation to continue the work. Phosphate-coated metals could be useful for people with surgically implanted devices like artificial hips, since they are safer than oxidized metal inside the human body.
"The new surfaces are likely to be corrosion-resistant, because phosphate is a good corrosion inhibitor. Phosphates are friendlier to biological molecules. They also promote the adhesion of one material to another," he said.
The surface chemistry studies have a direct impact on the interior walls of cylinders with high gas pressure, such as the cylinders used by hospitals, fire fighters and scuba divers. Sherwood's group works on coatings for cylinders that won't react with the gas.
The gas cylinder company Luxfer, which has supported Sherwood's research for years, funded instruments used to study the chemistry of the interior surfaces of cylinders.
"If you are a scuba diver, you want to be sure the gas is pure. It's very important to have that surface appropriately tuned to the particular gas," Sherwood said. "This is a fascinating problem. We are doing basic chemistry, but the understanding leads to very substantial improvements in practical applications."
Metals aren't the only materials with applications in surface chemistry. Sherwood develops something called "advanced materials," in which carbon fibers are weaved into a carbon or epoxy matrix. The result is a lightweight material that is just as strong as conventional metals.
Early carbon fiber-matrix materials broke easily under heavy stress. But fibers can adhere better to the matrix when its surface is chemically altered, making the material three times stronger. Sherwood's group tailors surface treatments to different matrices and fibers.
"We've now optimized the chemical attraction between a particular matrix and a fiber," he said.