I recently came across a photo of the car I used when going to college. It was an old Mercedes 190D. I had bought it during the unhappy national gasoline crisis of 1974, a circumstance of cursing car lines and closed stations and many hours of a good day were blown just to fill the tank.
Having a diesel car gave me certain advantages and one was the ability to fill up at empty truck stops. The car set me back $1,000 and my mom called it Rusty-Busty for good reason. I tried to stem the entropy that was dissolving my car during those salt-filled winters on the Long Island Expressway by welding three-inch steel channel underneath but to no avail. When I gave away the car years later for parts it still had the scorched interior from the time I inadvertently set fire to the rear door plastic with an errant arc.
The enemy of all iron and steel structures is rust. When you leave a screwdriver or pair of pliers or even a box of nails outside on a rainy night, inspection the next day usually reveals a smooth coat of orange hydrated iron oxides wherever the metal was moistened. The chemical reaction works fast and is normally presented as the following (leaving out the hydration step):
4Fe + 3O2 => 2Fe2O3
where the oxygen comes from the “air” molecule dissolved within the rainwater.
Once rust is given a foothold in iron and the oxygen combines with the metal at an atomic level, the bonds of the metal itself begin to weaken, allowing corrosion to seek more unprotected atoms and the process keeps repeating itself at a faster pace. If, as the biologists say, given enough time, iron mass will eventually convert entirely to rust and disintegrate.
Surface rust is flaky and friable, and it provides no protection to the underlying iron, unlike the formation of patina on copper surfaces. While it may be acceptable for an iron fence post to wither away over decades, it is downright dangerous for a bridge member to deteriorate and weaken the entire structure.
In 2016, estimates were made that showed the total direct cost of corrosion in the United States is averaging over $200 billion a year and this number is increasing. There are numerous websites that show the need for maintaining our bridges, buildings, pipelines, railings and signage from corrosion but it is a slow and painful job.
It seems that every decade a U.S. bridge collapses. In 1967 the Silver Bridge, a suspension type over Ohio River, collapsed due to corrosion of an eyebar. In 1983 the Mianus River Bridge in Greenwich, Connecticut on Interstate 95 fell when rust formed within the bearing of a pin, exerting a force on a hanger which was beyond design limits for the retaining clamps.
The ensuing investigation cited corrosion from water buildup due to inadequate drainage as a cause. During road mending some 10 years before, the highway drains had been deliberately blocked and the crew failed to unblock them when the road work was completed.
Even small people-bridges are affected by corrosion. On May 20, 2000, as hundreds of NASCAR fans left a stock car race and were crossing a pedestrian bridge at the Lowe’s Motor Speedway in Concord, North Carolina, the 320 foot long structure snapped in half, spilling people 17 feet onto Highway 29 below. Corrosion was ruled the cause.
Can anything be done to stop the rusting of structural steel? Yes, several things. First off, corrosion can be controlled with coatings, such as paint, lacquer or varnish that isolate the iron from the environment. The use of rust inhibitors in these coatings, such as aluminum powders, may help by forming an outer impervious oxide coating protecting the iron.
Secondly, the use of an externally applied electrical current, a form of cathodic protection, can help buried or immersed structures by suppressing the electrochemical reaction. The use of easily replaceable sacrificial anodes of more active materials cause that object to corrode instead of the iron. This is especially favorable for metal ships.
One interesting note: It was found that tiny amounts of the artificial radioactive element technetium-98 somehow forms a protective coating on steel. In one experiment, a specimen of carbon steel was kept in an aqueous solution of technetium ions (55 ppm) for 20 years and was still uncorroded. How it works is not well understood even in 2017.