Steel is strong, ductile and cheap, which makes it great stuff. Unfortunately, in moist air, ordinary steel rusts and just keeps on rusting. A chromium addition of at least 12 percent produces a steel that is non-rusting, but a lot more expensive. Some years back U.S. Steel found adding a few tenths percent copper and a little phosphorous to ordinary mild steel gave a semi-protective rust. They dubbed the steel Corten.
Many find the coating attractive. The untitled Picasso sculpture, right, (Photograph by J. Crocker) which stands in Daley Plaza in Chicago, makes good use of the steel. U.S. Steel used Corten for the exterior of its eponymous office tower. Many find the tower attractive, though some waggish Pittsburghers call the building “The Rusty Nail.”
The Scene of the Crime
In the current case, Corten steel was proposed for use in the gutters in a large urban convention center. In addition to collecting and diverting water, the gutters had to support the skylight of the building.
I was called into the case when the very famous architect and the other contractors took to arguing about the rusting of the gutters. The interior of the gutters was to be coated with a polyurethane varnish. No coating is perfectly impervious and argument centered on “holidays” (holes) in the coating and whether accelerated corrosion would take place at these points.
Coating a weather-resistant steel did not make much sense to me. Just to make sure, I contacted a graduate school classmate who spent many years at the U.S. Steel Research Lab. and asked for his opinion. He explained, rather wearily and for about the thousandth time, the steel only developed a semi-protective rust after repeated wettings and dryings. In other words, the stuff has to be left out in the weather to work. But, the architect was not alone. A Midwestern railroad used Corten steel in its rail cars as a cheap replacement for stainless steel. The substitution was not satisfactory as the steel developed rust holes. However, the steel was painted, which may have led to intensified local corrosion.
The Smoking Gun
Varnishing or painting Corten steel makes about as much sense as a school girl with a peaches and cream complexion slathering on a layer of cosmetics. But, the architect, the railroad and many dewy-cheeked school girls proceeded with the coatings.
Granted, the coating was a waste of money, I needed to find out if the polyurethane coating could accelerate failure. My expertise is limited to metals so I got a polymers’ graduate student to give me a tutorial. She explained there are literally thousands of polyurethanes that are used for everything from spandex athletic wear to floor finishes. I finally was able to opine that yes, the appropriate polyurethane coating would protect the gutter from rusting without giving accelerated corrosion at the holidays.
The center was constructed 20 plus years ago and is now being expanded. A recent newspaper article described the building as “... a leaky black glass convention hall ...” But I have heard no complaints about the gutters. Any rusting was not severe enough to let the skylight collapse into the building so there was no Calamity to report. The combination of Corten steel and a polyurethane coating seems to have served the purpose.