This pertains mainly to framing members--not much thinner sheet.

I've read authoritative sources which state that iron oxide (rust) on both iron and steel is in fact a protective "patina" that protects the metal from further corrosion. Some instances I know of where this occurs:

1) Cast iron radiators used in home/building heating systems. (I work on these systems so I'm quite familiar.) While it could be said that properly piped and completely sealed hot water systems become nearly anaerobic (no oxygen present) many of these began as systems open to the atmosphere. Steam systems would seem prone to extremely rapid corrosion as between boiler cycles the steam condenses into water (212F water) as air enters the radiators to replace the space occupied by the steam. The wall thickness of the cast iron radiators is typically about 1/8" when away from ornament/corners. Many of these radiators have been working for more than 100 years and I suspect have at least that much life remaining. While they are always quite rusty inside I have never encountered a failed radiator that did not fail due to either freezing or extreme impact. I've seen people ripping out these systems who area amazed when they survive being thrown out 2nd floor windows! There are even pressed steel (fairly heavy sheet--maybe 16 gauge) radiators pushing 100 years of age and still leak-free.

2) When touring in the "rust belt" I noticed many large Interstate and train bridges that by everything I can tell were originally left to rust naturally.

3) Homes often have exterior structures supported steel members that have never been coated. The home next door to mine was built in the early '60s (I knew the architect and he was a STICKLER) and has a large cantilevered deck supported by I-beams that have never been coated. The original redwood had to be replaced but the steel is still perfectly sound--if, of course, covered with rust.

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What is it with the frames of our cars that allows them to rust through? Are they too thin for the "protective patina" of rust to build up sufficiently? Is it a poor quality steel? Something else?

 

Hi SM,
I've thought that the 'chemicals' used on roads in the 60s and 70s contributed to the destruction of the steel on the frame and various chassis parts by rust.
I've also thought that the fact that areas of the birdcage that rusted seem to been areas with oxygen and a continuous supply of moisture.
????
Regards,
Alan

 

When it comes to "chemicals" on the roads that is pretty much my belief as well but those big bridges in the "rust belt" surely saw lots of salt and other things. I won't discount excellent engineering that forced runoff from the deck from contacting the structure.

When it comes to the birdcage with a continual supply of oxygen and moisture (you didn't mention heat which accelerates oxidation) I defer to the bottom of steam radiators and the top of hot water radiators both of which encounter long periods of "areas with oxygen and a continuous supply of moisture."

 

Hi SM,
I have a hard time comparing cast iron radiators and the steel used in frames and birdcages.
Sorry.
Regards,
Alan

 

I think cast iron objects appear to last longer because their parts are much thicker. Don't count on a patina to protect steel bridges. There was a fatal collapse of an interstate bridge a few years ago due to corrosion. And millions of dollars are spent on the Brooklyn and Golden state brides to maintain them. Iron products corrode when the carbon and the iron in the alloy create a galvanic path when exposed to moisture. A much slower but similar reaction to the videos we have seen of fire shooting out of a guys pants from his e-cigarette!

 

If I had a nickel for everytime I heard its a fiberglass car it won't rust. I believe Alot of original purchasers were not educated properly about the cars and drove them in all kinds of weather. I further believe what makes the cars appealing IE their shape and design is the Achilles heel. Function following form if you will. In the end it doesn't matter what caused it, only what you do to repair it and protect it. Won't be driving mine in the rain or snow. Would rather walk.

 

I agree but is it so hard to compare such with the steel used (and exposed to the weather) in bridges, homes and buildings?

Am not trying to start a pissing match in any way but for the life of me I don't understand the difference between uncoated steel/iron structures that last for decades without impairment and frames that (somewhat anecdotally) turn to Swiss cheese when stored in a barn with a dry roof but earthen floor. Is that a valid?

I can understand water trapped in the birdcage around the windshield as a great contributor to rusting of rather heavy steel but I still don't understand why the protective rust patina fails.

 

Engine blocks are similar to home radiators. Car frames don't have the benefit of the high heat to "bake" away the dampness. Cars also can go for years or decades parked while the chemicals and salt eat the metal.

 

Well I live right in the Rust Belt. In the Pittsburgh area we have more bridges than any other city. I live in Allegheny County which is the heaviest populated county and it includes The City of Pittsburgh. There are 1900 bridges in Allegheny County. It has been named the City of Bridges. Believe me they are in a terrible state of disrepair. We have so many bridges being reinforced and or replaced right now it's rediculous, but necessary. Obviously a bridge is usually over water and therefore moisture against the underside constantly.

On the other hand the United States Steel Building which is in Downtown is made of a special steel that was designed to rust and stay that way. It never had a protective coating of any kind. I've worked on and in the building and the steel columns and beams are in excellent condition, although rusty surface, and it was built in 1970. Obviously they had the technology in 1970 to make a pruduct that can handle the elements but I'm sure it would be too expensive to use in automobile manufacturing. Your car would last forever, and I'm sure GM didn't want that.

Ned

 

GM tried galvanized panels, but painting was a problem as the spangels would show thru the paint. Could be same for other types of steel...

 

NMT1957, The special steel that you are talking about is called "COR-TEN". This steel was used by coal hauling railroads for hopper cars, as an experiment. Unfortunately, the added cost of this special alloy made it more cost worthy to continue as they had in the past with steel and paint. I lived in Cleveland, Ohio most of my life and there is a special office building on Snow road, past West 150th street going to Cleveland Hopkins airport, that has the Cor-Ten unpainted beams on the outside! Lou.