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Corrosion of steel lintels


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A little while ago I did a brief visual structural inspection as a favor to a contractor. The building was built in 1975. The construction is brick over concrete block. As you can see from the photos, there has been excessive cracking and displacement due to corrosion of the steel lintels.

Any ideas as to why this is worse than typical for the age? The lack of flashing and weep holes is obvious, but that is typical around here. Looks like the lintels were never painted, but I've seen that before without excessive corrosion.

Looking at the clean separation between the mortar joints and brick makes me thing there was a lack of suction, so maybe hairline cracks? The condition is worse on the northern exposure, but I would expect that. One other factor of interest. There is a small concrete/concrete block manufacturer to the north less than 1/4 mile away. I'm not sure whether that could have much effect on atmospheric corrosion.

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I look at lintels all day, all year. I notice big differences in corrosion rates with little no difference in building type, age, or construction.

I see some really old stuff that's >100 years old, and it's fine. It's the bluish stuff, meaning there's a lot of chrome in there.

I can imagine some mid 70's steel being really crappy.

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Most of the steel used for modern lintels is most likely a 1020 steel hot rolled product. The primary difference between the products you encounter is the surface treatment of the steel and the installation of the wall flashing.

You can purchase galvanized steel lintels but I rarely see them in my market. Stainless steel angle is also available.

If the flashing is screwed the only thing that will stand up to the water intrusion is stainless steel. If the water intrusion is that bad you have other problems.

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That sounds good, but what about the buildings that have no flashing and no corrosion, and the one's that have flashing and major corrosion? And every combination in between?

Are you saying that all steel is the same quality?

Maybe, because there's no way for me to know differences in steel, but some stuff goes to blazes and some doesn't and there's no apparent condition that separates the two. Why?

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I think there's a correlation like you describe. Certainly.

So, nobody's going for differing grades of steel? It's all the same stuff?

Yes the grades differ as well as the production methods..

Just like colors differ with dye lots for pretty much everything made. If you have ever been in a steel plant,it is amazing how they mix the molten steel with additives. They toss in this and that with a little more of this until they get what they think is the proper mix. Yes it is now confirmed by a chemical analysis now, but the mix will still differ mill to mill.

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I think there's a correlation like you describe. Certainly.

So, nobody's going for differing grades of steel? It's all the same stuff?

There are certainly different grades of steel and different reasons why the steel corrodes. Most of us don't have the ability to know why the steel goes bad. We can just make educated guesses.

I did not intend to imply that your guess was any worse than mine.

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I know. I wasn't saying that. I see such huge variance in just about everything from mortar to brick to wood....I just think there's varying grades of steel that we aren't even aware of.

What Scott describes is why I have this idea; it's like baking a cake. They toss stuff in and make educated guesses mid recipe.

I was first alerted to this years ago when I was reading some old spec's for buildings. The architects and engineers back then would specify the grade of steel in the nice places.

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I have spent a number of years as a forensic metallurgical engineer in my life. The number of reasons that steel corrosion occurs are endless. Each individual case of excessive corrosion must be evaluated on its own history.

One should not try to make a general observation that encompasses all steel corrosion. The corrosion could be caused by stray electrical currents, variations in steel composition, as Steven suggested, the composition of adjacent materials and even to the air that the steel is exposed.

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I wish I could hire you as mentor. I look at steel lintels all day every day. I'd love to develop some guidelines in how to think about this.

If you have access to a mass spectrometer for chemical analysis, a scanning electron microscope for fracture analysis and a metallurgical lab equipped for processing specimens for microscopic analysis it would be my please to serve you.

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Kurt, I suspect that there are differences in steel that would explain the different rates of corrosion, but I did search quite a bit and could not find much good info on the topic. I found some references to increased amounts of manganese causing more corrosion. Manganese is added to hot rolled steel, so that is a possibility.

I was leaning towards poor brickwork because of the age and the complete lack of mortar residue at the horizontal joint that separated.

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