Steel Beam Flange Corrosion

[TylerM94]

Why does steel flake when it corrodes from a metallurgy standpoint? I was in a below grade steel framed parking garage where all of the beams were fireproofed. I noticed that several beams were visibly wet (fireproofing was soaking) and in some cases the fireproofing had popped off the beam. At one particular beam, the beam flange appeared to have expanded significantly to the point where the flange was composed of many thin layers of steel. I was able to chip these layers off with my finger. I am under the impression this is a pretty advanced form of corrosion. My question is, why from a metallurgy standpoint does the steel separate into these layers? Also, are there any good guidelines for repair of such members?

 

[Enable]

The material experts on this board will be better able to answer the metallurgical component of your question.

However, as far as repairs are concerned, I am afraid it depends highly on the particulars. You need to assess how much good material the base structure has left and from their decide what the best approach is. There really is no guide for these kinds of situations as they are inherently particular to the application / severity of corrosion / what can even be done given existing occupancy.

Some good reads for general metal rehab:

https://www.aisc.org/Field-Welding-to-Existing-Steel-Structures

Structural Renovation of Buildings: Methods, Details, and Design Examples

 

[bugbus]

I don’t pretend to be a materials person but my very limited understanding is that the hot rolling process tends to align and flatten any microscopic inclusions in the direction of rolling. This gives the steel a certain layered structure, with a certain weakness in the through-thickness direction which also would go some way to explain lamellar tearing. Wrought iron, as I understand, has this effect too but much more pronounced.

 

[hokie66]

The thin layers are iron oxide, not steel. When steel corrodes, the products of corrosion occupy a larger volume than the steel did. The same process which leads to spalling in a reinforced concrete structure. Yes, it sounds like the capacity of those beams are severely compromised.