rusting/rotting of steel beams above room subjected to large quantity of steam 1

[kacarrol]

Good afternoon,

I work in the HVAC department, so please excuse me if I make any mistakes when discussing structural items here.

I have a job where a room is occasionally filled with steam (think Russian sauna, but this is not a spa facility). The roof is supported by steel beams. Below the steel beams they have installed insulation board, vapor barrier and stainless steel plate (to keep the steam in the room).

This room gets the steam treatment about once a month.

I know that the vapor barrier installed is not containing the steam (no surprises there).

My question is, do you think the occasional steam treatment to the support beams would cause a lot of rust? The steam is made from fairly clean water (no chlorine).

The walls of this room are metal studs, they are just protected by the vapor barrier. I don't see a whole lot of rust on them at the ground level, but I couldn't see into the ceiling space. We have a structural guy working on this job, he just isn't available at the moment and my curiosity to know experienced structural peoples thoughts on this one is getting the better of me!

 

[JAE]

It would all depend on the coatings (if any) on the existing steel beams.

If the beams are plain (no coating) or have a standard shop primer then I would think that rusting could develop over time.

The rate of rusting would also depend on the intensity of the moisture, whether the steel drys out between steam exposures, any ventilation available in the interstitial space, etc.

 

[TomDOT]

Unless you have another source of electrolyte (salt) - rust should be pretty darn slow. Distilled water just doesn't corrode steel very fast.

I would still fix the vapor barrier, and ensure any accumulated moisture is removed.

 

[TehMightyEngineer]

Having seen TONS of rusted beams and columns due to steam leaks around old pulp and paper mills in my short career I've made the following observations:

Steam can and will cause highly accelerated corrosion, especially if the water is allowed to bead on the surface and is applied semi-continuously. This can cause failure of any coating system after a few years. However, infrequent steam causes no more corrosion than wetting the beam and allowing it to dry. Uncoated beams will rust but very little loss of thickness will occur provided it is a low salt environment. Coated beams subject to infrequent steam baths will possible have local corrosion anywhere the coating wasn't well applied or is damaged but overall will remain with no significant thickness loss or corrosion.

See the attached picture from a local pulp mill. Note the coating on the steel pipes and beams. This is about 50 year old steel and the coatings were not maintained under the steam bath they were getting. On the side opposite the steam you see minimal corrosion and a well adhered coating. On the flange dripping from steam condensation the corrosion has flaked the beam and the coating has broken away. Note also the coating failing on the pipes. If this coating were maintained they could probably stand this environment but because it's being allowed to fail they soon will go the way of the steel beam. They're only advantage is they're hot steam lines so limited condensation compared to the relatively cold steel. Also, the layer of pulp on the surfaces DEFINITELY doesn't help.

Short answer is coating integrity is important. Once the coating has failed the beam will rust in flakes. Each flake will act to hold water from the steam and the corrosion will accelerate. I've seen steel that has sat for 20 years without major issues getting bathed in steam but once the coating is allowed to fail and the flakes start building up the beam can rust through in a matter of months if salt or chemicals are present.

If possible I would prep, prime, and paint the beams (if not already done) and you likely have nothing to worry about. If this isn't possible then I would use a borescope and check the existing beams for corrosion and loss of thickness. If they're satisfactory I would seal the vapor barrier and call it a day. Keeping the steam and its condensation off of the steel is your most important task to avoid heavy corrosion.

Maine EIT, Civil/Structural.

 

[kacarrol]

Thanks for the input!

Especially TehMightyEngineer (star for you). It's good to know that the rust flakes would increase the rate of corrosion, I'll stick that tip in my back pocket.

I don't know if the beams have a coating. We shall see what comes out of this as the rest of the design phase plays out.

Thanks again,
K

 

[TehMightyEngineer]

Essentially anything that allows water to remain in contact longer with the steel causes corrosion. Acids and electrolytes (chlorine salt most commonly) speed up the process. Exposing greater surface areas to rust will increase corrosion rates. Thus, a pitted or cracked surface with dust, rust flakes, or shaped such that standing water can remain are the least favorable conditions.

Common corrosion prevention methods involve removing the oxygen or the water, providing cathodic protection (such as a zinc block in steel that is submerged in salt water), or providing an alkaline environment with limited moisture and oxygen (steel in concrete).

Maine EIT, Civil/Structural.

 

[TomDOT]

TehMightyEngineer - I'm going to have to disagree with you somewhat. A pulp and paper mill (at least parts of it) is a MUCH more aggressive environment than the one described by the OP.

Agreed that moisture, salt and acids are the main corrosion drivers, unless you start to deal with complexities like galvanic couples.

 

[racookpe1978]

Rather than isolate the area, ventilate it with a small or vent (HVAC) duct emptying dry cool sir into the closed area above the room to allow the occasional water vapor to cool. condense, and then evaporate. You are dumping a limited amount of vapor per month. Not galls per hour or per day.

The steel needs to be painted (preserved) first. AND - and it is important - any existing corrosion needs to checked, removed (wire-brush or equal) and painted. Painting OVER existing corrosion will fail.

 

[TehMightyEngineer]

Tom: Oh I agree, pulp and paper is probably one of the harshest environments I could think of for carbon steel. I wasn't implying that the OP had anywhere near that level of environment effects but rather that I've seen the effects steam, chemicals, and water can have on steel and pulp and paper makes a great environment to study accelerated corrosion from steam.

Maine EIT, Civil/Structural.