Steel Corrosion due to Atmospheric Moisture 2

[KenRad]

I'm an HVAC professional, trying to get some good information regarding room design to avoid steel corrosion. Over the years, I've heard people say that maintaining a room's relative humidity below 55% or 60% will minimize the corrosive effects of water vapor in the atmosphere. What I'd like to know is whether it's really relative humidity, or absolute humidity that's the key. I would suspect absolute humidity, but I've never seen any guidelines that refer to limits on either absolute humidity or dewpoint.

If the critical parameter is relative humidity, then raising the room temperature would help. But this obviously would not work if absolute humidity is the key.

Any info would be appreciated.

---KenRad

 

[CoryPad]

I can't point you to a reference, but absolute humidity is the real factor. Corrosion occurs like any other chemical reaction and is based on concentrations, so the higher the concentration of water in contact with the steel, the greater the corrosion extent.

The reason relative humidity is mentioned is because indoor temperatures are maintained to a relatively small window.

You will need less than 50 % RH at 22 [°]C to avoid corrosion of unprotected steel (again, no reference, just my anecdotal evidence).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[KenRad]

CoryPad,

Thanks; you confirmed what I had suspected, that reducing relative humidity without removing absolute moisture would not reduce the potential for corrosion. I never thought about why relative humidity was always referred to, but your reason makes sense.

KenRad

 

[moltenmetal]

Reducing relative humidity DOES work if the air might at some point have a dew point higher than the temperature of the steel itself...condensation of liquid water is far worse than atmospheric vapour phase moisture!

 

[strider6]

Usually the value used to asses the corrosivity of atmosphere is the RH. I've never seen absolute humidity. The limit assumed for control of corrosion is RH =60 %.
For reference see this docs from NPL:
Atmospheric Corrosion
"Control of RH Most atmospheric corrosion can be prevented by maintaining RH below 60%."

http://www.npl.co.uk/upload/pdf/atmospheric_corrosion.pdf

and in the Book "Corrosion" - Shreir - Chap "The atmosphere".

S.

Corrosion Protection & Corrosion Control

 

[bimr]

You want to be below 45% RH for corrosion prevention.

There is a load of information on the Bry-Air website:

http://www.bry-air.com/files/app_updates/CorrosionPrevention.pdf

 

[KenRad]

bimr,

45% seems excessively low to me, and in a southern US climate, would come with a large HVAC energy penalty. Since the number came from a company that sells dehumidification equipment, I'm skeptical.

---KenRad

 

[unclesyd]

You might want to read this paper as it points out a secondary cause of atmospheric corrosion of clean metal. It is quite interesting.

http://www.centurycorrosion.com/products/chlor-rid/section1.pdf

 

[saberblue]

One of the other issues will be how the steel is stored in the room - if the steel parts are not touching you can get better results but if the parts are touching then you need to worry about a galvanic cell as well.