Steel Corrosion due to Atmospheric Air
Steel Corrosion due to Atmospheric Air
(OP)
I'm 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
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





RE: Steel Corrosion due to Atmospheric Air
However, I'm puzzled as to why there's corrosion at all. I've got steel stuff laying around the house and office and have never come across much in the way of corrosion, except for stuff that is simply prone to rusting.
TTFN
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RE: Steel Corrosion due to Atmospheric Air
Store an old plane or tank out in the desert and it does not corrode.
Live on an island or right on the coast and you will see steel corrode. Not sure how much of a difference the carbon makes but I recently looked at some piping in an air craft hanger. They had galvanized in for the foam, but then were running black iron for sprinklers in mezzanine offices. Black iron is on its last legs.
Back home, on the great lakes I got a kick out of the city paying extra for weathered steel poles for roadway lighting along expressways. They bought them alredy rusted.
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Steel Corrosion due to Atmospheric Air
Steel in air conditioned buildings (eg in ceiling voids etc) never seems to get any special attention and never appears to corrode.
RE: Steel Corrosion due to Atmospheric Air
"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."
This has always been what I believed intuatively, but I was looking for some confirmation. The question arises in the design of manufacturing facilities, when trying to strike a balance between energy used by the HVAC, and sufficient dehumidification to prevent corrosion.
Some customers have standardized on certain zone setpoints (say 70F, 50%RH), with no basis other than "we've always designed to these conditions." I'm trying to come up with some good numbers, without excessive dehumidification, and the resultant energy penalty.
KenRad
RE: Steel Corrosion due to Atmospheric Air
Somehow I think 75F @ 45% RH would be better than 50F @ 90% RH though.
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Steel Corrosion due to Atmospheric Air
90% at 50 --> 48 grains
50% at 75 --> 67 grains
I would contend that the higher the temperature, the more active a given amount of moisture will be. There's a reason why the MIL-STD-810 Method 507 Humidity testing stresses a +30ºC change from baseline, and only a -10ºC change the other direction. The stated purpose of such this testing is, "to determine the resistance of materiel to the effects of a warm, humid atmosphere." The first item on the list of problems is, "Oxidation and/or galvanic corrosion of metals."
TTFN
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RE: Steel Corrosion due to Atmospheric Air
Perhaps it is not all just absolute humidity.
Moisture being able to quickly evaporate could be a factor
I have no proof but common sense suggests metal in a 90% RH environment would be more prone than something in a 45% RH environment, even though it is possible that 90% RH could actually be less grains of mositure in the air.
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Steel Corrosion due to Atmospheric Air
Take the examples to their extremes, 10%RH at 212ºF vs 99%RH at 32ºF, which one would you expect to see corrosion at?
My background includes semiconductors, so I'll use that as the baseline. Silicon dioxide is "grown" on silicon wafers by exposure to temperature, moisture, and oxygen. Since it has desirable properties, we don't call it rust or corrosion.
Silicon, in room temperature, regardless of the humidity or time, is limited to about 20 angstroms of oxide growth. At 1000ºC, even with less than 100% humidity, you can grow a micron of oxide in a few hours.
TTFN
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RE: Steel Corrosion due to Atmospheric Air
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Steel Corrosion due to Atmospheric Air
TTFN
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RE: Steel Corrosion due to Atmospheric Air
RE: Steel Corrosion due to Atmospheric Air
Surface corrosion is largely a result of having moisture condensed on the surface. This sets up an electrolytic cell with the air. Marine environments are worse as the condensate is a better electrolyte. High RH increases the incidence of condensation and greater corrosion rates as a result. Anything above 70 - 80% is thought to be unacceptable.
The 'constant' RH of 50%, is a spec largely driven by conservation of non-metallic objects. For materials that absorb moisture, the moisture content of the material is proportional to the Relative humidity of the air.
Cycling the RH causes the material moisture content to cycle, and over time mechanical damage is caused by swelling and contracting of the fibres.
RE: Steel Corrosion due to Atmospheric Air
h
http://en.wikipedia.org/wiki/Arhenius_equation
TTFN
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