Steel Corrosion due to Atmospheric Air

[KenRad]

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

 

[IRstuff]

Even so, raising the temperature will probably accelerate corrosion, regardless of the moisture content. I would go with reducing the temperature AND keeping the humidity low.

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

FAQ731-376

 

[AbbyNormal]

Not much of a temperature range inside of an occupied building.

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.

 

[marcoh]

I did a museum project a while ago which housed old planes where the same question was asked, ie relative or absolute humidity. The question was put forward to the boffins who pondered and analysed and came back with the response that constant relative humidity (and below 50% RH) was the critical factor.

Steel in air conditioned buildings (eg in ceiling voids etc) never seems to get any special attention and never appears to corrode.

 

[KenRad]

I posted this question on the Corrosion Engineering forum, and got the answer I was looking for:

"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

 

[AbbyNormal]

it sounds like he was saying that there was not much of a temperature range inside of a building

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.

 

[IRstuff]

It's interesting that you picked those two temperatures:

http://en.wikipedia.org/wiki/Absolute_humidity

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

FAQ731-376

 

[AbbyNormal]

They were deliberately picked.

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.

 

[IRstuff]

Almost all chemical reactions have a reaction rate that is temperature dependent. Additionally, for something like rust, the species has to diffuse down to the uncorroded metal to generate new rust. Diffusion is likewise dependent on temperature.

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

FAQ731-376

 

[AbbyNormal]

1000C sounds endothermic

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[IRstuff]

No, just big-ass electrically heated furnaces. My recollection is that they cranked about 16 kW into a 10-ft long, 10-in diameter quartz tube, where the wafers sat, and the gases flowed. We had 2 banks of 3 each, so there was a LOT of A/C involved, as well.

TTFN

FAQ731-376

 

[ChasBean1]

It's all about avoiding condensation on the steel. Regarding air temperature verus steel corrosion I agree with AbbyNormal that building-range temperatures are probably negligible relative to corrosion rates. The 1000°C example might be a different story. Intermittent wetting is a much bigger factor here. Air dew point has to be below the steel surface temperature. If you get cycles of cool nights followed by humid days and building infiltration, that's probably the most significant source (barring water leakage)...

 

[KiwiMace]

My understanding of metal conservation agrees with Chasbean1:

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.

 

[IRstuff]

Reading the OP, he was asking whether he should crank up the room temperature. Using the Arhenius equation and assuming a moderate activation energy of 0.7 eV, a 5ºF reduction in temperature results in a 23% reduction in the rate of corrosion, all else being equal.

http://www.siliconfareast.com/activation-energy.htm

http://en.wikipedia.org/wiki/Arhenius_equation

TTFN

FAQ731-376