Oxidation Prevention - humidity control parameter

[BronYrAur]

In the manufacturing of fuel injectors, I have been told (as an HAVC guy) that a relative humidity of 50% or less is needed to prevent rust. I have found that most people (including those in the HVAC industry who should know better) don't understand relative humidity (%RH). So I am questioning if the number of 50% RH is valid. Is RELATIVE humidity a good indicator of when rust will form, or should an absolute parameter (e.g. dew point) be used?

The trouble I usually run into is that someone will specify that the desired setpoint should be something like 75 degrees F and 50% RH. Then they decide that they want 70 degrees F and 50%. To those folks, 50% is 50%. In reality of course, the moisture in the air is quite different for those 2 scenarios.

So I am posting the question here as opposed to the HVAC forum to try to understand when the metal actually starts oxidizing? Is a relative humidity parameter really the best indicator?

 

[CoryPad]

Relative humidity is used by those studying corrosion. Here is one quote:

Most atmospheric corrosion can be prevented by maintaining RH below 60%.

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

 

[MintJulep]

I would guess that RH is not the best indicator, but that is has been empirically demonstrated over time to be a reasonably accurate proxy.

 

[EdStainless]

Within the range of conditions that we call ambient, RH is a fairly good indicator.
We used to require that RH be <50% at the lowest temp within the control range.
One facility that I know of uses absolute humidity, but they based it on 68F and 50% RH.

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P.E. Metallurgy, Plymouth Tube

 

[IRstuff]

The article explains pretty well that metals are alway oxidizing, and it's the rate of oxidation that's affected. The 50% value is pretty safe. In your example, if you were to specify per absolute humidity, the correct RH for 70F would be about 58% RH, but that would make for a very clammy environment. Moreover, since oxidation is accelerated by temperature, going to 70F should allow the absolute humidity to increase, which would make it even more clammy. So long as the 50% RH condition's temperature is not exceeded, then it really doesn't matter if you underspecify the humidity at lower temperatures.

TTFN
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