Rust prevetion - is absolute humidity versus relative humidity

[BronYrAur]

I'm an HVAC guy, so I know all about absolute versus relative humidity (%RH). What I don't know is which one is the governing factor in terms of rust prevention.

I do work with a fuel injector manufacturer who tells me that their assembly rooms must be kept at 50% RH or lower in order to prevent rust from forming.

I'm not entirely convinced that they understand %RH and how it is entirely dependent on temperature. For example, if I am sitting at 70 deg and 60%RH, I can achieve the desired 50%RH by heating the room up to approx 76 deg. Just like that, I dropped the %RH without removing a single drop of water from the air! This concept boggles some people.

So I ask you metal experts if you know what the real control parameter should be?

 

[blacksmith37]

Some materials are hydroscopic ;I don't know of any metals that are- .Sounds like the easy thing is set the parameters to give <50% at the lowest expected temperature.

 

[unclesyd]

No matter what you do with the RH in your range you will have to keep the components below the dew point. AS you know all tings are relevant so you have balance out the the temperature and humidity.
In our process we have large areas where we dry the air, cooling it in the process then we add moisture to prevent static electricity.

 

[redpicker]

Perhaps you would understand their requriement a little better if it was stated that the dew point must be at least 20 F below ambient; for example parts could be arriving from an adjoining warehouse that could be as much as 20 F below the assembly area. In this situation, a maximum 50% RH would be needed to prevent condensation on parts arriving from the warehouse.

But, the true situation is probably much simpler. Most hygrometers display in terms of % RH. In order for the QC folks to be able to verify the process parameters in the assembly area are met, they have to verify the humidity. While there may be some temperatures where a greater %RH could be tolerated, it doesn't matter becasue the QC folks will shut down the operation if their meter reads >50%. In this situation, the only way to get a higher % RH accepted would be to convince top management they need to change their QC procedures. Good luck with that.

rp