Not that this is totally applicable, but I keep something in my gun safe called a goldenrod dehumidifier.
Its just a metal tube with a heating element inside, that keeps the safe only a couple of degrees above the ambient temperature outside of the safe, and it is remarkable for preventing rust.
Basically, if there is any air exchange with the air outside, the air inside the container will be the same relative humidity, at the same temperature, but raise the inside temperature just a few degrees (relative to ambient), and the RH will drop significantly. Any heat at all will be better than none.
A safe temperature answer to your question though depends on your outside environment.
If you know a safe RH range (in your case, an acceptable maximum humidity), then Google for a psychometric table.
As an example:
If your ambient temperature (dry bulb) is 70F, at 80%RH, your wet-bulb is 66F, so heating to 83F would bring you back down to 40%RH.
Worst case weather scenario:
It gets cold overnight, and your metal cools down. Then warm humid air hits in the morning. Before your metal has warmed up, it may be below the dew point, and you will get condensation (just like how some windows fog in the morning). That's the absolute worst for corrosion, and just a little bit of heat keeps the condensation away, and you out of the danger zone.
Then again, I also keep my guns with a 2lb box of silica-gel designed to dry a closet, but that's really just a belt and suspenders. Hey, c'mon, nothing's too good for -my- guns. :)
I've also heard that VCI treated storage (like VCI treated paper wrappings, or plastic bags) can prevent corrosion, without the need to clean before welding, as the inhibiting vapors do not leave behind enough to make a difference.
I keep a VCI cardboard chip designed for fishing tackle boxes in all of my tool box drawers and feel that it helps in the nasty humid Long Island summers, but I also spray many of my tools with gun oil, so its hard to tell what works best.
Tommy,
You should be fine at 120 degrees. 100 is adequate enough to avoid moisture buildup, but the manufacturer's recommendation should be reviewed.
My feelings exactly Dave, but since I was not present and had no input on the order....I do not know the temp range. I have seen some that go pretty low in days gone by.
Thx all! Scott I rekon I need to shoot a call to Harris and see if they can give me a range/time guideline. You got a point.
The "moisture buildup" you want to avoid is condensate, or dew, however fine, forming because the temperature of the metal is at or below the dewpoint of the ambient air. If you are in a humid area, eg the Gulf, I would use 150 F min. to have a good margin of safety. We had issues once when employees were using the rod oven to warm their lunches; (No Comida!). If near salt spray, more extensive protection is needed. Salt deposited on the surface is hygroscopic and will take moisture from the air more or less independent of any dew point.
And regarding such as 4130 or any other hardenable low alloy, any rust on the surface is itself hydrated and can contribute undesirable diffusible hydrogen to the weldment. The rod metal must be kept absolutely clean of any deposited or corrosion byproduct contaminant.
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