If the 870 Parkerizing job wasn't done properly, that can be the issue. The military protocol calls for dipping in a mild warm solution of chromic acid at the end of the actual phosphating. This has the effect of putting chromium on free iron particles in the pourous phosphate matrix, preventing them from rusting by the same oxide layer formation
process that inhibits rust in stainless steel.
The problem with that process is that chromic acid is highly toxic and subject to a lot of EPA controls. This may make it too expensive or otherwise unattractive to use if your shop isn't already set up for it. I don't use it in my phosphating projects for this reason. I hit the finished work with an acid neutralizing degreasing cleaner, like Formula 409, then quickly rinse it and put it into boiling distilled water (distilled to avoid water spots later; if you have both a water softener and a reverse osmosis system that follows it, that RO water may well be good enough). Boiling water converts any red rust that may have started to black magnetite (bluing) which doesn't rust easily. The heat, as it steams off to dry, puts a microscopic blue layer on any exposed steel surface, too. With the part thus prepared, water displacing oil handles the rest. It's extra bother, but you can do it.
For dehumidifying a cheap material is montmorillonite clay. This is in a lot of military spec desiccants. It is
sold as oil absorbent and probably is what a lot of kitty litter is, too, but I don't want the deodorizers; just the clay. Spread it out on a cookie sheet and put it in your oven at about 450°F and let it bake for an hour. When it finally is cool enough to handle, pour it into a paper bag and tape it shut. The bag should have extra room in it as this stuff swells as it takes on water. It'll take on about 18% by weight at 40% R.H. Not fabulous, but cheap.
Here's a comparison of different types.
If you put a jar with a perforated lid full of
magnesium chloride in the bottom of the safe, it will take on water any time the RH is about about 32%, tending to regulate RH around the jar until it reaches a saturated solution. I do not know, however, what the capacity and recharging schedule (drying) will look like.