Unless a safe is sealed air tight, dehumidifiers cease working after a very short period of time, perhaps just a few days. Moisture will penetrate the cracks and quickly saturate the dehumidifier at the outside ambient humidity level.

Dehumidifiers do help level-out fluctuations in humidity level, such as those caused by temperature cycling. But, something like an old towel inside the safe (or anything that absorbs moisture) will do that.

Long-term storage requires that the metal surface be sealed from both moisture and oxygen and that the sealant not be volatile.

J.B.