A kilo turns pink in only 24 hours? It sounds like you have a serious humidity issue in your basement, even beyond the possible issues with the firearms. That much moisture can lead to mold and other issues with the house.
May I suggest a much bigger/ better dehumidifier for the whole basement, and add a golden rod to the safe.
You may want to get a humidity meter like this one on Amazon to find out truly what the relative humidity is in your basement:
Try to keep the humidity in the safe around 40% or a bit lower.