About 15 years ago the missus did an OU geology course, and left a bottle of dilute HCl near the router shelf in my workshop. They rusted overnight, which prompts me to ask: what's in the drawer underneath? Not suggesting that you might have a chemistry lab, but something in there might be the culprit. Other things than water cause oxidation.
Failing that, bung in a few sachets of Silicagel Do Not Eat and bake them occasionally in the oven. Serve when tender.