In alt.home.repair, on Sun, 27 Jan 2019 21:24:04 -0500, Wade Garrett
wrote:

When changing the batteries in my programmable Honeywell MagicStat 5/2
thermostat, I noticed a thin layer of dry blue chalk-like deposit on one
of the negative terminals in its two-AA battery compartment.

No leakage from the AA alkaline battery, no deposit on the other
negative terminal, thermostat at least 20 years old.

The deposit came off fairly easily- most with a Q-Tip, the rest I
scraped off with a small knife.

Never seen a blue deposit before- wonder what it i

Copper oxide? But the rest I can't even guess at.