On 1/27/2019 9:24 PM, 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

Potassium hydroxide usually leaks on the negative terminal forming
potassium carbonate. It is alkaline so it is easily cleaned with an
acid like vinegar, then cleaned with water.