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

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.

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.

On 1/27/19 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


Alkaline batteries leak...it's what they do.
When you become sick and tired of cleaning up battery compartment messes, give rechargeable Eneloops a shot. I've got some from 2011 that are still going strong.

https://www.amazon.com/Panasonic-K-K.../dp/B00JHKSN5I

On 1/27/2019 10:41 PM, Ed Pawlowski wrote:
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.Â* It is alkaline so it is easily cleaned with an
acid like vinegar, then cleaned with water.

Google says potassium hydroxide is the electrolyte and the negative
electrode is zinc and the positive electrode is manganese dioxide.
Neither carbonate is blue and maybe it is from a metal in the contacts.

https://en.wikipedia.org/wiki/Alkaline_battery

Wade Garrett posted for all of us...



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

Blue crustys, a leak from the battery. If one gets them on their vehicle
connectors they are are referred to as green crustys or green pus.

--
Tekkie