HP41CX, corrosion on (-) of one battery

[Uli]

„You have to worry about the calculator terminals. If it is clean, just replace the batteries. Your bateries are old, they can leak at any moment.“

Thanks for your idea. But that is to simple :-)
I have changed the batteries after this issue occured first time. The first batteries were fine and the actual batteries are very new.

Battery contacts in calculator are looking fine as well.

The corrosion on the battery is brown (not green) and it was not easy to clean it. It looks like corrosion from iron and not from copper .