Why do positive terminals on DC devices exposed to water corrode more than negative terminals?
Normally one would expect the negative electrode to get smaller and the positive to get bigger, as the technical defined direction of current from plus to negative is wrong. In reality its via versa, charge is phyically transported from minus to plus - so ions are transported from the negative towards the positive electrode.
&quot;dry&quot; Effects:
https://en.wikipedia.org/wiki/Electromigration
&quot;wet&quot; Effects:
https://en.wikipedia.org/wiki/Electrolysis
What you see should be caused by the reaction of the spring of the battery holder and the batteries electrode. Different metals cause a small voltage.
https://en.wikipedia.org/wiki/Galvanic_series
If then, like you say in your application it gets wet, you essentially build a small battery. (so you speed up the process a lot)
I guess you can improve like using a less reactive metal for the battery holder spring like e.g. a gold coated one.
And of cause using a (more) watertight housing would also help :-)