It is not a question of what type it is or not. As gliders grow older, they
will show wear. And corrosion sometimes too. Mostly depends on how it has
been treated and stored, but also on for instance water bags being tight,
if it has been sitting outside for some time tied down etc.

The PIK-20 case is not alone. I have found similar corrosion issues in the
wings of a Standard Libelle and DG300. A thorough annual will usually flush
out any issues. If in doubt, use a boroscope to access hard-to-get-to
areas. Such as the wings from the opening inside the airbrake boxes.