yes, the flange is plastic. no, the coolant didn't cause the leak. the
fact the VW used plastic on for a part that sits on a hot metal engine and
has boiling coolant/water flowing through it ... THAT's what causes the
flange to deform (mine was visibly warped). imho.

the new, metal replacement flange is thicker, but does NOT have a hole
cut into it for the o-ring. so you'll have to use a flat type of gasket.

joel