Assuming the post is Stainless Steel
, you'll be most concerned with corrosion
at the lower bearing just above the rudder
itself. SS corrodes in the absence of oxygen which can occur at the bearing. It should be obvious but it can also be cracks which may be harder to see. The yard should have someone who is familiar with this type of corrosion
to back you up if you feel you need it. Probably wouldn't even charge you.
You'll have to drop the rudder, at least partially, to see the problem area.