Don't look for corrosion on the brass valve...look for it on your aluminum engine, which is now effectively a sacrificial anode for your valve. Isn't that good news? :-}
Even though the original type of stainless (?type 316) valve is more expensive than the brass one, I like to think of these cost decisions more as a ratio. How much is a proper marine valve replacement versus how much is a new engine? That way I don't cringe as much.
Remember, too, for your own interest, that in de-zincification, the brass LOOKS fine and dandy. It's just that one day it will simply break without warning; and since Murphy lives, it will happen at the worst possible time. (I really hate Murphy...why does nothing ever break at the best possible time?) ;-}
Regards,
Leander
leander@worldnet.att.net