Stubborn zinc just won't come off

[Ryban]

The zinc on my 5HP 2-stroke Mercury was looking pretty eaten away, so I tried to replace it this week. Unfortunately, I can't get the darn thing out.

It's a little rectangular zinc with a retaining screw that fits into the side of the shaft. The screw came out just fine, but the remaining 50% or so of zinc is firmly corroded in place. I've tried hitting it with penetrating oil, and I've gone at with a hammer and a flat-head hoping to pry it out. No luck.

Anyone have some further suggestions before I go chisel it away?

[captstu]

Barnacle buster dissolves it. Leaves threads clean