Whats going on with these Gil Exhaust Manifolds?
 

Whats going on with these Gil Exhaust Manifolds? They all seem to be having some sort of corrosion under the powder coating in the same area. They are pressure tested good but can't figure out whats going on. Running in Bracish water and flush on a regular basis. Thanks
https://cimg2.ibsrv.net/gimg/www.off...1a83e5a91.jpeg
https://cimg3.ibsrv.net/gimg/www.off...14a9209cb.jpeg
https://cimg4.ibsrv.net/gimg/www.off...16e5f951c.jpeg

That is where the casting holes are. In your first pic, you can see the faint circle. After years and years of heat cycles, they prob got microscopic cracks in the powdercoat. Tiny amounts of moisture and air get in and the corrosion just builds over time.

What do you mean by casting holes? Please explain.

Thee are made similarly to an engine block to create the hollow water jacket. The holes are plugged and then they get powdercoated
https://www.offshoreonly.com/forums/...manifolds.html