YANMAR 1GM 10 Exhaust port corrosion

Does anyone have any ideas why

Does anyone have any ideas why the exhaust port in my 1GM10 seawater cooled engine has corroded severely. The head was new 18 months ago and has about 100 hours use.The previous head was beyond repair with the same fault.The exhaust elbow is not affected.

"The water cooled exhaust elbo

"The water cooled exhaust elbow is leaking at rthe flange (top) where the inner and outer pipe are welded. Then when you shut down the engine sea water will sit in the head at the exhaust valve area. The first elbows were made of copper then they replaced that with stainless steel. The weld at the top of the elbow will break through and cause leaking. Remove the elbow and have it glass beaded and you may see the fault. Another cause is water in the exhaust line flowing back into the exhaust port of the head after shut down. Get a catalogue on the Vetus Exhaust systems, Go on line) and they have excellent diagrams showing the proper water cooled exhaust systems installations.
Regards, Ron"