Cylinder head corrosion

[30boat]

I was given a 90's Mercury 2.5 outboard because it was a non runner.Easy to see why.I've never seen this much corrosion on a cylinder head.It just desintegrated.What's more the rest of the engine is spotless with no signs of corrosion anywhere.The owner fresh water flushed it everytime it was ran so I'm baffled as to what might have caused this.A new head is on the way from the states and hopefully I'll have a perfectly good little outboard soon.

 

[Billjratt]

Is it possible there was not a good electrical connection through the headbolts?
Maybe it had been rebuilt at some time with too much hermetite or similar, then the head would be jumping up and down (electrically) every time the plug fired, which is also the period it is likely to be full of salt water.
Just a thought.