D1-30 heat exchanger/exhaust manifold

Hello,
we have found out that our heat exchanger/exhaust manifold has corroded near the exhaust outlet. The corrosion is quite extensive.
When we bought the engine, there was only fresh water (no coolant) in the cooling system and it was there at least a year. The engine has not been apparently used at all during that period. Perhaps this might have started the corrosion, but probably was not the sole cause.
We are now suspecting the exhaust pipe design. We have this one: http://www.marinepartseurope.com/en/volvo-penta-explodedview-7746500-25-8329.aspx and it seems that some water might get sprayed up into the exhaust mainfold.
Any thoughts on this?
Would their exhaust riser (http://www.marinepartseurope.com/en/volvo-penta-explodedview-7746500-25-9785.aspx) be a better option? I am wandering what does it look like on the inside of the riser...

Many thanks
Petr

The internal design is probably the same.
As exhaust and raw water gets 'mixed', you will have corrosion.
Which of the two to chose might depend on boat type and engine installation, then particularly exhaust system.
The 'high riser' makes it more difficult for the water to rush back into the engine when stopping. Might be useful if a long hose and no exhaust/water lock close to the engine, and the outlet above the engine.

Thanks a lot. I started to think about custom high riser, because we have a very marginal slope from the manifold to the lower point of the exhaust hose. Being an engine greenhorn I never really though about it. (Being naive at the same time and thinking that "certified engineer from Volvo Penta" would do the thinking for me...)

Use this as a rough guide:

Hi There, I have just had my d1 30 over heat this weekend. I found the problem to be the exhaust manifold also, it has a hole in it as the exhaust comes out. My engine was new approx sox years ago and has always had antifreze in it since new. I am not impressed with Volvo.
Regards
Peter