Yamaha T9.9 Corrosion

[s199810]

I have a high thrust 9.9 outboard, and it corroded open at the back just above the exhaust hole, see the photos. Put my hand at the corroded opening when the motor is running and I can feel exhaust gas coming out of it, but point a flash light to it, I can't see it connects with the exhaust passage. Checked the service manual, it doesn't tell what that area is. Does anybody have an idea why exhaust gas comes out of that area? I don't know how important that area is, and am thinking to just seal it with expansion foam. It corroded open a couple of years ago. It started with a small corrosion crack and became larger and larger, and eventually completely open like now. I don't know why it corroded like that as well.

 

[s199810]

Further to my question above, found this photo in another thread, showing the actual photo of the upper casing after it's removed. My guess of the function of that small void is, it surrounds the exhaust tube so to isolate the heat from the larger void which contains the oil pan? Otherwise the oil pan could be got overheated by the radiation heat from the exhaust tube? But this hypothesis is not correct also as if you see the service manual, there is wet exhaust tube going right through the centre of the oil pan. Can anybody thred light of what that small void is designed for? Thanks!