This is the galvanic corrosion. The reason of this is the iron cast elbow next to aluminium exhaust
housing. Gasket is the innocent part that has no role in this corrosion event.
The iron cast elbow gets rusted very fast since the salty water and exhaust gas passed through it when the engine
runs. It is like puting an iron in to the sea water. Then liquid and gas flow through this elbow become irregular, because of the rust deposits on the gas and sea water flow surfaces in the elbow(mixer). Sea water starts to splash back drops to the aluminium exhaust housing.
When you put two or more different metals like aluminium and iron close to each other in a closed housing and splash some salty water on them, galvanic corrosion occur even if you install a thin gasket between them.
This is a common problem for each Volvo Penta
D1-20, D1-30, D1-40 engine. When you look at to Yanmar
engine at this point, they use a longer stainless steel
elbow instead of an iron cast short elbow. This is the most important differance between the two brands which makes Yanmar
more preferable and reliable.
This video shows a tuned 316L stainless steel elbow installation
to D1-30 engine of my boat