Fresh water boaters can get 12 to fifteen years on an exhaust system, not for those of us in salt water. The salt eats the iron in a very short time.
We recommend that one or both exhaust riser be pulled at EVERY annual service and be inspected for rust and corrosion.We also recommend that the entire exhaust system be replaced every 5 to 6 years. That sounds a bit excessive and it can get a bit expensive, however, it is far less than a new engine, way less.
Imagine tossing a cast iron fry pan into the bay and leaving it in the salt water for 5 years..... that is what your cast iron exhaust system looks like. Flushing with fresh water and products like Saltaway help a great deal but it will not stop the corrosion.
These are exhaust risers from a Mercrusier 5.0 2006 model (6years old) The customer assured me the motor was flushed evertime they used it. In this pic they are laying upside down, the flat square flat surface is where it bolts to the manifold. The square hole in the center is exhaust and the round holes fore and aft are cooling water. Note the rust and corrosion in the exhaust chamber. The round pipes so badly rusted is where the water and exhaust join and dump over board. The slots for the water are completely rusted over. No water could get out causing the mortor to over heat.
This is looking down onto the exhaust manifold where the riser bolts to it. The cooling water flows thru the two holes on either side. Again note the rust and corrosion inside the exhaust chamber.
Somewhere inside the manifold or the riser the wall between the exhaust chamber and the cooling water has cracked. When the motor was running the exhaust pushed the water out like it is intended. When the motor was shut off all the water in the manifold and riser ran trough the crack into the exhaust down into the engine cylinders and sat on top of the pistons. Look in the exhaust ports in these photos and you will have an idea of what the inside of the motor looks like.When they tried to restart several weeks later.... the motor is frozen. A complete new motor and exhaust system was needed to get them going again.
The exhaust parts are like a tube inside of a tube. the inner tube carrying hoot exhaust gas and the outer carries the cooling water, they join at the riser and dump out. Because they are cast insand molds and not precise the thickness of the walls can vary a great deal, thin then thick. There is no way to inspect them. I can not say exactly when it will fail but it WILL fail.
Flush it often, use Salt busting rinses and consider adding a flush valve to make things eaiser. Have them inspected and replace them before you need a new motor.