exhaust manifold failure
 

Last Saturday I had a problem. At he end of the day when I throttled up
a bit to slide the boat up the trailer there was a big cloud of white
smoke/steam from the exhaust.

Pulling the port riser I found a bit of coolant puddled in one exhaust
port of the manifold. Removing the manifold showed about a tablespoon
of coolant sitting on the same exhaust valve while the others were dry.
Only that plug looked a bit washed/wet.

I'd just replaced batteries due to hesitant starting after heat soaking
and now feared head gasket failure and partial hydro-locking as a
possible root cause of all this. At least the oil was still nice and clean.

But good news... pressure testing the manifold revealed a leak in the
suspect exhaust runner near the exhaust port so the heads can stay on
and I might be back on the water next weekend.

The engine is a small block 5.7L with closed-cooled aluminum manifolds
and ~800 hours on it. It's only spent about 3-4 weeks in salt water
over a 12 year life and while I understand that's a very long life for
raw water cooled iron manifolds I'm kind of surprised at this failure in
FWC aluminum. What's a typical lifespan under these conditions?

I located the area of failure by stethoscope and rather than a distinct
crack it appears more like a rough porous region. Anyone seen this
before? Do exhaust gasses eat aluminum?