manifold lifespan

[cdavisdb]

I just changed the manifolds and risers on my 5.7 V8. Total time on risers was 4 seasons, almost 5 years total. Manifolds are original, I think, but looked near perfect in 2009. 2014 should be way over time for the manifolds and a little over time for the risers, but look at the pics below. I've done this job on other boats at 3 and 4 years and the risers normally look pretty awful, major rust pile. The manifolds seem to last a little longer, but by 4 years the water passages are rusted near shut in some areas, overall look pretty nasty.

Look at these. Way less rust than I'm used to seeing. They don't look to me like either manifolds or risers are anywhere close to needing to be changed. These are Volvo parts

I wash mine more thoroughly than most folks. After every salt water use. Usually wash twice, a week apart, always do this if the trip is more than a day. I discovered that letting the fresh water sit in the engine for a week picked up a lot of salt, the discharge is noticeably salty when I start flushing the second time. Also, the boat gets in the water far less trips than most people.

Questions:

Am I imagining things and this looks normal for the amount of time? Or does this look exceptionally clean to you guys?

Is the metal in volvo manifolds and risers different, more corrosion resistant?

Would my type of use and/ or washing schedule result in less corrosion?

Even if they look pretty good, will cast iron get pitting that will go through the walls anyway, such that how they look doesn't matter?.