Connor,
There are some high-nickle versions of cast iron that Volvo might be using which are probably more corrosion resistant, depending on what other elements are in the alloy. I know Chrysler Corp. used it in a lot of their V-8 engine blocks, although I think that was mainly because of it's higher strength, so you'd also like to see a lot of chromium in the alloy for good corrosion resistance. Only way to know for sure would be to put one of those manifolds in a SEM (Scanning Electron Microscope), which analyzes the wavelengths of the electron emissions and shows what elements are present in the alloy.
I'm sure your 2-step flushing procedure helps if it gets more of the salt out, but cast iron still rusts pretty quick, so I think going beyond 4 years would still be pretty risky without at least taking a look at them. It only takes 1 thin spot to kill an engine!
If those expensive Volvo parts really are that much better, maybe it's worth investing in a borescope from Harbor Freight
http://www.harborfreight.com/catalog...lt?q=borescope, especially if you could also figure out a way to justify also using it in your business like Strick does with his vet business! Don't know what the resolution is on the HF scopes, but the 9mm Olympus and Cole-Morgan aerospace scopes I've used give you 5X magnification that make a 10 mil crack in a turbine blade look like the Grand Canyon! They're very handy tools that we used for some amazing inspections with that saved a lot of engine teardowns. By just pulling the risers, I think you could get a better look inside both the hot and cold side of the manifolds with a borescope than you could by actually pulling the manifolds for a 1X eyeball inspection! That would allow you to switch from "scheduled" to "on condition" maintenance, which is how the military and airlines maintain the hot sections of aircraft engines! Denny