316 SS is generally not used for very high temperatures because volatile moly oxide can accelerate oxidation in confined spaces. So , of the two alloys , 904 is preferred.

904L has even higher Mo than 316, so not a good choice for very hot sections.
Higher temps are often handled either with a 439 ferritic or 309/310 austenitics.
If you are talking about cooler sections where deposits and condensation are likely then 904L has somewhat higher corrosion resistance.
Often you see lower alloys used in the hottest sections (304L or 409 or even just steel) and higher alloys used in cooler sections (316L or others).
In places where condensation is very likely and there is real risk of serious corrosion (near the coast) you will even see alloys like 29-4C (superferritic) used.

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You don't like aluminium, do you?

* Finding a solution is great * Knowing how to implement it is fantastic * Believing it is the only one and best is naive ?

they don't make aluminium chimney systems.....

I know nothing about 904L. I'm building stacks for some of my boats out of 316L. I'm going to send them out for electro-polishing but that maxes out the budget. We see surface temps exceed 500°F (gold color) which sensitizes them so they require a lot of maintenance to keep shiny. I wonder how 904L would compare. How does cost compare?

500F won't sensitize them, but it will start to form surface oxidation which in high Cl environments will lower the corrosion resistance.
TBE, do any locations ever reach 600F? If not then you could go to 2205 as an option.
A lot more corrosion resistant and not much higher price.

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P.E. Metallurgy, consulting work welcomed

Tug Boat eng - 904L is around 2-3 times the price but its like hens teeth at the moment......

Given the Ni and Mo content 904L should be expensive.
There are better alloys for hot corrosion resistance.
More Cr helps but higher Mo doesn't really.
I might look at 309/310 before 904L for this type of application.
If the hot end never gets over 600F I would even use 2205.
In exhaust the hot end is easier, because it is dry. As you get to the cold end where you likely have condensation you have real problems.
In auto exhaust they often use plain steel or Aluminized steel at the hot end, 409 in the middle, and 439 in the colder end.
In marine applications I would consider alloys like AL29-4C for the cold end.
Using multiple alloys is very common in exhaust ducting.

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We once had a HCl gas leak in our lab, and all the nice shiny 316 surfaces got clobbered.

TTFN (ta ta for now)
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