Metallurgy continued - AL294C Stainless Steel

What's the benefit?

The question in all these kinds of debates is probably one of cost versus benefit. There is no doubt that Al29-4C is tasty and performs admirably under a variety of conditions - but is it needed when combusting the usual variety of gas-based fuels?



Based on my very limited research, it seems like Al29-4C is of particular use when dealing with very low pH flue gases such as condensing oil (especially when the source fuel is up at or beyond the allowable 1700PPM sulfur limit - may result in a pH at 2 or lower). Under these conditions, crevice corrosion, etc. is a really nasty problem. But in a condensing gas boiler, the pH doesn't drop below 4 (IIRC) and hence the issue is not as pressing.



Most corrosive environments are at their worst when the corrosive medium is allowed to condense/dry on the spot instead of being washed away. In a condensing boiler, it's fairly likely that the condensate will clean the surface semi-continuously, especially in top-down designs like the Prestige and the IBC. In the flue, it's more likely for stuff to dry on the spot after the the boiler shuts down. See how the requirement for Al29-4C vs. 316Ti or 316L varies by fuel and chimney liner manufacturer.



My guess is that going slightly thicker on the HX matieral and/or upgrading to 316Ti is likely cheaper in the context of increasing a boiler HX longevity than switching to a very expensive metallurgy like Al29-4C. Plus, I'd wager that the majority of HX failures out there have something to do with Balance of Plant (BOP) failures, such as clogged sumps, fan disk failures, etc. Thus, it's the BOP where I'd pay the attention, with a special eye on detecting failure modes before they can ruin the HX.



Now, if I was running my boiler on a sour-gas well in my back yard...or a local landfill...  oh, I'm not, so I guess I won't worry about that. But if I were to run those kinds of fuel a super-resistant HX might become a requirement.