316L patches on 304L ductwork subject to diluted H2SO4

[AN1970]

I'm trying to figure if 316 patches welded on a badly corroded 304 body of a temporary quencher would accelerate corrosion. The duct, acting as a quencher takes 1000F sour gas (exhaust of a thermal oxidizer processing ~50,000 ppm H2S and ~40,000 ppm methane landfill gas) and cools it down to 170F using water quench (diluted H2SO4 and likely some H2SO3 present in the condensate - pH 1.5; at least 10% oxygen from air co-quench also present int he gas phase and likely in the condensate). The 304 body exhibits signs of severe pitting and impingement with multiple patches welded to cover rusted-through holes (it's a "make it last until a Hastalloy replacement arrives" job...). Repositioning the primary condensation area down from the original sweep section helped for now but we are trying to slow the corrosion rate on the previously less corroded section that is under impact now.
Any advise will be appreciated!

 

[gr2vessels]

Ha, why bother with this 'issue' of accelerated corrosion. The 316 won't accelerate any existing corrosion rate, if anything will last a bit of extra time compared with 304. I assume you use dual certified 316/316 L, check your certificates. Keep the patches handy and the welder close. Anything else is waste of time and effort/money.
Slowing down the corrosion rate has nothing to do with the use of 304 or 316, they corrode about the same in your environment. Changes in your process could make difference. Fluid velocity has a lot to do with the corrosion rate, the quench water impingement, etc. You don't disclose to much of your process and equipment design, hence only general (and trivial) answers.
Cheers,
gr2vessels

 

[EdStainless]

Don't waste you money, just use 304L for patches.
There is no advantage to using 316L in this application.
You ought to check some condensate and see what the Cl and Fl content is. I am putting my money on at least a few thousand ppm.
I hope that you went with C22 and not C276, if you have Cl or Fl present you will need all of the help that you can get.

Remember that the acid will condense first at near 100% and then gradually dilute as the water condenses, and that in the range of 40-60% concentration there are no metals that are fully resistant.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[racookpe1978]

What if they spray "additional" water in earlier (further upstream) to dilute the acid down more, cool the whole mix down earlier in the pipeline, reduce fluid speed by increasing mix rate (might be self-defeating by adding the extra water), and to dissolve the oxygen into the water?

 

[AN1970]

Thank you all for your advice!
To EdStainless: we are developing a source gas testing spec, will incorporate testing for all halogens in addition to sulfur species, N2, CO2 and methane.
To racookpe1978: there is no way to introduce water earlier due to high temperatures (that would simply mean relocating the condensation point). We are using partial air quench to control water usage and initial contact temperatures.

A follow-up question of sorts: in your experience, have you come across the need to install a conductivity break between a Hastelloy ductwork and a vessel made of 304L? We will be connecting the new Hastelloy quencher to the scrubber tower made out of 304L.

Thank you!

 

[EdStainless]

If you make the alloy transition far enough down the stream then you will not need isolation.
You may want to use another alloy for the intermediate step.
For example C-22/Alloy 20/304L, this might let you use less Ni alloy and still get great corrosion resistance.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[AN1970]

Thank you Ed!

 

[Guest102023]

Is the question really about possible galvanic corrosion in the 304/316 couple? If so, I wouldn't worry, the effect is minor.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[AN1970]

Attached are photos of the inside of the old 304 duct (we finally replaced the quencher duct). Have anyone seen this type of corrosion pattern? The loss of metal thickness is highly localized and very severe.