Coatings for wet and hot corrosion?

[YungPlantEng]

We have a spray nozzle that quenches a hot sulfur dioxide gas stream from 2450 to 180 F. There is significant corrosion at the nozzle tip and fusion zones where the tip is welded to the pipe assembly.

Part of this corrosion is related to molten salt accumulation (na2so4, nacl, etc.) where chlorides and sulfides collect and flux out alloying elements. There is also the (hypothesized) concern of sulfuric dew point corrosion related to startup/shutdown, and thermal cycling.

The existing alloy is c276. We previously tried a thermal spray coating (catastrophic corrosion at the very tip but was protected above 4” from the tip) and an aluminide diffusion coating (reasonable improvement except to tip section where white salt deposits and significant corrosion was apparent).

External refractory is silicon nitride (cracks and replaced yearly) and while we have investigated a ceramic nozzle there are concerns about cracking that don’t present with the original specified c276. Hence why we’re still evaluating coating types.

Looking at trying an AlCr diffusion coating next but just not sure of its capability to resist acid corrosion. Let me know if you guys have any ideas.

Also included a photo showing how the deposits present pre cleaning.

 

[SJones]

Any investigation of zirconium alloys?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[YungPlantEng]

The thermal spray coating that corroded catastrophically was a zirconia-yytria coating. It is believed that a combination of ferric/sodium chloride and sulfide build-up on the nozzle tip would cause a similar issue with zirconia.

 

[YungPlantEng]

Here are some images of uncleaned and cleaned lances showing how the corrosion products present. Usually colors are black, green, blue. Nozzle always show similar wear/corrosion patterns at the tip where weak sulfurous water (2% H2SO4, high chlorides/sulfides, 130 F). The spray cone itself doesn't get attacked but there is a possibility of the welds being related more to crevice corrosion than chromium depletion of the weld area.

 

[TugboatEng]

What is the spray cone made from? Perhaps the entire nozzle should be made from that material?

Have you considered making an all ceramic nozzle? Silicon carbide for example? Provided there are not rapid temperature changes this may prove to be a durable solution.

 

[YungPlantEng]

We could likely coat the rest of the nozzle with that material but the spray tip is the predominant failure mode. We believe the thermal shock from expansion would cause cracking with SiC / SiN

 

[EdStainless]

Have you looked at Ultimet?
Full SiC would require a redesign of course to minimize tensile loads, but this would likely be the most effective answer.

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P.E. Metallurgy, consulting work welcomed

 

[YungPlantEng]

Haha I did. They don’t have experience bonding to C276 so there would likely be some experimentation and there’s no nice way to reduce tensile stresses on a vertically overhung nozzle. Or maybe there are but I’m not a structural/mechE by trade and it’s difficult to convince my company to put any r&d into this (even though they’re fine with me live trialling coatings that end up costing multitudes more).

 

[EdStainless]

Make it from solid Ultimet, not coating.

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P.E. Metallurgy, consulting work welcomed

 

[YungPlantEng]

My apologies I thought you were talking about Ultramet. Could we potentially laser clad ultimet to the C276 to avoid a potential catastrophic failure if there were some unforeseen mechanism? Here's another picture of the pretty scaling on the nozzle for your troubles - keep in mind this was about 2 months' corrosion on the C276.

 

[EdStainless]

Get a bar of Utlimet (the Co based alloy) and make a nozzle from it.

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P.E. Metallurgy, consulting work welcomed