Will Heat Treating 304 SS help mitigate corrosion forming in the weld/HAZ in piping in SO3 Service?

[jeevesme]

Hey guys, I have a question that I cannot find a direct answer to and thought you guys may be able to help me.

We have some 16" 304ss piping that we have replaced 3 years ago because the welds/HAZ were corroding and causing leaks. We are now in the process of replacing the piping yet again because of this same issue. Our sister plants simply have a 1 year PM to replace it. My question is, if we heat treat the welds, will we gain corrosion resistance (assuming the welds were properly welded)?

We typically use 308 to weld 304 to 304. Would we gain anything by using 316 or an incoloy?

Thanks in advance!

 

[EdStainless]

What is the C content of your material?
If this is 304L, then I would look into welding with 304L (or straight grade)filler and then annealing the pipes.
You may find that it isn't worth the trouble, but if the only corrosion that you have is weld/HAZ then this should address it.
Though welding 304L pipe with 304L filler should be enough to prevent sensitization.
You may want to have some testing done to verify what the root cause is.

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P.E. Metallurgy, Plymouth Tube

 

[jeevesme]

I believe it is dual stamp 304/304L which we usually weld with 308L. As for the C content, I am not sure because we did not keep the MTRs.

 

[metengr]

308L is the correct filler metal.

 

[jeevesme]

Edstainless, What type of testing would you recommend?

Would using an Inco filler yield more corrosion resistance?

 

[Dhurjati Sen]

Dear jeevesme,

What is the service & what are the parameters?

Regards.

DHURJATI SEN

 

[blacksmith37]

If you mean polythionic stress corrosion cracking ? ; a stabilize at about 1600 F for over an hour will reduce the risk. This treatment is impractical for many structures. Using 321 and matching filler ( I don't remember the fillers) would be an alternative.

 

[metengr]

You don't need an overmatching filler metal, like Inco Weld A for joining 304L. Your weak link regarding corrosion resistance is the heat affected zone in the 304L.

 

[EdStainless]

Welds and HAZ in 304L should be able to pass A262 pract E without problems.
If the HAZ passes but you have attack in the welds then looking at 321 for filler might be an option.
You could also do some pitting tests in 1000ppm NaCl at pH=5 to see where the weak links are.

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P.E. Metallurgy, Plymouth Tube

 

[weldstan]

You may wish to purchase extra low carbon ELC) 304L. Solution treating pipe spools with flange construction might be considered. Grade 321 should also be considered as replacement pipe.

 

[jeevesme]

@weldstan, it looks like we can get 321 for about 10% above what 304L would cost which is definitely doable. At 40% increase in cost, would 316L be better?

 

[EdStainless]

Without knowing the specific service we cannot say. It depends on the service.
Where are you in the world? Can you get 316Ti? (Europe)

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P.E. Metallurgy, Plymouth Tube

 

[weldstan]

If your sister plants are replacing 304/304L every year, chances are highly likely that your alloy selection is wrong. A superior, more expensive alloy that gives 10 to 20 years service would seem more cost effective.

 

[Chumpes]

hello
upgrade depends upon the corrosion mechanism that was identified... what is it ?
what are operating conditions ?
possibility of free water ?
intergranular attack ?
wrong heat treatment during fabrication causing sensitization ? high carbon grade ?
any chance to have the ASTM IGC test performed in welding procedures ?

304/304L is one of the weakest stainless steel, better use SS 316/316L or 316Ti in water and SS 321/347 at higher operating temperature.
rem : filler metal matching with SS 321 is SS 347

 

[EdStainless]

There are many environments where the lower Cr in 316L is a negative for performance.
Make sure that you know some details about the environment and failure mode.


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P.E. Metallurgy, Plymouth Tube

 

[blacksmith37]

If it is polythionic cracking , 316 will not help, L grades are little help. Polythionic occurs in as little as a few hours during shutdown if any moisture is present. Our technician made a movie of the cracking , took about an hour with time lapse (done 50 years ago). Old guy story : Exxon licensed sulfur plant technology from Amoco ( long ago). Exxon corrosion guys put in 304 or 304 L . Pieces of pipe fell out during shut down (two trains). Exxon came back to Amoco and were shown where it said use 321 or 347 in the license info.

 

[jeevesme]

Thank for all the help so far guys. It is much appreciated! Here is some more information that I have:

Service: SO2/SO3
Temperature:120-125F
There is a possibility that water could get in the system.
Unsure whether the corrosion is in the weld or haz or both.

EDIT: We are located in the US. We have not had the ASTM IGC test done. I will recommend it to them when we pull the piping this year (I was not part of this group the last time). Unfortunately I do not have any specifics on what the corrosion type is(cracking,pitting etc).

 

With due respect to the solutions offered above, I suggest you take a different, more holistic approach.
Instead of selecting an alloy from first principles, determine (i) what degradation mechanisms can potentially affect your equipment (including shutdown and layup conditions)?; and (ii) what are current industry standard materials of construction and fabrication best practices? Root cause failure analysis by a subject matter expert should be your first action. (Hint: failure analysis cannot be performed well by crowd sourcing.)

API RP 571 Damage mechanisms affecting fixed equipment in the refining industry is a good starting point (but it does not make one an equipment designer)
In your situation I strongly recommend expert 3rd party consultation.


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[pradipgoswami]

Quote:
Service: SO2/SO3
Temperature:120-125F
There is a possibility that water could get in the system.
Unsure whether the corrosion is in the weld or haz or both.
EDIT: We are located in the US. We have not had the ASTM IGC test done. I will recommend it to them when we pull the piping this year (I was not part of this group the last time). Unfortunately I do not have any specifics on what the corrosion type is(cracking,pitting etc).

Answer:
In presence of water both SO2/SO3 has the potential to turn to sulfurous and sulfuric acid. Both of these acids are highly corrosive. Search in the web for "isocorrosion curves" on both of these acids various temperatures and concentrations. and the best material for the service.Both of the stated factors along with other process parameters contribute heavily on repetitive corrosion failures.

As the failures are repetitive, some more insights would be advisable before choosing the replacement.

If polythionic acid is suspecetd, NACE RP-170 could be a good resource, in addition to API-RP 571 already suggested.
Thanks.



Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
ca.linkedin.com/pub/pradip-goswami/5/985/299

 

[jeevesme]

Thank you for all of the replies. We do have a materials engineer but since this is such a large company, but getting an answer in a timely manner however, is something to be desired. Things tend to move much quicker here than they should.

In any case, we are going with sch40 321 using 347 filler. We will be cutting and sending out a sample for testing and everything will be documented this time.