Pickling of stainless steel - to polish welds or not

[CheemaJ]

Hi,

The plant I am working in has experienced some very rapid and localised corrosion on 316 stainless steel process pipework. It is almost entirely around the heat affected zone. See attached document showing some photographs of this. The duty is a clean in place return line. To mitigate the issue going forward we are going to replace the pipework with a new welding standard (to reduce oxidation) and also pickle and passivate before putting into service. On welds that our fabricators can access (i.e. union joints) they tend to mechanically polish the welds. I was wondering if there is any advantages or disadvantages to doing this given that all the welds will be pickled anyway.

Thanks in advance.

 

[EdStainless]

Since the shielding looks pretty good (minor heat tint) I don't think that mechanical prep is needed.
Mech prep can help, but it can also just embed debris into the surface if done wrong.
You are better off enforcing ID shielding (limit allowed heat tint) and then pickling to remove what is there.
You can go to AWS for such guidelines or the ASME BPE standards.
I presume that you are looking at using pickling paste for this?
We used to wash (med pressure) with a mild alkaline solution (baking soda in water) after this, then rinse with VERY clean (DI) water.
Then you will passivate the entire system?

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P.E. Metallurgy, consulting work welcomed

 

[CheemaJ]

@EdStainless: there are several welds which cannot be accessed to use pickling paste. There is a supplier nearby which specialises in immersion pickling. Rather than do some by hand and immerse others it is cost effective to send all pipework to their works for immersion pickling. I am not 100% sure what their process is to remove the acid afterwards, am due to go there on Tuesday to go over this. We were not planning on passivating the entire system afterwards - as I was under the impression the newly installed pipework would self passivate. Unless I am missing something? Regarding the polishing I will instruct them to leave welds as laid.

 

[EdStainless]

If the piping will be in sections you could tank them to pickle.
This should be a warm Nitric/HF solution. There are usually two or three steps of rinsing.
This will change the overall appearance slightly. If the material has been made correctly then it may not even be noticeable, but if you have some under-annealed welded tube the welds will become rough and very visible.
The word passivate has two meanings.
The material will naturally re-passivate and have the correct corrosion resistance.
The passivation treatment is a cleaning operation, principally to remove surface iron contamination. Many people do not trust that the system can be assembled without picking up further dirt.

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P.E. Metallurgy, consulting work welcomed

 

[Dhurjati Sen]

It seems you have not addressed the corrosion issue properly.

Have you done PMI of the welds?

Have you checked that somebody had checked the electrodes during fabrication?

Were the welders qualified?

Have you thought of material upgradation, say 321 or 317 ?

May be you have done all of the above.

DHURJATI SEN
Kolkata, India

 

[EdStainless]

DS, I don't think that you meant to say 321. That sure isn't an upgrade, it is a downgrade.
If these are begin made with filler (this picture looks like it but, but is it needed?) then going to 317L could help. There are about a dozen various weld filler grades, just make sure that it is low C. And take one with N and 4.5% min Mo if possible (317LMN).
The next step from there is either a 6%Mo superaustenitic (254SMo, AL-6XN, 25-6Mo) or 904L. But these are usually not needed.

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P.E. Metallurgy, consulting work welcomed

 

[Dhurjati Sen]

Thanks for the correction Ed Sir.

I had meant to say material substitution / upgradation, say 321 or 317?

It is very heartening that my posts are read thoroughly by the experts like yourself, Latexman, Ironic and many others.

However, the OPs rarely answer my posts, not to talk about reading those. Getting consultancy through my posts is like a century plant, that blooms after 100 years.

DHURJATI SEN
Kolkata, India

 

[EdStainless]

321 and 347 are stablilized versions of 304, neither has much corrosion resistance. The are throwbacks from the days when low C was difficult and expensive as a way to fight sensitization in welds. Today you buy 321 (Ti) or 347 (Nb) and you get both the additive and ultra low C, so why bother. The low C (<0.02%) is effective at eliminated the Cr carbides in the HAZ.
For corrosion resistance (pitting and crevice) you need more Cr, and/or Mo, and/or N. Everything else is just for phase control. The best examples that I know are alloys like 317LMN and 33 (33%Cr, 2%Mo, 0.5%N).

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[MJCronin]

EdStailess....

Idle question ... How does the current price of C276 compare to the materials you suggested as a solution ?

I was told that over the past 5 years, since the Chinese have begun supplying this, C276 has dropped in price considerably.....

MJCronin
Sr. Process Engineer

 

[EdStainless]

MJC
C276 is priced based on the Ni and Mo prices. The variations in price are largely driven by product forms (plate, bar, tube). It costs about the same as 625, and is twice the price of AL-6XN.
The price landscape is odd today. 316 is about 25% more than 304. 2205 is a little more than 316, but if your correct for the strength then the end costs are often equal.
317 and its derivatives have all become custom order only because the higher Ni and Mo drive higher prices and they have no better corrosion resistance than 2205.
If you can tolerate C276 that may have some issues then use the Chinese material. But if you are doing a lot of welding or heaven forbid electropolish then don't risk it.

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