Internal Pickling and Passivation of Super duplex stainless steel SA 790 UNS 32750 Pipes 2

[Salmet]

Dear Members,

In one of our desalination plant projects we are being advised to perform Internal Pickling and Passivation of Super duplex stainless steel SA 790 UNS 32750 pipe welding joints, for untreated sea water application. Is it really required internally? I will be thankful for your kind input on it.

 

[EdStainless]

If internal is where the Seawater will be then that is where you should focus.
Is there any heat tint on any of the welds?
Have you visually looked at the ID of all of the welds?
Pickling of the ID of welds is usually done with a pickling paste (RedOne) as they go.
That way they only need to be able to reach back one joint length.
Any oxide on the welds will appreciably lower the corrosion resistance.

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P.E. Metallurgy, consulting work welcomed

 

[SJones]

Pickling should buy an increase in the pitting temperature. Particularly useful if the seawater is at some tropical temperature. If the piping is small diameter with little access to weld roots, use a spooling design such that spools can be pickled in a bath. Site joints can then be flanged.

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.

 

[Salmet]

Thank you Edstainless and Sjones for your kind responses. Earlier we did a lot of super duplex piping without internal pickling and passivation for untreated sea water application @ 65 bars operating pressure at maximum 55 oC operating temperature. Since almost 3 years we did not had any complain of leakage etc. In this case we did external pickling and passivation by a patent chemical system.

After the completion of joints the root pass had uniform bright color with a little tint. Can we assume that if welding of super duplex piping is done with proper heat input control, then we don't need pickling and passivation from inside? Most of piping job is not accessible from inside.

Regarding the reduction in corrosion resistance by oxide layer, isnt't it the special characteristic of super duplex stainless steel that it provides special resistance to pitting corrosion under extreme corrosion environment.

I will be thankful to you for your point of view.

 

[EdStainless]

The surface oxide layer is formed by taking Cr out of the underlying metal.
The result is 2-5% less Cr in layer right under the oxide layer.
Did the WPS for this project require A923 as part of the qualification?
Detrimental intermetallic phases also greatly reduce corrosion resistance.
This is not a G48 test, it requires different sample preparation and acceptance criteria.

Have you done a cleaning and inside inspection on the 3yr old pipe?
If not, then I suggest that you would be very surprised by the extent of ID weld pitting.
Fortunately, with duplex SS the corrosion will progress as far the next change in structure and then it must start again.
The unfortunate side is that pits are autocatalytic and once they form, they will always continue to grow.

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P.E. Metallurgy, consulting work welcomed

 

[Salmet]

Thank you EdStainless,

For the project I mentioned above, A923 was not part of qualification. Process team haven't yet done the inside inspection or cleaning.

So, in a situation where most of the welds are inaccessible from inside, would proper pickling/passivation by conventional HF+HNO3 mixture will do the job?

Even if done properly and neutralized fully, can it assure that working at high pressure (65 Bars or more @ 55 oC operating temperature )will prevent pitting at untreated sea water service?

 

[EdStainless]

Without an A923 and a G48 as part of the qualification you cannot be assured of anything.
You could try pumping Nitric+HF, it is usually trapped between pigs to minimize the amount of acid needed.
And you need to pump slowly.
It will be an uneven pickle because of temperature variations.
And you might need to repeat it a few times to get full coverage and enough residence time for effective cleaning.



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P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

Jesus Elvis has left the building. The passivation and or pickling should have being done at the detail level first. Or am I missing something.

 

[EdStainless]

1. The welds should have been shielded well enough that no heat tint was visible.
2. Weld and welder qualifications should have been done on as welded coupons for pitting corrosion.
3. If welds needed to be pickled it should have been done with paste as each weld was finished.
The horses are long gone on this one.

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P.E. Metallurgy, consulting work welcomed