Stainless Steel - Pickling & Passivation - Pipe weld inside 1

[thandion]

Dear experts,

My question is, after welding our clint wants us to do pickling and passivation on the welded A312 TP316L stainless steel. But they want to do pickling on the inside of pipe as well (weld area). The thing is we are purging weld from inside until weld is finished and cooled down, so there wouldnt be any heat changes at the inside. And natural chrom-oxide layer could happen easily.

Is this really mandatory? Your urgent response is appreciated.

PS. weld is only from outside, oneside bevel.

Thanks a lot.

 

[EdStainless]

Do you have standards for the resulting heat tint?
Will you be visually inspecting every weld?
If there is no tint at all then passivation only should be good.

There are standards that cover this sort of thing, try the ASME BPE code.

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Plymouth Tube

 

[thandion]

There is no specific extra standard other than internation ones.
Yes we will visually inspect all welds.
For inside of pipe, isnt natural oxidation is enough, do we need passivation treatment.

Could you give me the section from ASME BPE, couldnt find specific detail for that.

Thanks,

 

[gr2vessels]

It can be done, but it depends on the method of application and the pipe service requirements. It's not a simple flushing of the pipe with pickling solution.
It could also be the Client doesn't trust you welders and needs to be sure you are not ruining the piping. It also depends on the size of the pipe and what type of pickling you might use. Use the search button on top of the page, this topic has been discussed a few times earlier.
Cheers,
gr2vessels

 

[thandion]

Pipes will be according to ASME B31.3.

Diameter is varies between 1/2" to 20". Could find specificly for inside weld, thats why I opened a new topic.

Thanks,

 

[SJones]

Is this really mandatory?

If it was in the technical specification or purchase order, yes. Even with good purging practice there could be detrimental effects on corrosion performance and pickling usually creates the optimum surface conditions. On the other hand, if carried out incorrectly, it can do more harm than good.

http://www.stainlesspickling.co.uk/docs/cleaninghandbook.pdf

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

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

 

[EdStainless]

https://www.asme.org/products/codes-standards/bpe-mp-2012-discoloration-acceptance-criteria

If you have a standard color to work to and you will inspect and document each weld then passivation alone should be good enough.
Yes, you need to passivate, not to form a passive layer but to clean the surface so that it can naturally form a defect free passive film.

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Plymouth Tube

 

[moltenmetal]

Unless this is bent pipe without elbows, this is not a small thing to ask...

Short spools can be pickled by immersion. Otherwise you're talking about a circulating solution process as the only practical option, and that's not easy to accomplish both safely and effectively.

 

[thandion]

As stated in the ASTM A380; stainless steel has a very thin and stable oxide film rich in chrome. This film reforms rapidly by reaction with the atmosphere if damaged. If stainless steel is not adequately protected from the atmosphere during welding or is subject to very heavy grinding operations, a very thick oxide layer will form. This thick oxide layer, distinguished by its blue tint, will have a chrome depleted layer under it, which will impair corrosion resistance. Both the oxide film and depleted layer must be removed, either mechanically, or chemically (acid pickle with a mixture of nitric and hydrofluoric acid).

Therefore if there is no oxygen left inside of pipe (due to purging with argon), there is no need to remove thick oxide layer which is the purpose of this practice. Thats why I asked this, because it doesnt make any sense. Is there any other reason that I missed?

 

[EdStainless]

The problem is that even a tint that is a dark gold color will ahve significant Cr depletion under it.
The second problem is that the natural passive film will only form, and function correctly on a clean surface. Even small embedded particles of SS or Fe will lead to local corrosion initiation and pitting.
Passivation is a cleaning operation.
Even if you welds are very bright you should do a full system passivation of the finished system to assure optimal corrosion resistance.

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Plymouth Tube

 

[moltenmetal]

Austenitic stainless steel pipe is frequently used in the as-welded condition, with removal of exterior heat tint only. If weld roots are deposited by GTAW with an internal inert purge, that pipe is frequently put into service without further pickling or passivation of the weld roots. EdStainless is of course correct that passivation is required to develop optimal corrosion resistance by removing potential future pit initiation locations etc., that does not imply that it is absolutely required for all services.

 

[EdStainless]

Sorry, yes I was speaking of high purity or very aggressive systems.

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Plymouth Tube

 

[Ripz]

Using backing gas removes the possibility of internal oxidation i.e Heat Tint. So no pickling is required internally. SS self passivates on contact with air, so no chemical induced passivation is needed.

You client sounds either inexperienced, has extremely aggressive service, or overly cautious to the point of madness.

 

[EdStainless]

You missed on option Ripz, that is that they are handling a high purity fluid.
Anything darker than a light gold tint on welds is unacceptable in UHP water service.
To do this with backing gas requires a level of care that most people outside of pharmaceutical and semiconductor fabrication have no concept of.

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