Discoloration in an SS pipe

[lganga]

We have a problem of internal dicoloration in a duplex SS pipe. That appears after welding the external reinforcement or saddle of the pipe support.
Can it be avoided with an argon internal atmospher or Nitrogen atmospher in the internal side while welding the saddle?.
Our client has complaint about it.
Regards
Luis

 

[MikeHalloran]

Yes, you can purge the inside of the pipe.

In fact, you must either purge or pickle or mechanically remove the stain.

The brown discoloration associated with welding heat in an air atmosphere compromises the corrosion resistance. I'm sure Special Metals has some papers about it. Search on "heat tint".





Mike Halloran
Pembroke Pines, FL, USA

 

[bimr]

The Nickel Institute has fabrication guidelines that will prevent the occurence as well as tell you how to repair the discoloration:

Practical Guidelines for the Fabrication of Duplex Stainless Steels, 2nd Edition (16000)

http://www.nickelinstitute.org/en/TechnicalLiterature/Other Series/DuplexGuide_16000_.aspx

 

[lganga]

You gave me a very usefull answers.
About cleaning internally(because the dicoloration is only internal) there will be places not able to be reached,
Can be trust in a chemical cleaning with a liquid solution and pasivation?.
A mechanical cleaning and passsivation is standard in a buttweld. But for this case not all weld affected sectors can be cleaned in a mechanical way
Regards

 

[MikeHalloran]

You're generally better off with a purge if you can't easily clean and inspect the inside.



Mike Halloran
Pembroke Pines, FL, USA

 

[stanier]

You need to pickle and passivate using a mixture of nitric and hydrofluoric acids. Cleaning is carried out by circulating the fluid until the chemical reaction changes. Bohler and Sandvik publish welding guidelines on their websities that address the cleaning of weldments. You also need the following standards:-

ASTM A380
ASTM A370
ASTM A923
ASTM E92
ASTM E562
ASTM G48

this practice is common place in desaliantion plants where duplex and super duplex stainless steels are common place.

If sea water is used for hydrotesting and remains stagnant in the piping then corrosion will occur.

Refer tot he gold Coast Desalination Project debacle where many millions of dollars were spent replacing the stainless steel because of these issues.

http://www.couriermail.com.au/news/...gain-for-repairs/story-e6freoof-1225881524588

http://www.couriermail.com.au/news/features/corrosion-hits-desal-plant/story-e6freoyo-1225700238785

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