Post Weld Treatment of Stainless Steel
Post Weld Treatment of Stainless Steel
(OP)
Hi All,
I am after some specific advice on post weld treatment of stainless steel.
I am stick welding 304 S.S. plate (3mm) with 316 electrodes.
I want to pickle and passivate these parts.
I have access chemicals to make up my own solutions etc and would like to do so.
I think I can passivate finally in 10% by weight citric acid solution.
Immediately post weld what do I do? I presume it would be foolish to clean up with a wire brush (non stainless). So do I pickle once cool? What solution can I make to pickle in?
I am assuming:
Weld
Cool
Pickle (no idea of solutions to use)
Rinse
Passivate
Rinse.
It seems (and I could be wrong) that some people are pickling and passivating with 1 solution?
I would like to avoid Nitirc Acid if possible.
Thanks for being such a helpful community!
Regards, Iain.
I am after some specific advice on post weld treatment of stainless steel.
I am stick welding 304 S.S. plate (3mm) with 316 electrodes.
I want to pickle and passivate these parts.
I have access chemicals to make up my own solutions etc and would like to do so.
I think I can passivate finally in 10% by weight citric acid solution.
Immediately post weld what do I do? I presume it would be foolish to clean up with a wire brush (non stainless). So do I pickle once cool? What solution can I make to pickle in?
I am assuming:
Weld
Cool
Pickle (no idea of solutions to use)
Rinse
Passivate
Rinse.
It seems (and I could be wrong) that some people are pickling and passivating with 1 solution?
I would like to avoid Nitirc Acid if possible.
Thanks for being such a helpful community!
Regards, Iain.





RE: Post Weld Treatment of Stainless Steel
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Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
RE: Post Weld Treatment of Stainless Steel
Steve, thanks for the reply.
The stainless steel needs to appear reasonably good but most importantly, it needs to be corrosion resistant. I am thinking of changing to 316 base material as well. So I need to ensure the welds have the same resistance to corrosion as the base material which I believe will require passivating to restore chromium oxide surface.
This object is buried in moist soil and so corrosion resistance is the key aim, although consumer satisfaction with appearance is initally a little important.
Regards, Iain.
RE: Post Weld Treatment of Stainless Steel
RE: Post Weld Treatment of Stainless Steel
I would talk to a first-line SS suplier about this. Not assuming you don't know what you are doing, but I have seen too much pickling of pressure vessels that was not much more than glorified car washing.
RE: Post Weld Treatment of Stainless Steel
I presume that your gas shielding is good enough to prevent heat tint on the welds and base metal.
I have a paper that addresses the effectiveness of various surface cleaning methods, I'll hunt for a copy for you.
My first thought is use a clean SS wire brush or flap wheel, spot pickle with paste if there are 'problem areas', and then warm citric passivate the whole thing.
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Plymouth Tube
RE: Post Weld Treatment of Stainless Steel
Thanks for your comments.
Edstainless.
You are suggesting that I wire brush the welds and then citric acid dip the whole object?
Is there a process I can add before to remove the 'colors' from the welding process?
Regards, Iain.
RE: Post Weld Treatment of Stainless Steel
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
RE: Post Weld Treatment of Stainless Steel
You might want to try Blue Away polish for the light heat tint, It works well on heat tint left by TIG welds,
http://www.competitionchemicals.com/blueaway/
RE: Post Weld Treatment of Stainless Steel
Sorry, I missed the welding method. Wire brush to remove heavy debris, use picking paste, then passivate to clean after all forming is done.
You need to think about a better welding method, MIG or TIG with wire feed. With good enough gas shielding to give you clean bright welds.
= = = = = = = = = = = = = = = = = = = =
Plymouth Tube
RE: Post Weld Treatment of Stainless Steel
My stick welding is very clean. There is no wire brushing necessary at all. The only reason I use to wire brush is to burnish the heat treatment colors. My welds are clean and bright, there is however a heat treatment band along side the welds.
I am considering a Sulfuric acid pickling and a citric acid passivation.
Regards, Iain.
RE: Post Weld Treatment of Stainless Steel
Like SJones stated the stainless steel is not immune to soil corrosion. I would wrap the plate with appropriate tape to insulate from soild and moisture.
If your plate is 304 you should be using 308 filler metal instead of 316 filler metal. If you are thinking about switching take Ed Stainless's advice and use lean duplex instead of 316 for better corrosion resistance.
Good luck to ya,
Robert
RE: Post Weld Treatment of Stainless Steel
SJones, good information. Star for you.
RE: Post Weld Treatment of Stainless Steel
Stainless steel passivates - forms that thin chromium oxide film - just by exposure to air. All you need do is remove iron contamination and heat tint (the blue color each side of the weld).
There is no technical reason for a separate passivating step.
One might passivate for Sales Reasons, if one must please someone who does not know that stainless passivates in air.
In the northern parts of the USA we use great quantities of chloride salts on our roads in the winter. Buried stainless near a road may corrode from these salts seeping through the soil. Under these condtions 316 stainless may not be adequate.
RE: Post Weld Treatment of Stainless Steel
Filler metal composition will also play a large role in as-welded corrosion reisitance.
If you MUST perform PWHT can you afford a bright anneal and fore-go the pickle/ passivate?
I appologize for adding more questions than answers!
RE: Post Weld Treatment of Stainless Steel
RE: Post Weld Treatment of Stainless Steel
Normally it takes only a very short dip into HNO3/HF pickling solution to remove heat tint. A problem sometimes arises when the scale redeposits so the best approach in this case is to use a pressure rinse or better yet a steam lance.
If the subject Stainless Steel is in a media where there is a measurable corrosion rate there is no need to remove the heat tint and passivate. Sometimes the heat tint/oxide is beneficial as in our case where we force an heat tint/oxide on Stainless Steel used in one part of our process.
RE: Post Weld Treatment of Stainless Steel
Attach the positive end to the part, and gently wipe the soaked pad over the discolored area. Be careful not to rub too hard or you will wear through the fiberglass and short the unit out. This will instantly remove any heat tint on the part, even dark blue.
B.E.
The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor