Post Weld Treatment of Stainless Steel 4

[gibsoni]

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.

 

[SJones]

What is the aim of the proposed surface treatment? If it is only to remove weld discoloration, and the subsequent surface appearance requirements can tolerate some later rust spotting, then simply sand or invest in a stainless wire brush. If you must have optimum performance and appearance, you will only need to pickle, otherwise known as acid descaling. Unfortunately, this could involve some potent chemicals. The pickling may need to be followed by a cleaning to remove smut, again with some potent chemicals, to get a 'white pickle'.

http://www.outokumpu.com/applications/upload/pubs_113142858.pdf

Steve Jones
Materials & Corrosion Engineer

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

 

[gibsoni]

Hi Steve ad others,

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.

 

[stanweld]

Review ASTM A-380.

 

[Guest102023]

You have access to the chemicals, but do you also have access to the safety equipment, facility, and means of disposal?

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.

 

[EdStainless]

Look into switching to a lean duplex instead of 316. Same cost, better welds (corrosion resistance is nearer the base metal), higher strength, better overall corrosion resistance.

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

 

[gibsoni]

Hi All.

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.

 

[SJones]

The wire brushing is capable of removing mild heat tint colours but may be not the heavy scale that will occur with SMAW. Also, bear in mind that the stainless steel is not immune to soil corrosion. So, you will go to great lengths to clean it up, only to get it trashed when it is buried!

Steve Jones
Materials & Corrosion Engineer

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

 

[unclesyd]

on a lot of our SS fabrications we will bead blast for a uniform surface.
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/

 

[EdStainless]

I believe that the Outokumpu paste is called Red One. Great pickling paste.

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.

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Plymouth Tube

 

[gibsoni]

Thanks everyone.

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.

 

[qcrobert]

If your welds are very clean as you say with no wire brushing and you only need to remove the heat burnish then just use Prep&Etch solution (from Home Depot) and wash thoroughly with water.

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

 

[21121956]

Hello everybody:

SJones, good information. Star for you.

 

[JamesCKelly]

For best corrosion resistance you do need to remove the heat colors, also any embedded iron. The iron comes from contact with tools, clamps, steel or cast iron tables, shear blades, et cetera.

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.

 

[vanci]

Use the lowest heat input possible [(VxAx60)/Travel in inches per minute]. Allow interpass to cool to amient temps before depositing additonal beasds on multi pass welds.

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!

 

[Guest102023]

Heat tint is an oxide layer that has depleted the underlying metal surface of important alloys like chromium. It must be mechanically removed before chemically treating (pickling).

 

[unclesyd]

I have to take exception to to the above post concerning pickling not removing heat tint, heat tint can be removed by pickling and not by passivating.
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.

 

[berkshire]

One of the quickest ways to remove heat tint is to use reverse electrolisis, A copper pad wrapped with fiberglass cloth dipped in phosphoric acid,is attached to the negative lead of a battery charger.
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