Pickling to remove oxides before welding is clear. You don't eant to contaminate the molten metal.  What you're trying to do after welding is first remove oxides caused by welding. This can be done mechanically or by acid. If you mechanically remove the oxide, then you also need to remove the de-chromized areas which result from chromium leaving the matrix to form sulfides and oxides. This, too, requires acid, but since it isn't removing oxide, it's not called pickling. It's called passivation.
 If you remove the weld oxide by pickling it does the passivation also.
  Some specifications may predate the understanding of the phenomena and not help you understand why these things should be done.

Thanks for the response.

I have more questions, hope you all can help me understand the process a little bit more.

1. Pickling is for cleaning the stainless steel parts (removes oxides on surfaces). Passivation is for enchancing the corrosion resistance of stainless steel(forms chromium based passive layer). Am I right?

2. Is it true that I must clean the stainless steel parts before doing passivation, by either mechanically cleaning (grinding, sanding) OR chemically cleaning (pickling)? In another word, passivation does NOT clean the parts.

>If you remove the weld oxide by pickling it does the >passivation also.

3. What do you mean? If pickling will also passivate the stainless steel at the same time, why sometimes people will do both (pickling and then passivation)? to further improve the corrosion resistance property?


4. Is it common to just "locally" pickle the welds or passivate the welds? How pickling and passivation are done? by spraying, or by submersing the whole part in a tank full of chemicals? The stainless steel frame of the equipment I am working on is as huge as a cnc machine.

Thanks a lot!

Lee

Don't pickle the weld. Clean it with a wire brush. You might happen to passivate with an acid solution, but don't take it to the point of a full pickle. Pickling is a crude way to remove scale from large areas (e. g., extruded pipe ID/OD). It leaves hydrogen in the grain boundaries.

njlee,
In answer to your last post:
1. Yes.
2. Yes.
3. Pickling is sometimes done with reducing acids such as hydrochloric or sulfuric.*  
An oxidizing mixture of nitric + hydrofluoric (at least 5:1) can also be used.
Passivation is normally done in a strongly oxidizing nitric acid solution, which is considered to leave a more protective passive layer, and is required by many specifications.  Warm citric acid solutions can also be used for passivation.
*An inhibitor should be used with these to prevent hydrogen pickup.
4. Both processes are done by immersion in tanks of solution when the parts are small enough.  Local pickling can be done with acidic pastes.  Local passivation can be done with gels.

If the scale is light, or if it is largely removed by wire brushing (SS wire is best) or bead blasting, you can electropolish to get 100% scale removal and passivation.  Again, this is done in tanks for small parts, or by ‘brush electropolishing’ for weld seams in large parts.  Electropolishing (after mechanical polishing) is a common requirement for pharmaceutical tanks and piping and for medical parts.  See Thread338-60778 and Thread330-50008.

kenvlach has given a complete and correct response. But, I have to disagree with something posted by JimMetalsceramics, who has been making excellent contributions lately.
  By only mechanically removing scale from stainless steel welds, one will end up with significantly poorer than expected resistance to pitting corrosion. This is because the formation of the oxide "pulls" chromium from the underlying metal leaving a layer less than a micron thick which is depleted by a few percentage points in chromium. Acid removes this layer. Secondly, welds of stainless steel usually contain about 0.010% sulfur. This precipitates as CrS aroound inclusions and depletes the matrix there of chromium. This is why pits form at inclusions and why welds have poorer pitting resistance than the base metal. Again, removing the chromium-depleted regions with acid takes away the nucleation sites for pitting.
 Welds have critical pitting temperatures about 5 to 10C lower than the base metal, so they need all the help they can get.

Sanding or grinding the weld may be an option. I don't like using acids to etch away surfaces, especially with heat affected zones (large grains).

You mention medical pharmaceutical requirements.  In most situations when my company supplies material into pharmaceutical applications the surface must be electropolished.  Such a smooth finish is required to enhance the cleaning process and prevent materials from clinging to the vessel or piping walls.  Is this required?  

Hi All,

Thanks for the responses.

#4 polished surface finish is one of the requirements. After reading all the good comments from you, I realize that I have to do passivation to promote the corrosion resistance. However, I think what I am trying to find out is can mechanically cleaning processes (such as sanding, grinding, polishing) replaces pickling? Or, maybe I should ask is it common to do both before passivation?

If pickling is so aggressive/crude, will it degrade the #4 polished surface finish?

Thanks again.

Lee

No, the metal removal from pickling is too little to degrade the appearance. Don't try to avoid the pickling. Welding and sanding seriously degrade corrosion resistance. I just saw an experiment in which 316L tubing was welded and abraded and then tested for critcal pitting temperature. The temperature dropped from over 10C to 2C. This was the equivalent of taking the moly out of the 316, or reducing the chromium by 5%. It reduces 316 to 304 corrosion resistance levels. Treat that weld well.

I'd like to know the surface finish of the weld after sanding. The pitting resstant may have been impaired solely from a rough sanding job. My welding professor felt that acid was used far too much without understanding the drawbacks and that people don't polish because they don't want to do a painstaking job.

Surface finish was Ra 15 micro-inches. To avoid damage from surface grinding you must go to a finer finish by far, like from 1200 grit. Grinding leaves surface crevives on a micro scale and very high residual stresses which by themselves lower pitting potential by 150 millivolts.
Electro-polishing will work as well as acid but your professor is not correct if he's teaching the avoidance of acid is harmless.

The idea is that if you polish fine enough, you avoid hydrogen entrapment and grain boundary etching from the acid. Most people don't want to take the trouble. To them acid is a quick fix.

Be careful not to spread myths.
 
1. Austenitic stainless steel are not embrittled by hydrogen picked up by pickling. That is what happens with martensitic steels. It is not a danger in this austenitic alloy system.
2. Grain boundaries, or any other metallurgical feature, which is deficient in corrosion resistance because of its local composition, will be attacked by acid pickl;ing preferentially. You do it initially, under control, by pickling so it won't happen out of control later in service.
3. Abrasion makes a pretty surface and it's harmless for carbon and alloy steel. Don't think that means it's ok for stainless. All that "brushed" stainless you see on appliances has badly compromised corrosion resistance. That is why smart materials specifiers have changed over to rolled-on finishes which look brushed but are really bright annealed.

njlee,

Depending upon the size of your part, you can have stainless cheaply polished to submicron finish after removing the outer skin. Do you have a drawing?