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What is solution annealing, pickling, and passivation?

What is solution annealing, pickling, and passivation?

Please explain to me the solution(s) used in annealing, pickling, and passivating SS304, 304L, 316 materials.  What is the purpose of this treatment?  Is there literature out there that I can download which will go into the details of why this process is used?

After the process is there any detrimental effect to mechanically polishing the surface.

Thanks for your time

RE: What is solution annealing, pickling, and passivation?

solution annealing of SS dissolves any precipitated carbide phase at high temperature, then rapidly cools so that carbides will not be present to lessen corrosion resistance.

Pickling removes existing surface oxides (as from heat treating or welding), surface contamination such as iron from steel tooling, and removes the near-surface layer of metal which is often depleted in chromium due to prior heating.  Procedures given in ASTM A380.

Passivation is usually accomplished in nitric acid solutions (or increasingly with heated citric acid solutions).  It selectively dissolves Fe & Ni atoms from the surface layers, and oxidizes the remaining Cr atoms, forming a corrosion-resistant surface oxide high in Cr2O3. See 'HOW TO PASSIVATE STAINLESS STEEL PARTS'

The highest degree of passivation is obtained by electropolishing prior to passivation.  See 'Comparing the characteristics of surface-passivated and electropolished 316L stainless steel"

RE: What is solution annealing, pickling, and passivation?

Re mechanically polishing the surface.  Must be prior to passivation.

Processing sequence may be:
1) Clean SS of any lubricant which may contaminate surface when heated.
2) Solution anneal.
3) Mechanical descaling (wire brushing, temper rolling of sheet) to fracture & partially remove heat treat scale.
4) Pickle (acidic solution).
5) Mechanically polish.
6) Electropolish (optional).
7) Passivation.

RE: What is solution annealing, pickling, and passivation?

Ken has done a very good job.  The other thing that you will hear if you listen enough is that people use these various operations for other reasons as well.  When you have specific questions, aks.  We will help you sort them out.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

RE: What is solution annealing, pickling, and passivation?

Lotsa good info out there:
Check out ASTM A-967 “Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts”

Tuthill & Avery, “Specifying Stainless Steel Surface Treatments,” Advanced Materials Processes, Vol. 142 (6), 1992

RE: What is solution annealing, pickling, and passivation?

Thanks to everyone for your help!
Can anyone go through the details of the solution annealing (what solution is used, temperature, how long?) and then the acids used for the pickling and passivation. Are the pickling and passivation processes carried out at one time or must they be seperate?  

The product we produce is a stainless steel well screen used in the water well industry.  After dipping the 6 meter screen in a Nitric and Hydroflouric mixture for about 30 minutes, we then wash the screens using soap.  Finally the screens are mechanically pollished with brushes.  Feel free to critique this method.

Now you probably can understand my choice of "call names"  

RE: What is solution annealing, pickling, and passivation?

Hi nottoobright,
Please search for & read some info on the heat treatment of stainless, solution annealing stainless, sensitization...

'Solution' means refers to the microstructure of the stainless steel.  The SS is heated hot enough (usually in air, but a protective atmosphere or vacuum can be used if you are willing to pay more) to dissolve carbides such as Cr23C6 into the (solid) austenitic (fancy way of saying that the metal atoms have a face-centered cubic microstructure) alloy, and then cooled fast enough that the carbon does not have time to precipitate and grow into carbide phase.  Thus, the carbon is said to remain in solution (within the solid alloy).
A solution annealing procedure is given in this thread:
Need for solution annealing

Re your procedure:
"The product we produce is a stainless steel well screen used in the water well industry.  After dipping the 6 meter screen in a Nitric and Hydroflouric mixture for about 30 minutes, we then wash the screens using soap.  Finally the screens are mechanically pollished with brushes."

1) It is necessary to know the fabrication details -- are you welding, bending, cutting, grinding...?
2) It is pointless to use soap after pickling in nitric + HF (pickling solution).  If oil or dirt is presnt, clean prior to pickling.
3) 304, 304L, 316 & 316L can be passivated in 25-45 vol.% nitric acid for 20-30 minutes, after any brushing. Is this brushing for appearance or for removing weld scale? Weld scale removal should be done prior to pickling; it may reduce the necessary pickling time.

Passivation (+ rinsing & drying) is normally a final process.  


RE: What is solution annealing, pickling, and passivation?

Hi Kenvlach
In answer to your question, we start with a raw material, for example, of a 4mm diameter SS304 wire.  It comes in 100 kg coils.  Some is straightened and cut to 6 meter lengths to be used as rods (ribcage) and some is run through a series of rollers to change it to a triangular shape ("v-shape")The dimensions change to 3 x width 2.5mm.
The triangular wire is then continuously wrapped around the ribcage and electonically welded at each junction.  A slot is left between the wraps to allow for water in, sand and particles filtered out. (too much info?)

Normally not necessary to anneal the wrap, never had a problem before.

Rust protection is something we've never had a problem with before either, however, as we move into the "big leagues" we're running into people who know a lot more than we do and sometimes, to be quite honest, like to baffle the customers with bull. (can i say that on this website?)  I'm trying to figure out what's true and what's not and at the same time improve  my own product.

From what I'm reading out of the replies, my pickling and passivation may be a little lacking.  My mechanical cleaning (purely for cosmetic reasons - a nice shiny stainless steel screen) may be degrading the level of protection.

Anything you can tell me is much appreciated

RE: What is solution annealing, pickling, and passivation?

I would suggest
1) Clean prior to welding, to remove any wire drawing or forming lubricant that may contaminate the welds,
2) Pickle, brush and passivate.  This sequence gives a more uniform appearance (than brushing first).  Use stainless steel brushes.

I presume that you are doing resistance welding and that the screens are used in drinking water wells.  If there are more severe service conditions, e.g., chloride in the water, a limited stress relief heat treatment at 290 - 425oC (550 - 800oF), should be carried out after welding. The stress relief reduces the occurence of stress corrosion cracking (SCC), and the relatively low stress relief temperature minimizes 'sensitization.'

Use annealed wire rather than cold-drawn, if not already doing so.  A high temperature solution anneal would improve corrosion resistance, but does not seem necessary.
It would be less expensive to switch to the low carbon grade 304L, which would reduce the need for annealing (solutionizing of carbide).

RE: What is solution annealing, pickling, and passivation?

Hydrofluoric is kinda nasty stuff - the citric acid processes are safer & more eco-friendly

RE: What is solution annealing, pickling, and passivation?

Are you using "L" grade wire?  Is it actually wire or is it slit strip? (most used i nthis application is strip slit from sheet, it costs less than square wire) This might determine what benifit you would see from annealing.
The wire that you get is annealed, right?  Then you don't re-anneal after forming?  This would be the typical process.  You want the wire work hardened some in order to raise strength and erosion resistance.
If you haven't seen issues with weld pitting corrosion or stresss cracking then I wouldn't worry about that part of the process.

I am with Ken, I think that you would be getter off washing, and maybe even pickling prior to welding.  The cleanliness of the welds should be your first concern.
Are the welds discolored?  You need only to pickle enough to clean residual oxides from the welds.  You really do need to use nitric/HF for this in order to disolve the oxides.  We run agressive pickle, 25%nitric, 5%HF at 130-140F.
Just water rinse after.  Pickling is better at removing organics than any wash job.
Polish, and follow the polish with a passivation.  Look in ASTM A967 for options.  Straight nitric or citric are the most common.  We use warm (120F) 25% nitiric for passivation.

By putting the passivation last you will prevent the rust spots or blushing that come from Fe and polish residue surface contamination.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

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