When Should one specify Pickling and passivation? 2

[bmoorthy]

Dear All

I have many times(Almost always) seen the specification that specify "Pickling and passivation" as a requirement where ever and when ever Austenitic Stainless steel is used.

I wonder whether it is objective to ask for Pickling and passivation so very mechanically in all situations.

Surly there has to be some guideline and some international specification/Standard that give the rules (Or rule of thumb) as to when an item is required to be Pickled and passivated.

Can some give the basis on which one can specify Pickling and passivation.


Regards

 

[israelkk]

For military and aerospace the requirement is to use passivation per QQ-P-35

 

[unclesyd]

P&P is a way to insure the cleanest and most corrosion resistant surface available with a particular SS. If an item needs to be clean and pretty then you need to P&P.

I've never seen a compilation of the rules or guidelines in requiring pickling and passivation. A lot, probably most, of the callouts are based on experience.

Here is a short list of generalities.

The pharmaceutical industry.
High purity chemicals and solvents.
Architectural applications.
Purely esthetic reasons, like appliances.
It is often used as a cleaning procedure.
Cleaning after hot forming, removing mill scale.

Some places where it is called for it really has no effect on the say the corrosion rates. If a process will corrode SS there is no need for P&P except for insuring a high integrity on the initial welds.

There is quite a leeway on ways and means to P&P a SS and again it depends on the results required for a particular set of environmental condition the metal will be exposed to.

I will not get into specifics of determining when and where P&P is needed nor the particular processes but if you are confronting a particular situation that you would like more information about please post it.

 

[bmoorthy]

Yes indeed. We have Inconel 625 overlayed (ENi Cr Mo-3) Strip cladded Slag Catcher and some Scrubber and Seperators with the same combinations.

There are some SS Vessels in Chemical injection (MEG) packages and some SS in Water service.

The client spec does not specify. Now as a contractor where all should i specify P & P ?

 

[Capntom]

I agree with most of what has been said. Passivation removes the free-iron from surface of SS. The surface looks a little cleaner but surely not "pretty". To get the Shiny polished look the item should go through the next step of "electropolishing". The electropolishing step reduces the peaks and smoothes out the surface revealing a shiny look. The smoother surface also increases its corrosion resistance. Visually you can detect if a part has been electropolished, you can not detect if a part has been passivated

 

[PAN]

Some stainless steel fabricators in my country use sand blasting for surface cleaning instead of acid pickling.

I'm not sure about the reference standard. However, it looks clear surface during visual inspection.

 

[EdStainless]

But after blasting you should always passivate to remove impurities from the surface.
Blasting, grinding, polishing, cutting, buffing, all should be followed with passivation.

You cannot detect if a surface has been passivated, pickled or electropolioshed with out destructive tests. How a surface looks is no criteria. A surface that has been 'flash EPed' or buffed will look bright, but have very poor corrosion resistance.

ASTM A380 is a good ref. It give a list of possible solutions to use and some basic guides. The only tests mentioned are strictly for free iron on the surface, not stability of passivation.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[skylerf]

Some interesting replies. Let me throw my two cents into the ring.

I don't understand why a print would state pickle and passivation in the same note. Normally pickling is used to remove heat treat scale by solublizing the scale/metal in a strong acid bath. Since metal is soluble in the acid bath, it will provide a passive or iron free surface. So, to follow this process with a passivation step does not make sense.

I agree with the reply concerning electropolish. Electropolish will also provide a passive or iron free surface. So, to follow this process with a passivation step would also be redundant.

 

[EdStainless]

It is possible to pickle and leave an active surface. Passivation requires the formation of a Cr oxide layer, if the pickle bath is strongly chemicaly reducing then there may not be a pssive film after pickle.
We passivate after EP in order to remove any residuals from the EP process. Any other cleaning may damage the passive surface, at least using Nitric acid will assure the maintainance of passivation.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[skylerf]

I agree that Nitric Acid is a strong oxidizer, which does produce the Cr oxide layer in cycle, but Cr will naturally oxidize with oxygen from the atmosphere. So, is it necessarly a requirement that a passivation process produce a Cr oxide layer for the surface of the material to be passive? Or is the requirement for a passivation process to produce a surface that is absent of free iron? There are many ways to achieve a surface absent of free iron beyond the typical Nitric Acid passivation step.

Passivating after the EP process is interesting. I believe the residuals that you are refering to is residual acid and not residual iron. Is that correct?

 

[bmoorthy]

This is some thing new

So Do we need to Passivate a SS Pressure vessel after Pickling or not?

I was under the impression that Pickling and Passivation are twins (when ever Pickling is done Passivation is done, such was my assumption).

Is it a correct assumption?

 

[EdStainless]

The Cr oxide layer that forms on exposure to air is not very thick or stable. There is a lot of Fe in it.

You really want a layer that is not too thin, not too thick, very rich in Cr vs Fe, and free from organics (C).

Often pickling leaves a poorly developed passive layer as well, since that is not the goal of the pickling operation.

After EP there are the possiblities of sulfur and phos compounds on the surfaces. These compounds have limited solubilities and don't help the surface chemistry.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[IFRs]

Passivating after welding is almost always needed to prevent surface rust or corrosion.