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Super Duplex SS Time required for natural passivation
3

Super Duplex SS Time required for natural passivation

Super Duplex SS Time required for natural passivation

(OP)
Dear all,

Does anybody know or has any experience with Super Duplex SS?
Assume that I scrape it or machine it. How long should I wait for the chrome layer to buildup? Is it hours , days or months?

I don't want to put the pipe in service and see any rust!
And passivation is not an option.

Thanks,

Herc

RE: Super Duplex SS Time required for natural passivation

seconds.
The real problem is that if you scrape it with steel then you will embed metal into the surface.  The rust is that embedded material corroding not the DSS.
Avoid metal tools and you will be fine.......

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Plymouth Tube

RE: Super Duplex SS Time required for natural passivation

(OP)
Well  I think that I am stuck with steel forming tools.
.Any coating to suggest?
Do you know of an article somewhere that explains why it takes seconds?

Some people are pretty adamant saying it takes 2 weeks to repair.

Thanks

RE: Super Duplex SS Time required for natural passivation

I have done corrosion testing where we were in aqueous solutions, reached in and struck the surface and then watched the conditions return to full passivisation.  It starts right away and is done while you stand and watch.
Look at HDPE coatings.  Heck, pick up truck spray bed liner would work to protect the surfaces.

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Plymouth Tube

RE: Super Duplex SS Time required for natural passivation

(OP)
Ed, you say to use HDPE for the forming dies?? I don't think that would hold much. I have heard something about nanocrystalline powders that they use on forming corrugated SS.

RE: Super Duplex SS Time required for natural passivation

We use thin plastic coating on some forming tooling.
As long as you don't slide against it it will take a lot of load.

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Plymouth Tube

RE: Super Duplex SS Time required for natural passivation

(OP)
Ed,

I think that it shall see some sliding...

Will keep that in mind though! thanks

RE: Super Duplex SS Time required for natural passivation

why is passivation not an option?  pickle/passivation only takes 2 days for vendors in my area.

 

RE: Super Duplex SS Time required for natural passivation

Passivation, the natural formation of a hydrated iron-chromium oxide layer, is instantaneous. What some people erroneously call passivation is actually the process of iron removal through an acid wash. It's a good thing but is only beneficial if the surface is already contaminated with iron, which can prevent natural passivation from occurring.  

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Super Duplex SS Time required for natural passivation

The chemical passivation process is not forming a surface oxide film but rather a cleaning operation.
SS will naturally form a passive chrome oxide layer when exposed to air.

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Plymouth Tube

RE: Super Duplex SS Time required for natural passivation

(OP)
So you guys are concuring to the instantaneity of the formation of the passive layer. I would like to see some scientific explanation too. Meaning .. Some equations or experiments that show the rate of formation.

Regarding pickling and passivation...
I thought that pickling was removing the free iron and a small layer of metal from the surface. Agreed? And that happens with some acid + nitric or citric acid.

If what you are saying is true than why are we passivating in just nitric or citric afterwards?
 

RE: Super Duplex SS Time required for natural passivation

Pickling is done to remove the oxide and low chromium layer which is caused by oxide formation. That gives the surface the same chromium level as the bulk material. You should note from this that grinding or blasting away the oxide does not necessarily remove this chromium depleted layer, which results from chromium diffusing to the surface to join with oxygen during high temperature exposure. Pickling dissolves and removes the lower chromium content surface layer  rather than just removing the oxide.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Super Duplex SS Time required for natural passivation

(OP)
Mcquire
I understand and agree. So the question remains. Why do we passivate? Is it to enhance the speed or something else of the chrome layer formation? And if yes, that brings me back to my original question. How fast would it selfpassivate.

RE: Super Duplex SS Time required for natural passivation

We passivate to remove foreign impurities from the surface which might hinder normal passivation. In the absence of impurities natural passivation is virtually instantaneous. I'm sorry not to be able to say what fraction of a second it takes for full surface coverage, but you get the idea.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Super Duplex SS Time required for natural passivation

(OP)
Wouldn't pickling remove all surface impurities?

RE: Super Duplex SS Time required for natural passivation

If you pickle in a nitric HF solution, neutralize and rinse there is no reason to follow with a passivation, if it was going straight into service.
However in most manufacturing operations there are cutting, machining, forming, handling steps that may introduce surface issues.  Following these with a passivation will give you an optimal surface for corrosion resistance.

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Plymouth Tube

RE: Super Duplex SS Time required for natural passivation

I agree with Ed on all points. It is hard to completely rule out that contact with steel during processing hasn't left iron traces on the surface.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Super Duplex SS Time required for natural passivation

A lot of shops forming SS ,use " Die Rubber " which is actually a poly urethane material. Stretched across the dies to prevent steel contamination of the material being formed. This maybe what Ed is referring to.
 A related question: What is the difference between  Passivation ,and electro polishing?
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

RE: Super Duplex SS Time required for natural passivation

Electro-polishing will elegantly accomplish everything passivation will insofar as removing iron contamination and it also guarantees than any de-chromized layer on the surface is removed. By smoothing the surface and removing asperities it also maximizes corrosion resistance.
And, it looks pretty.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Super Duplex SS Time required for natural passivation

You can rub the surface with citric acid which will lift the iron from the surface and remove the source of of the rust

RE: Super Duplex SS Time required for natural passivation

I would check first that you actually do have any iron contamination. A ferroxyl test (ASTM A380) on some finished parts would show whether you have a problem.

http://uk.linkedin.com/pub/terry-roberts/28/a9a/a10

RE: Super Duplex SS Time required for natural passivation

Please,   

I need some help to eliminate porosity in valves  of duplex steel (cd3mn).

What's the better deoxidation for this kind of DSS.

A lot of thanks.

RE: Super Duplex SS Time required for natural passivation

Attention, H-28 McG, and others.
Time to passivation.
After extended discussions and a couple of well researched papers this year at NACE I have changed my mind on the issue of time to passivation.
When corrosion testing samples of SS there is a detectable change improvement in corrosion resistance over 72 hours after sample prep.  There is no discernible difference between 72hrs and 240hrs, but there is a measurable difference between 0, 24, and 48 hours.
This effect held regardless of the sample prep methods, grinding, blasting, pickling, passivation, and various combinations all showed the effect.
In the real world this not much of a deal since it is rare for parts to go into service in less than 48 hours from finishing.
However in the lab it is a concern.  Either test everything ASAP and get more conservative results, or let all samples sit 72hr min and get slightly better results.
I know, this should prob go into another thread.  When I get a copy of the paper to read again I will start a thread on this subject.
Ed

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Plymouth Tube

RE: Super Duplex SS Time required for natural passivation

(OP)
Ed, valuable info. do we have this paper from NACE?

I did some research myself in scientific research and I only found instantaneous formation of the layer in a fraction of a second.  

RE: Super Duplex SS Time required for natural passivation

People do realise that those passive films vary in thickness and nature depending on the solution that the metal is in however a thicker film in one solution in no benefit in another solution. It won't be stable.    

RE: Super Duplex SS Time required for natural passivation

(OP)
Cloa you are right about your comment. Thicker film doesn't always mean better film. I found a scientific paper from 1985 I think from Cambridge that was debating on whether the Cr is building an oxide or the Fe is getting dissolve in the acid. Very interesting.

Ed,

I went through 3 years of NACE publications but I didn't find anything specific on the subject of this thread. Do you have any leads ? I got like 20 articles to skim though!

RE: Super Duplex SS Time required for natural passivation

The paper is C2012-0001644 by T. Ladwein and M. Sorg
It is from this year.
Ed

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Plymouth Tube

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