Super Duplex SS Time required for natural passivation 3

[Hercules28]

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

 

[EdStainless]

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

 

[Hercules28]

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

 

[EdStainless]

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

 

[Hercules28]

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.

 

[EdStainless]

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

 

[Hercules28]

Ed,

I think that it shall see some sliding...

Will keep that in mind though! thanks

 

[eyec]

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

 

[mcguire]

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/

 

[EdStainless]

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

 

[Hercules28]

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?

 

[mcguire]

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/

 

[Hercules28]

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.

 

[mcguire]

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/

 

[Hercules28]

Wouldn't pickling remove all surface impurities?

 

[EdStainless]

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

 

[mcguire]

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/

 

[berkshire]

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

 

[mcguire]

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/

 

[edstimator]

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