Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number) 10

[Jason8zhu]

Dear All,

PREN is a vital number to roughly evaluate the
corrosion resistance of CRA(Corrosion Resistance Alloy),
in which Chrome(Cr) and Nickle(Ni) are importance element.

In calculating PREN,percentages of Cr/Mo/Nitrogen and even tungsten(W) are included.
Would anybody share with me, why Nickle(Ni) is excluded ? Thanks.

 

[EdStainless]

The only things that have an impact on the resistance to chloride pitting (the only thing that correlates to PREN) are elements that are either passive film (oxide) formers or ones that promote the formation/reformation of those films.
While Ni (and Mn) are vital to the phase formation and stability of the alloy they play no roll in the surface passivation and Cl pitting resistance.
In fact if you look at a phase stability diagram (Schaeffler) you will see that most of the elements that factor into pitting resistance are Cr-like ferrite promoters. There are exceptions as N helps pitting resistance and is an austenite stabilizer.

In any environment other than one where Cl pitting is the concern the PREN is not a relative measure of corrosion resistance at all. So if you are in high pH, or clean acid, or strongly reducing, or any corrodant other than Cl you need some other test to guide alloy selection.

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P.E. Metallurgy, Plymouth Tube

 

[Jason8zhu]

thanks very much. Your guidance help understand the real meaning of PREN and now i got it.

 

[EdStainless]

My only other guidance is that PREN is a estimation.
The real issues is usually how the material has been processed.
The PREN is based on mill annealed, blasted, and pickled samples being tested in the lab.
It is very common to find alloys with 3-4 point difference performing exactly the same in the field, or for the lower one to work better.
Forming, heat treating, welding, and cleaning are all vital to optimizing corrosion resistance.

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P.E. Metallurgy, Plymouth Tube

 

[mcguire]

Just to add to what Ed explains, is that the surface condition is really important because the crucial composition right at the surface is changed by pickling, annealing atmosphere, etc.

The passive film on stainless is only about 2 nm thick, but it is a semiconductor, just like silicon, TiO2, Ge and others. There are normally n-type defects in the film. Increasing the density of such defects make it is more conductive, and less protective against corrosion. This is characteristic of an air formed passive film or one on a lower Cr alloy. Additions of Mo and N to the alloy created P-type defects to the semi-conductive film, canceling the n-type defects and giving it much lower conductivity of the corroding species to the metal surface. Chlorides are bad because the create n-type defects in the film, enhancing electrical conductivity and thereby permitting corrosion to occur by mass transport through the film.

If this is news to you, don't feel lonely. It is also new to all the textbooks,mine included, which have a lot of puzzling hand waving about the causes of corrosion resistance. Only recently have researchers looked at the problem from a solid state physics instead of an electrochemical viewpoint, and the new approach ties it all together neatly.

Michael McGuire

 

[CoryPad]

That is new mcguire, thanks!

 

[mcguire]

And nickel has no effect on the defect structure of the semi-conductive film, because it is not a net positive valence contributor like molybdenum so it doesn't help stabilize the passive film's insulation properties, to answer the original question. It only helps after corrosion has reached the base metal at which point it lowers the corrosion current.

And another implication of this is that both hydrogen and carbon should mimic the effect of nitrogen as they are cationic interstitials. Studies of collossal supersaturation of stainless by carbon have in fact shown this to be the case, although historically carbon has been viewed unfavorably because it generally acts as a chromium depleter. Hydrogen is lost too quickly to be much help, so we alloy with nitrogen.

Michael McGuire

 

[Jason8zhu]

Thank Mcguire and all friends on this thread. All your guidance not only enlighten my puzzlement but also embark me to learn more on alloy corrosion in seawater corrosion.

 

[ispengg]

McGuire,

Thanks for the very informative post. Its new!

Though late for the post, I want to get clarified on few things on role of Nickel in Chloride stress corrosion cracking. I have read in some of the books on corrosion of stainless steel that, boiling MgCl2 testing has shown that minimum 42% Nickel is required to avoid Chloride Stress corrosion cracking in Austenitic Stainless steel.Hence UNS N06625 or N08825 is selected for chloride containing process environment.I believe for stress corrosion cracking the breaking passive of layer is initiation point,what is the role of Nickel in reducing SCC?

Can you please refer literature or books which you are referring in the post regarding new developments in the research of stainless steel.

 

[mcguire]

In my opinion, stress corrosion cracking initiates at a corrosion pit in stainless steels. (There are some other forms of failure which are confused with SCC, such as grain boundary corrosion accompanied by stress, which I don't consider SCC.) I believe, but cannot prove, that the pit acts as a point of accelerated corrosion and as a mechanical notch, permitting crack initiation and growth.

The rate of the corrosion IS influenced by nickel level. Nickel reduces the corrosion current, retarding the cracking process. This is where nickel is beneficial: in the crack growth phase where corrosion fuels the process, not in the initiation phase where pit formation rules. So, I don't doubt that a threshold level for nickel above which there is no SCC may exist.

I have no doubt, but again cannot prove, that SCC is a variation on hydrogen embrittlement. I can prove it in martensitic alloys, but I no longer have the ability to do the crucial tests in austenitic stainless. There are too many well-established charlatans in the field to bother trying to prove it.

Michael McGuire

 

[ispengg]

McGuire,

Thank you...

I want to read further on the solid states physics theory of the passive films which you explained earlier. Can you please refer the literature or book which I can buy.

Thank you

 

[mcguire]

If you Google "stainless steel passive film semiconductor", you find a number of worthwhile papers. I don't think you'll find anything in books on the topic. There is similar work for titanium, whose TiO2 is an N type semiconductor.

Michael McGuire

 

[EdStainless]

McG, I'll grant you this much, you cannot have CSCC in an austenitic alloy without there being active corrosion.
Though in samples that we have run the cracking does not always initiate at a discernible pit, but if it cracks you will find pits on the sample someplace.
If the alloy fully resists crevice corrosion in an environment then it won't crack in it.

But some high Ni (~45%) austenitics that have fairly low pitting resistance still will not CSCC, so there are other mechanisms at work.

Ni plays a key roll in corrosion resistance in acid environments, and also in how the oxide film forms and adheres at high temps.


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P.E. Metallurgy, Plymouth Tube

 

[CoryPad]

mcguire,

What are your thoughts on cathodic protection and SCC? The traditional view is that SCC and Hydrogen Embrittlement are different, and one reason why is that cathodic polarization suppresses SCC but accelerates HE.

 

[EdStainless]

My thoughts, are that it depends on the alloy greatly.
Entirely different in a ferritic or martinsitic alloy than in an austenitic.

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P.E. Metallurgy, Plymouth Tube

 

[CoryPad]

Thanks Ed

 

[mcguire]

In martensitics, cracking occurs equally under cathodic and anodic polarization. In ferritics, essentially not at all. In austenitic (stainless steels) HE doesn't occur because (again my theory) because of the scarcity of certain dislocations at crack tips. Anodic conditions can replicate the high concentrations at crack tips, however.
I see the embrittlement step as the delayed, asymmetrical entry of hydrogen to the triaxial stress zone of the crack causing a decrease in stress in the crack travel direction, thereby increasing the shear stress above that required for fracture. This can be demonstrated on a simple table top experiment.


Michael McGuire

 

[EdStainless]

In Cl stress cracking of autenitics there is great doubt as to H being involved at all.
At elevated temp I can crack 316 SS stressed samples in 100ppm Cl and 10ppb S at 100% Y.S., but you can't do the same with hydrogen charging.
You do need something other than just Cl at very low levels, at high levels straight Cl will work fine.

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P.E. Metallurgy, Plymouth Tube

 

[mcguire]

I don't disagree with your facts. However, crack tip electrochemical reactions generate an extremely high partial pressure of hydrogen in a very localized zone, i.e. the crack tip. Hence the destabilization of the balanced triaxial tension.


Michael McGuire

 

[nickypaliwal]

Although Nickel does not contribute to PREN. Why are nickel alloys considered highly resistant to CLSCC ?