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





RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
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
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
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
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
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
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
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
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
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.
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
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
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
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
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
Michael McGuire
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
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
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
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.
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
Entirely different in a ferritic or martinsitic alloy than in an austenitic.
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P.E. Metallurgy, Plymouth Tube
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
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
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
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
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
Michael McGuire
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
SCC resistance is a very complicated issue. You will recall that alloys with little to no Ni (ferritic, which is interesting since these alloys are very susceptible to hydrogen cracking) and alloys with a lot of Ni (generally over 40%) are virtually immune to CSCC.
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P.E. Metallurgy, Plymouth Tube
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
RE: Why NICKLE is not taken into consideration in PREN(Pitting resistance equivalent number)
Do we need to consider same chloride content in water, gas and Oil outlet line from test separator for Material Selection ?
Though simulation shows gas phase in gas outlet but there can be water carryover in the phase.
Also about water outlet ?