Pitting resistance equivalent number PREN 5

[GAshby]

I have come across pitting resistance equivalent number (or Pitting Index) which as been mentioned in some of the replies here and understand that it is an empirical scale. I have managed to find no reference to the source of this information only various equations to calculate the value from the metal composition. Does anyone know the source or what the values equate to and examples of the environment the PERN values can be used in?

Cheers
Gwyn

 

[TEV]

Gwyn:

See technical paper:

http://www.avestapolarit.com/upload/documents/technical/acom/acom92_1.pdf

 

[mcguire]

Gwyn
Just to add to the information given in TEV's reference, the PREN formula holds true only in the absence of sulfur, which is generally not the case. In the normal range of sulfur, e.g. up to 0.015%, the PREN appears to be reduced approximately by the factor of 1000 X ( %S ) for alloys such as 304 and 316. This, too, can be altered by the presence of elements which react with sulfur. A firther complication is surface condition, e.g degree of roughness, pickled or not, etc.
The formula does correctly depict the roles of Cr, Mo, and N, but does not fully represent pitting corrosion resistance.

 

[TEV]

Mcguire's caveat on sulfur is correct. Machined stainless steel products will typically have about 0.025 wt percent sulfur, added to improve machinability. Tubing products typically about 0.010% S for welding penetration, and other products may be low, near 0.001%, or ?.

PREN is a concept for relative performance, particularly in chloride pitting environments. It is not exact, and there is a lot of scatter, depending on details of corrosion exposure, material composition (i.e. sulfur level and others), and processing history (welded?, passivated? surface condition, etc.)

 

[bruv]

Another reference is

http://www.avestapolarit.com/upload/documents/technical/acom/acom91_1_2b.pdf

ALso, take a look at thread338-25788 , also on this subject.

 

[TEV]

A tip of my hat and a star to Bruv. He was there first. His thread pre-dates my joining Eng-Tips, and I should have done a search to find it.

 

[mcguire]

Gwyn
In reference to your original question, the PREN equation relates best to the saline solutions in which stainless is prone to pit. I wouldn't apply it to organic acids, temperatures far above boiling, or microbiologically-induced corrosion with great comfort except to the extent the rank order of alloys' resistance should stay the same.

 

[bruv]

Thanks TEV

One of these days I'll get round to writing up an FAQ on the subject.

 

[mcguire]

bruv
I have an FAQ on the subject. See if you think it says enough.

 

[GAshby]

Many thanks for all your help.

Gwyn

 

[bruv]

mcguire

That'll teach me to do MY research properly...