PREN equation 3

[mead]

I have seen the formula for pitting resistance equivalent number (PREN) presented with different values of the multiplier for percent Nitrogen. I have seen 13,16 and even 30 used. Which is correct. Also, Does anyone know where this formula was first presented? Was it by ASTM, ASM, NACE? Alot of literature on ss refers to it so it seems to be an accepted concept. But where did it come from. Thanks

 

[bruv]

Somewhere I have seen the information you want - when I find the link again I shall post it. If I remember correctly, the formula is somehow related to the type of stainless steel it is being used for.

The general formula in use for duplex stainless is %Cr + 3.3(%Mo) + 16(%N). I have seen it with 1.5(%W) added in for super-duplex type alloys but this is rare.

 

[bruv]

Bingo

Two good links to start with are:

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

and

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

The second one give 1982 as the first date for PRE numbers

Chris

 

[mcguire]

bruv has found some good references. The formula is more a useful correlation than a rule because it tends to over simplify.
I posted a FAQ on this site giving the mulitiplier if 16 for nitrogen because the latest technical papers used 16. But, clearly it is not correct for ferritic stainless steels, since nitrogen is essentially insoluble in ferrite and combines with chromium to LOWER corrosion resistance. Likewise the formula overlooks the complicated role of sulfur which is often present in stainless and has a notably negative effect. Titanium in small amounts can greatly increase corrosion resistance by combining with sulfur rendering the sulfides insoluble and unable to initiate pitting.
Silicon, copper and nickel also have some beneficial effect.

 

[bruv]

The origin of the PRE formula appears to date to 1982 - it's in the following reference: Duplex Stainless Steel Conference Proceedings, St Louis 1982, ed RA Lula, published by the ASM. ISBN 0-87170-166-9. Check pages 603-629. The authors were working with duplex alloys, and used 16 as the multiplying factor for N

McGuire - Do you think that small amount of Ti would have a similar effect in Ni-base alloys, for example cast Inconel 625 and Hastelloy C type alloys?

Thanks

Chris

 

[mcguire]

It would except that these alloys are usually very highly refined and contain diminishingly small amounts of sulfur. The same is true of duplex stainlesses, which are virtually impossible to hot roll if their sulfur exceeds 8 ppm.In this case titanium would tie up nitrogen and hurt pitting resistance.
The effect is also dependent upon whether the sulfides are exposed on the surface. Usually they are removed by pickling or passivation before service. If fresh sulfides are exposed, as by surface abrasion, i.e. polishing, then they can promote pitting.
This normally only happens for appearance-oriented applications, such as for appliances or architectural stuff.

 

[mead]

My thanks to all for the helpful information. The responses have put another question in my mind.

mcguire - Can polishing of electro-polishing (in order to obtain desired surface roughness)in 316L expose surface sulfides?

Also, I am looking for reference information that puts forth a maximum Chloride pppm that 316L and 304L can be used for. I am trying to definitively prove to our operations department the negative effects of certain process changes.

Thanks in advance.

 

[mcguire]

Although electropolishing can and does remove metal and thus can expose sulfides, it will also remove the sulfides already exposed as well as those it exposes...net result is fewer exposed sulfides.
As far as a maximum chloride level, let me refer you to the FAQ in this section. The maximum level of chlorides is a function of pH, temperature, alloying levels within the limits of the specification, surface condition and other anions and cations in the environment.
If you can pin down all these variables, a permissible max on chlorides can be established for a given time span. I'm not sure I can do it, however.