PREN equation
PREN equation
(OP)
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





RE: PREN equation
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.
RE: PREN equation
Two good links to start with are:
www.avestapolarit.com/upload/documents/technical/acom/acom92_1.pdf
and
www.avestapolarit.com/upload/documents/technical/acom/acom91_1_2b.pdf
The second one give 1982 as the first date for PRE numbers
Chris
RE: PREN equation
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.
RE: PREN equation
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
RE: PREN equation
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.
RE: PREN equation
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.
RE: PREN equation
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.