Duplex VS Chloride

[PAN]

Is duplex stainless steel susceptible to chloride stress corrosion cracking (compare with martensitic stainless steel)?
Please advise me about some references for further study.

 

[rustbuster1]

It depends on the duplex SS and the environment.

The duplex SSs are significantly less susceptible to chloride SCC than the austenitic, 300-series, SSs. The alloy manufacturers such as Avesta-Sheffield, Usinor, Sandvik, VDM, etc. have tons of comparative information to help you decide which duplex SS is the solution to a problem with SCC of austenitic SS. Even the duplex SSs have a ladder of performance depending on their Cr and Mo contents. This puts Ferralium 255 and 2507 ahead of 2205, with 2304 coming lower than 2205 in chloride pitting and SCC resistance. Conversely, if the SCC is from hot caustic solutions, the 2304 may be superior to 2205.

In other words, there is no simple answer. But one thing is sure: duplex SSs are less susceptible to CSCC than the 300-series austentic SSs.

 

[mcguire]

Duplex stainless can, like any other stainless alloy, pit if the pH, chloride concentration and temperature are sufficiently adverse. The subsequent risk of SCC is minimal, however, because cracks which form in austenitic grains cannot propagate through surrounding ferrite grains. Hence, the resistance to SCC failure. Martensitic stainless can't come close to the SCC resistance of duplexes, nor can austenitics.

 

[MJCronin]

PAM,

There exists within the metals literature, an index called the "pitting index".

The units of this index are relative, i.e. one alloy verus another. I believe Alleghney Ludlum (?) the makers of AL6Xn uses the pitting index to compare it's products against 304SS in a paricular liquid.

If you know something about your liquid environment, you can use the index to compare alloys. Everything comes down to money, the dupex stainless steels are very expensive...

PAM, I suggest that you also try

http://www.hghouston.com

(The Hendrix Group) for another Engineering Forum and past discussions about duplex stainless

Hope that this helps

MJC

 

[MJCronin]

PAN,

OOOOppps !!!

I made a slight error.... the parameter is called the "pitting resistance equivalent number" (PREN)

You can get more info on this at this link:

http://corrosion-doctors.org/Localized/Pitting.htm

Use GOOGLE and search "PREN and "duplex stainless"........ you should find out a lot

MJC

 

[mcguire]

The PREN equals (%Cr)+ 3.3(%Mo)+ 16(%N). Duplex grades have a lot of nitrogen, which gives them a big edge. The above formula is from the literature and is widely quoted, but it neglects the very negative influence of sulfur on pitting, and therefore SCC, resistance. Duplex stainless is essentially devoid of sulfur, because sulfur makes it too fragile to hot roll successfully. So producers drive sulfur down to under 8ppm. Typical austenitics have .001 to .008% sulfur.
If you use a duplex write into your spec that you want nitrogen at the high end of the specified range for the alloy in question. For 2205 it should be 0.15% minimum.
Producers often minmize nitrogen, at the expense of corrosion resistance, to get better hot rolling characteristics.

 

[rustbuster1]

It's funny how quickly one can digress in these responses. PREN is a useful predictor of PITTING resistance for stainless steels. It is derived from correlation fits for critical pitting temperatures of various stainless steels. It is NOT reliable for predicting SCC, even though some alloys with low PREN numbers tend to have lower critical temperatures for SCC too.

Duplex SS, as already stated above, are very much more resistant to CSCC than common austentic grades. Duplex SSs are very cost comparable with 304 and 316. Your reference was to martensitic SS. This is an interesting comparison because martensitic SS in the hardened form is susceptible to hydrogen SCC and not susceptible to chloride SCC.