Why does PREN be greater than 32 for sea water?

[j2kabba]

I read the Q&A elsewhere - for seawater, PREN of stainless steels must be greater than 32.

I want to know why PREN must be greater than 32. Why 32?

Another question, how is the lifetime of 316L stainless steel piping in 40C, seawater cooling condenser tubing?

Thanks,

 

[SJones]

For seawater, it's more usual to go for a PREN greater than 40. PREN is an empirical formula related to alloying content and corrosion performance. The thresholds are set by observations of alloys that perform acceptably and those that don't.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[j2kabba]

In AL6XN.com homepage, I found "For seawater, PREN must be greater than 32" in FAQs.

I am wondering why PREN must be greater than 32. Is PRE 32 special criteria for seawater?

Would you please let me know why PRE must be greater than 32 ?

Thanks

 

[Metalguy]

The "lifetime" of 316L in seawater will depend on flow. If you can be sure the flow will be fast and constant, it might survive. Otherwise you can expect pitting fairly quickly.

"I'm that dog who saw a rainbow, only none of the other dogs believed me." from "Kate and Leopold"

 

[mcguire]

PREN of 32 is somewhat arbitrary. Some experts would say 35. Others would note that if crevices are possible, you need a PREN of 40.
The guideline is based on lots of experience.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[EdStainless]

First you have to look at how the PREn is being calculated. What multipliers are being used. Is the moly times 3.3 or 3? Is the nitrogen times 16 or 30? That is one reason for differing guidlines.

If you put 316 into seawater that is stagnant and you allow a biofilm to form it will last some where in the range of days to weeks. If it is kept very clean, no film or silt, and the flow velocity is high then it will last for years. But one bad shutdown where someone forgets to flush the tubes with fresh water and leaves them with a little bit of seawater in them, and they will pit in just days.

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