What is the PREN of super duplex stainless steels?

PREN stands for the pitting resistance equivalent number, which is a value calculated from the alloy composition that indicates its typical resistance to pitting corrosion. The industry specifications NACE MR1075 and Norsok M650 would require the three main super duplex stainless steels (Ferralium 255, UNS S32750 and UNS S32760) to have a PREN >40.

There can be differences from production heat to production heat, given the slight variability in content of the elements that are included in the PREN calculation, namely chromium, molybdenum and nitrogen. In addition, the specification of the super duplex grades is different too. As the PREN formula does not take in to account elements such as copper, which is proven to improve the corrosion resistance of Ferralium 255 in various acids, a threshold of PREN >40 seems like a sensible target.

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