Pitting resistance equivalent number
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Pitting resistance equivalent number (PREN) is a measurement of the corrosion resistance of various types of stainless steel. Exact testing procedures are specified in the ASTM G48 standard.[1] In general: the higher PREN-value, the more corrosion resistant the steel.
The PREN-value is calculated using the following formula: PREN = 1 x %Cr + 3.3 x %Mo + 16 x %N (w/w) Steels with PREN-values above 32 are considered seawater (corrosion) resistant.
Furthermore a PREN-value ≥ 40 for duplex steels is called for in the DIN EN ISO 15156 as well as the American NACE – a standard for use in hydrogen sulfide environments known in the oil and gas extraction industries. Exception: stainless steels with molybdenum content ≥ 1.5% may have a PREN-value ≥ 30. In these norms the PREN-value takes into account tungsten [W] in the alloy and is defined with the formula: PREN = 1 · %Cr + 3.3 ( %Mo + 0.5 · %W ) + 16 · %N
References
- ^ "ASTM G48 – 11 : Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution". ASTM. Retrieved 28 September 2012.Full text of standard not available free of charge online but available in libraries
steels with PREN less than 32 are not sea water corrosion resistant. It depends on type. ferritic stainless steels require a minimum PREN of 35, duplex a minimum of 40 and super austenitic a minimum of 45 <Kovack and Redmond. Paper 267, Proc. Conf. Corrosion '93, NACE.> These alloys also need to be manufactured and heat treated correctly to be sea water corrosion resistant to the expected level. PREN alone is not an indicator of corrosion resistance.
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