Pitting resistance equivalent number (PREN) is a predictive measurement of the corrosion resistance of various types of stainless steel. In general: the higher PREN-value, the more corrosion resistant the steel. 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.
Steels with PREN less than 32 are not sea water corrosion resistant. It depends on type:
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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. The value should be calculated for each heat to ensure compliance with minimum requirements, this is due to the chemistry variation within the same alloy compositional limits.
PREN Formulas (w/w)
The PREN-value is calculated using the following formula: PREN = 1 x %Cr + 3.3 x %Mo + 16 x %N
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
NOTE: There is another formula presented that takes Silicon [S] into account as well. Silicon presents at half the amount as molybdenum, meaning you need twice as much silicon to replace the same amount of molybdenum.
This formula would be: PREN = 1 x %Cr + (3.3 x (%Mo + (1/2 x %Si) ) + 16 x %N
Pitting Resistance Measurement
Exact pitting test procedures are specified in the ASTM G48 standard.