Pitting Resistance Equivalent Number (PREN) is generally used to indicate the pitting resistance of stainless steels. In general: the higher PREN-value, the more corrosion resistant the steel. A PREN of 32 is considered the minimum for seawater pitting resistance. 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.
PREN is calculated as follows for austenitic stainless steel and duplex stainless steel respectively:
This formula suggests that molybdenum is 3.3 times more effective than chromium at improving pitting resistance, which is true within limits. Chromium must always be present in stainless steel to provide basic corrosion resistance. Molybdenum cannot provide this basic resistance, but it significantly enhances a stainless steel’s corrosion resistance, as the formula shows.
The table shows that ferritic, austenitic and duplex stainless steels with different levels of pitting resistance are available, and the figure below shows that it is possible to select several different grades with similar pitting resistance. For example Types 444 (ferritic), 316 (austenitic) and 2304 (duplex) have similar resistance to pitting corrosion in chloride environments.
PREN Values for Various Grades of Stainless Steels
|Grade||EN No/UNS||Type||Typical PREN|
|316L 2.5 Mo||1.4435||Austenitic||26|
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