As discussed in earlier post <<What is PREN value for pitting corrosion?>>, pitting corrosion is one of the most common localized corrosion attack and most destructive form of corrosion in metal and alloy. Out of so many type of alloy, how to differential the pitting resistivity of particular metal and alloy compare to the other? 

Pitting Resistance Equivalent Number (PREN) is used. PREN is an index common used to measure and compare resistance level of a particular metal and alloy to pitting corrosion.
PREN can be calculated , using the alloy chemical composition, to estimate relative pitting resistance of metal and alloys. Common equation for PREN calculation as followed:

Common equation for PREN calculation as followed :

PREN = %Cr + m.(%Mo) + n.(%N)

Per experiments, m range from 3.0 to 3.3 whilst n range from 12.8 to 30.

For Ferritic grades Stainless Steel, the formula employed is :

PREN = % Cr + 3.3 (% Mo)

For Austenitic grades Stainless Steel, the formula employed is :

PREN = %Cr + 3.3(%Mo) + 30(%N)

For Duplex (austenitic-ferritic) grade Stainless Steel, the formula employed is :

PREN = %Cr + 3.3(%Mo) + 16(%N)

For high Ni-Cr-Mo alloys e.g. Inconel 625, Hastelloy, etc, the formula employed is :

PREN = %Cr + 1.5(%Mo + %W + %Nb)

Legends:

Cr – Chromium, Mo – Molybdenum, W – Tungsten, Nb – Niobium

Pitting is one of main problem for material expose to seawater. Minimum PREN required for material expose to seawater is 40. Duplex Stainless steel, Super duplex stainless steel, etc are exhibiting PREN > 40.