HOW TO GAUGE RESISTANCE TO PITTING

BECAUSE THE RESISTANCE OF A STEEL AGAINST PITTING AND CREVICE CORROSION IS PRIMARILY DETERMINED BY THE AMOUNT OF Cr , Mo AND N₂, AN INDEX FOR COMPARING THE RESISTANCE TO THESE TYPES OF ATTACK IS OFTEN USED IN MANUFACTURER'S DATA SHEET. IT IS CALLED THE " PITTING RESISTANCE EQUIVALENT NUMBER (PRE OR PREN). ON OCCASIONS, IT IS ALSO REFERRED TO AS THE LCR ( LOCALIZED CORROSIVE RESISTANCE) NUMBER. IT IS DEFINED IN WEIGHT % USING THE EQUATION GIVEN BELOW :-

PRE = (%Cr) + 3.3 * (%Mo) + 16*(%N₂)

THE HIGHER THE PRE NUMBER, THE BETTER THE PERFORMANCE OF AN ALLOY IS EXPECTED TO BE IN CHLORIDE ENVIRONMENT.

THE PRE VALUE FOR ALLOY 2507 IS 43, THE SAME AS THAT FOR THE 6% Mo SUPER AUSTENITIC ALLOYS CONTAINING N₂. ALLOY 2205 HAS A PRE VALUE OF 35, WHILE FOR TYPE 316L, IT IS 24.