Pitting resistance rises with chromium, molybdenum, tungsten and nitrogen, summed as PREN = Cr + 3.3(Mo + 0.5W) + 16N in weight percent. Super duplex grades, with about 25% chromium and 3% molybdenum, reach PREN 40 to 45. Specifications that set a minimum do not all use the same formula: ASTM A1082 requires 40 for S32760 bolting without tungsten, while A479 bar and A182 forgings in the same alloy must reach 41 with half the tungsten added to the molybdenum.
A high PREN is not proof of a good part. Intermetallic phase from wrong heat treatment can lower the critical pitting temperature by 27 °F (15 °C) or more, and a weld's can be 9 to 27 °F (5 to 15 °C) below the base metal's. G48 itself sets no acceptance criteria, so an order names the method, temperature, time, surface preparation, whether edge attack counts and the criterion; for duplex, ASTM A923 Method C is a ready-made acceptance test.
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model dataMethods A to F, what G48 does and does not predict, and the effect of surface preparation from ASTM G48-25. The PREN formula, its nitrogen factors, the super duplex range of 40 to 45, the CPT definition, the CCT typically 15–20 °C below the CPT, and the CPT lost to intermetallic phase and in welds from published molybdenum-industry guidance. S32760 PREN minimums from ASTM A1082, A479 and A182; the B8MLCuN (S31254) example from ASTM A193 Table 1 at minimum chemistry (19.5 + 3.3 × 6.0 + 16 × 0.18 = 42.2). A923 Method C: 24 hours, 10 mg/dm² per day, 25 °C for 2205 and 40 °C for 2507-type super duplex. The coupons, crevice formers, bath, beakers, microscope and balance are generic and the coupon size is illustrative; the temperature steps carry no figures; pits and crevice attack are drawn schematically; the masses and corrosion rate shown are an illustration, not a test result.
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