pitting resistance equivalent number
A number, calculated using a given stainless steel alloy's chemical composition, which aims to predict its resistance to pitting corrosion.
PREN: predicting a stainless steel's pitting corrosion risk
Pitting resistance equivalent number, or PREN, is a calculated index that forecasts how well a stainless steel will resist localized corrosion attack, particularly in chloride-rich environments like seawater or de-icing salt spray. It uses the alloy's chemical composition, specifically the percentages of chromium, molybdenum, and nitrogen, to produce a single number that lets engineers and material buyers compare alloys without running years of field trials.
The most widely used formula is PREN = %Cr + 3.3(%Mo) + 16(%N), though variants exist with different weighting coefficients. A higher PREN number indicates better resistance. Austenitic stainless steels typically score between 20 and 45; duplex grades often exceed 35; super duplex and super austenitic grades can reach 60 or higher. These numbers are empirical, derived from salt fog testing and actual marine exposure data, so they hold genuine predictive power within the ranges tested.
The index emerged because pitting occurs not uniformly across an alloy's surface but as random micro-failures triggered when chloride ions penetrate the passive oxide film at weak points. Chromium stabilizes this film. Molybdenum tightens it further. Nitrogen, when dissolved in austenite, hardens the matrix and aids passivity. PREN captures these three factors because they are the dominant chemistry variables controlling pitting behavior; other elements matter far less for this specific failure mode.
Practical limits and pitfalls
PREN is a screening tool, not a guarantee. It does not account for surface finish, welding defects, crevice geometry, bulk water temperature, pH, or oxygen levels, all of which influence actual pitting performance in service. A Grade 316L stainless with PREN around 30 will resist pitting better than a Grade 304 at PREN 18, but only if both are properly cleaned and maintained. Welds, scale, and crevices under gaskets or deposits remain vulnerable regardless of PREN.
Designers use PREN to short-list candidate alloys for a given duty, then validate the choice with site-specific corrosion testing or proven field history. A heat exchanger in a coastal power plant, for example, might demand PREN greater than 40 to justify the cost; a pipe in neutral industrial water might need only 25. The number sits alongside microstructure, cost, strength, and fabricability in the final material decision.