Pitting Resistance Equivalent Number (PREN)

Stainless steel does not resist corrosion by being noble. It resists corrosion by growing a chromium-rich oxide film a few nanometres thick that reforms instantly when scratched. Chloride is the ion that breaks that film locally without breaking it everywhere, and the result is pitting: a tiny anode inside a vast cathode, driving downward at a rate that has nothing to do with the alloy's general corrosion rate. PREN is the industry's attempt to rank alloys for that specific failure by composition alone.

The coefficients are fitted, not derived. Chromium builds the film and gets a weight of 1. Molybdenum, which stabilises the film and helps it repassivate once broken, is worth 3.3 times as much per unit mass. Nitrogen — cheap, and a potent austenite stabiliser besides — is worth 16 in the common form. Those weights come from regression against critical pitting temperature measurements, which is the real test: the temperature at which a given alloy first pits in a standard ferric chloride solution. PREN is a proxy for that measurement, and its whole justification is that the correlation holds well across the grades it was fitted to.

The version implemented here is PREN = %Cr + 3.3(%Mo) + 16(%N), the general-purpose form. A variant weighting nitrogen at 30 is widely used for duplex and super duplex grades, where nitrogen contents are high enough that the coefficient choice materially changes the answer, and a third form subtracts a manganese term. Two PREN figures are comparable only if they came from the same formula, and a specification demanding "PREN ≥ 40" without naming which formula has not actually specified anything — a 25Cr–3.5Mo–0.25N duplex scores 39 on the 16 coefficient and 43.8 on the 30, on either side of the line.

The landmarks are worth carrying. Type 304 sits around 18–19: fine in potable water and mild atmospheres, unreliable anywhere chloride can concentrate, which includes under a gasket, beneath a deposit, and in the crevice under a pipe clamp. Type 316 reaches about 24–25 on its 2% molybdenum, which is a real improvement and still not a seawater alloy. Standard duplex 2205 lands near 35. The 6% molybdenum superaustenitics and super duplex grades clear 40, which is the conventional threshold for ambient seawater service. The numbers are ordinal more than cardinal: PREN 42 versus PREN 40 is not a meaningful distinction, while PREN 42 versus PREN 25 certainly is.

Three limits, and every one of them has sunk a project. PREN is a composition index and knows nothing about heat treatment or fabrication — a super duplex welded with uncontrolled heat input precipitates sigma phase and intermetallics and will pit at a fraction of the resistance its analysis promises, which is why welding procedure qualification matters more than the certificate on the plate. It addresses chloride pitting and crevice corrosion only, and says nothing about chloride stress corrosion cracking, sulphide stress cracking, or general corrosion in reducing acids, where a high-PREN duplex can perform worse than a plain 316. And it describes the alloy, not the crevice: geometry, deposits, gasket materials and stagnant conditions concentrate chloride and drop the effective resistance far below what the bulk water suggests. The best alloy in the world will pit under a poorly chosen gasket.