Hangbo Alloy Group
Shanghai · Est. 2012
PREN predicts pitting resistance of nickel alloys in chlorides. Learn how to read it, its limits, and where high-PREN grades earn their cost.
When a buyer asks whether an alloy will survive seawater or a chloride process stream, the number that comes up first is PREN - the Pitting Resistance Equivalent Number. It is a useful screen, but it is also one of the most misread figures in alloy selection. PREN ranks pitting and crevice resistance in neutral chlorides; it says nothing about acid attack, stress corrosion, or galling. This guide explains what PREN actually measures, where it helps, and how to use it alongside the real service environment.
The common form is PREN = %Cr + 3.3 x %Mo + 16 x %N. Chromium builds the passive film, molybdenum slows its breakdown in chlorides, and nitrogen both stabilizes the film and suppresses pit initiation. Higher PREN means a higher critical pitting temperature and a lower tendency to nucleate pits in chloride-bearing water.
PREN is an empirical ranking derived from weight-loss and potentiodynamic tests in neutral chloride; it is not a thermodynamic constant. Different laboratories quote slightly different coefficients, but the 3.3 and 16 multipliers above are the most widely cited for austenitic and super-austenitic grades.
PREN is reliable for relative pitting and crevice resistance in neutral-to-slightly-acid chloride solutions such as seawater, cooling water, and flue-gas scrubber liquors. It is the right first filter when comparing stainless and nickel alloys for the same chloride duty.
PREN does not predict resistance to reducing acids (sulfuric, hydrochloric, phosphoric), stress corrosion cracking, or intergranular attack - those depend on nickel, copper, and tungsten content that the formula ignores. A high-PREN 6Mo super-austenitic can still fail in hot sulfuric acid where a lower-PREN Hastelloy C-276 shrugs it off. Always pair PREN with the actual corrodent list.
A common rule of thumb: PREN above ~32 resists brackish water, above ~40 resists open seawater, and above ~45 stays passive in warm, aerated seawater with crevices. But temperature, oxygen, and crevice geometry move those lines, so treat PREN as a shortlist tool, not a guarantee.
In practice, choose the lowest-PREN grade that clears the duty, then confirm with the corrodent list and any prior service data. When the environment is mixed - chlorides plus acids plus high temperature - lean on the molybdenum- and tungsten-rich nickel alloys covered in the Alloy 31 FGD guide rather than chasing PREN alone.
Open seawater with crevices generally needs PREN above about 40, which is why 6Mo super-austenitics such as Alloy 926 and nickel-copper 90-10 copper-nickel heat-exchanger alloys are common in condensers and desalination. Below 32, expect pitting in warm aerated seawater.
No. PREN only ranks chloride pitting. Strong reducing acids are handled by nickel, copper, and tungsten content the formula ignores, so a high-PREN stainless can fail where a lower-PREN Hastelloy C-276 or Alloy 31 succeeds. Always match the alloy to the specific acid, not the PREN.
Nitrogen raises PREN (coefficient 16) because it stabilizes the passive chromium-rich film and makes pit initiation harder and repassivation faster. That is why modern super-austenitic grades add 0.2-0.4% nitrogen to reach PREN values once reserved for nickel alloys.
As a dedicated nickel alloy manufacturer serving chemical processing, marine, oil & gas sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.