What PREN means and what it does not prove

The pitting resistance equivalent number is a useful way to rank stainless steels. It is also one of the most over-read numbers in materials practice.

The number, usually written as chromium plus 3.3 times molybdenum plus 16 times nitrogen, is a weighted sum of the elements that most improve resistance to pitting. By this measure 316L sits above 304, and duplex grades sit higher still.

That ranking is the legitimate use. When the question is which of two grades is more likely to resist pitting in a chloride environment, the number usually orders them correctly, instantly, from composition alone.

The trouble begins when it is asked to carry more. It is a relative ranking, not a rate, so it does not tell you how fast a steel will corrode or whether it will pit at all in a given service. It addresses pitting and says little about crevice corrosion and nothing about stress-corrosion cracking. The nitrogen coefficient varies between sources, so two quoted numbers are not always calculated the same way. The relationship was developed for wrought, solution-annealed material and does not account for welds, surface finish, and microstructure.

The honest reading is that the number ranks inherent pitting resistance and little more. A high number is a reason to consider a grade, not a guarantee that it will survive. The Materials Simulation Lab presents it with its formula, inputs, and limits for exactly this reason.