Passivation & PREN
Why stainless steel doesn't rust—and how to calculate pitting resistance
AI generated Stainless steel doesn't rust because a wafer-thin, self-healing of chromium oxide forms on its surface. How well this layer protects against pitting can be estimated with the PREN formula: , , and nitrogen make the difference. That's the formula behind every stainless steel choice.
The whole secret of stainless steel is an invisible layer: the . As soon as the steel contains at least about 10.5% , a layer of chromium oxide only a few nanometers thick forms spontaneously on the surface. It is dense, adherent, and—crucially—self-healing: if it gets scratched, new chromium oxide forms immediately in the air.
This process is called . Without it, stainless steel would be nothing but expensive structural steel. The is the reason stainless steel stays bright where structural steel rusts.
But the is not invulnerable. Its number one enemy is chlorides (salt water, road salt, pool water). They break through the layer locally—and then occurs. How well a stainless steel is armed against this can be estimated with a simple formula: the (Pitting Resistance Equivalent Number).
PREN = %Cr + 3.3·%Mo + 16·%N
The formula says: not only counts, but above all (factor 3.3) and (factor 16). A few examples:
• (304): PREN ≈ 18 + 3.3·0 + 16·0.05 ≈ 19 — okay for normal atmospheres, unsuitable for chlorides.
• (316): PREN ≈ 17 + 3.3·2.2 + 16·0.05 ≈ 25 — significantly better against chlorides.
• (Duplex 2205): PREN ≈ 22.5 + 3.3·3.1 + 16·0.17 ≈ 35 — very resistant, for seawater.
• Superduplex: PREN > 40 — for the most aggressive chloride environments.
The rule of thumb: PREN below 24 = only little chloride, 24–32 = coastal/mild chloride, above 32 = seawater and aggressive media.
For you as a welder, the PREN formula is doubly useful:
1. Material selection: You can estimate whether a stainless steel is up to the environment—instead of hoping on the data sheet.
2. Filler selection: The filler must have a PREN at least as high as the . If you weld 316 (PREN 25) with a 308L filler (PREN ~19), the weld bead is the weak point and rusts first. That's why: Mo-containing → Mo-containing filler.
And after : The heat tint next to the weld bead is weakened . It must be removed (pickling, grinding) and re-passivated—otherwise it will rust right there.