THE P.R.E.N. FACTOR

In highly corrosive environments, stainless steels can suffer a type of localized deterioration known as "pitting." Unlike the greenish patina that forms on copper and its alloys—which acts as a protective barrier—pitting is a microscopic perforation that develops on the metal's surface, as if an invisible, tenacious insect had dug a nest. This phenomenon does not affect less resistant metals such as carbon steel, brass, or bronze, but rather manifests itself specifically in alloys that, due to their composition, should be highly resistant to corrosion.

Pitting is primarily caused by the action of chloride ions (Cl⁻), present in media such as hydrochloric acid (HCl), regardless of its concentration, and in seawater, where sodium chloride (NaCl) is dissolved in large quantities. Although chromium or chromium-nickel stainless steel demonstrates excellent resistance in most environments, its vulnerability to halogens—especially chlorine and fluorine—requires additional solutions. To reinforce protection against these agents, molybdenum (Mo) is incorporated in proportions that rarely exceed 5% by mass. This element, belonging to group 6 of the periodic table along with chromium and tungsten (W), forms a more robust passivating layer than that of chromium, capable of resisting saltwater attack for long periods.

It should be noted that pure metallic chromium, with a purity equal to or greater than 99.999%, is highly resistant to seawater, especially if it has been passivated with nitric acid (HNO₃). However, in commercial stainless steels, the chromium content ranges between 10.5% and 35%, making it insufficient to replicate the resistance of the pure metal. Furthermore, although chromium is very stable against oxidizing agents such as nitric acid, its protective oxide can be damaged by halogenated compounds, such as hydrofluoric acid (HF), which attack it particularly effectively.

Molybdenum, on the other hand, is not resistant to nitric acid, but it is resistant to hydrochloric acid, and its oxide provides an excellent barrier against most corrosive substances. Its inclusion in stainless steels responds to this ability to compensate for chromium's limitations in certain environments. However, its high cost limits its use to the minimum quantities necessary to ensure the desired functionality. Its role is not to replace chromium, but rather to complement it where it is insufficient.

To evaluate a stainless steel's resistance to pitting, the P.R.E.N. (Pitting Resistance Equivalent Number) index is used, which quantifies the alloy's ability to resist pitting corrosion. The basic formula is:

P.R.E.N. = 1 × %Cr + 3.3 × %Mo + 16 × %N

A more complete version includes tungsten:

P.R.E.N. = 1 × %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N

Nitrogen (N), although not always listed in chemical composition tables, significantly improves resistance in saline environments. Let's look at some practical examples:

For AISI 304, with 18% chromium, without molybdenum, tungsten, or nitrogen: P.R.E.N. = 18

For AISI 316, with 16% chromium and 2.5% molybdenum: P.R.E.N. = 16 + 3.3 × 2.5 = 24.25

For AISI 904, with 22.5% chromium and 4.5% molybdenum: P.R.E.N. = 22.5 + 3.3 × 4.5 = 37.35

These values ​​allow for establishing resistance thresholds: a P.R.E.N. ≥32 indicates stability in marine environments, while a P.R.E.N. ≥40 guarantees absolute inertia even during prolonged immersion without access to oxygen. AISI 316, with a P.R.E.N. of 24.25, falls below the marine threshold, but this does not imply that watches made with this steel will corrode at sea. In fact, 316L is widely used in high-end watches, even in models designed for diving. The difference lies in the context of use: a watch, even if occasionally submerged, is exposed to freshwater after each dive, which eliminates chlorine residue. In contrast, a boat propeller or offshore structure remains submerged for years, facing a much more aggressive environment.

So, unless you live underwater—and aren't a merman or a mermaid—your 316 stainless steel watch is more than ready to accompany you on your aquatic adventures. And if you ever decide to design marine structures, you already know that P.R.E.N. will be your best ally in choosing the right steel.