Pitting Resistance Equivalent Number

What is Pitting Resistance Equivalent Number?

The Pitting Resistance Equivalent Number (PREN) is a measure used to evaluate the resistance of stainless steels and other corrosion-resistant alloys to pitting corrosion. The higher the PREN value, the greater the material’s resistance to pitting, particularly in environments containing chlorides, such as marine settings or other highly corrosive chloride-rich environments.

There are several PREN formulas. They commonly range from:

PREN = %Cr + 3.3 × %Mo + 16 × %N
to:

PREN = %Cr + 3.3 × %Mo + 30 × %N.[4]
There are a few stainless steels which add tungsten (W), for those the following formula is used:

PREN = %Cr + 3.3 × (%Mo + 0.5 × %W ) + 16 × %N
All % values of elements must be expressed by mass, or weight (wt. %), and not by volume. Tolerance on element measurements could be ignored as the PREN value is indicative only.

What is Pitting Corrosion ?

Pitting corrosion is a localized form of corrosion that typically occurs in small areas on the surface of a metal, forming tiny holes or pits. These pits gradually expand and deepen over time, potentially leading to material failure. The main characteristic of pitting corrosion is that it is concentrated in specific areas of the surface, while the rest of the surface may remain relatively intact.

Materials Prone to Pitting Corrosion

Stainless steel: Particularly austenitic stainless steels (such as 304 and 316), are prone to pitting corrosion in chloride-containing environments, especially when the molybdenum content is low.
Aluminum and aluminum alloys: These materials can also suffer from pitting in chloride environments.
Copper alloys: While copper alloys generally have good corrosion resistance, they can experience pitting corrosion in certain conditions (e.g., in chlorinated water or seawater).

Preventive Measures for Pitting Corrosion

Selection of pitting-resistant materials: Choosing materials with a high PREN (Pitting Resistance Equivalent Number) value, such as those with higher chromium, molybdenum, or nitrogen content, can enhance the material’s resistance to pitting.
Surface treatment: Passive treatments, surface polishing, and coatings can improve the material’s corrosion resistance and reduce the likelihood of pitting.
Environmental control: Reducing or controlling the concentration of chloride ions in the corrosive medium, adjusting the pH, or lowering the temperature can effectively reduce the risk of pitting corrosion.

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