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Pitting corrosion, punctiform local corrosion on the surface of stainless steel in chloride environment and it will continue to deepen and perforation. Pitting damage can be subtle and sudden, often resulting in safety incidents. 

Pitting Resistance Equivalent Number, referred to as "PREN" or "PRE value", is a numerical value calculated based on the mass fraction of some elements in the metal. It is an important parameter for the quantitative evaluation and marking of the comprehensive corrosion Resistance of stainless steel. It can be used to evaluate the ability of stainless steel to resist Pitting corrosion in the chloride environment. It is generally considered that resistant to seawater corrosion PREN over 32. ASTM G48 specifies a precise pitting test procedure.

There are several PREN formulas. The most common is: 

PREN=%Cr+3.3×%Mo+30×%N

The chromium in stainless steel guarantees the basic corrosion resistance of the metal. Molybdenum can greatly enhance the corrosion resistance of stainless steel by 3.3 times that of chromium, as shown in the formula. Different grades can have similar pitting resistance. For example, 444 Ferrite stainless steel, 316 Austenitic stainless steel and 2304 Duplex stainless steel have similar pitting resistance in chloride ion environment. For safety and stability, materials with PREN values greater than 40 such as S31254 super stainless steel and S32750 super duplex steel are commonly used in Marine environments.

How to choose the appropriate stainless steel mainly depends on the corrosion strength of the application environment and the mechanical properties that required. When more than one alloy meets these requirements, other factors such as physical properties, processability, availability, and price need to be considered.

The formula holds for 300 series stainless steel, it reflects the relative ability to resist pitting initiation rather than pitting diffusion, ignoring inclusion (manganese sulfide), surface condition, heat treatment and other relevant factors, can be a " broad" indicator.

PREN of commonly-used stainless steel: