What is Pitting Resistance Equivalent Number – PREN

What is Pitting?

Pitting corrosion is a localized form of corrosion that causes small, deep holes or pits to form on a metal surface. This type of corrosion is highly localized and can lead to rapid material failure with minimum material loss.

Causes of Pitting

• Chloride Exposure (Salt & Marine environments)
• Poor Material Selection (low-grade)
• Improper surface treatment & coatings
• Moisture & High Humidity
• Chemical Exposure
• Manufacturing Defects & Surface Contamination
• Extreme Temperatures & Thermal Cycling
• Stagnant or Low Oxygen Environments

Types of Pitting

Check out the major types of pitting below:

How to Calculate PREN?

The following equation is a commonly used formula to calculate PREN:

PREN = %Cr + 3.3*(%Mo) + 16*(%N)

• Cr = Chromium
• Mo = Molybdenum
• N = Nitrogen

The higher the PREN value, the better the alloy resists pitting corrosion. Please not that for this specific type of corrosion nickel & copper offer minimal corrosion resistance.

A minimum PREN value of 32 is considered to protect against seawater pitting corrosion.

NSI Stainless Steel & High Nickel Alloy PREN Values

Check out Nickel Systems PREN values for our stainless steel and high nickel alloy comparison:

NSI MATERIAL’S PREN COMPARISON TABLE

Note that these Nickel Systems materials are those materials which are commonly used when PREN values are being considered.