What is A Pitting Resistance Equivalent Number (PREN)?
A look at how pitting resistance equivalent numbers (PREN) are determined
and what they do -- and do not -- tell you about a material
Pitting corrosion resistance is an essential attribute to consider when choosing components and materials for a piping system.
However, resistance levels change based on metallurgic composition and with the sheer variety of options available today, having a clear idea of corrosion resistance just by name or grade is nearly impossible.
This is where pitting resistance equivalent numbers (PRENs) -- also known as pitting resistance equivalent values (PRE-values) can help.
“The PREN (or PRE) numbers are useful for ranking and comparing the different grades, but cannot be used to predict whether a particular grade will be suitable for a given application, where pitting corrosion may be a hazard.”
While you should always consult an engineer to ensure your materials meet all requirements, we’re going to outline how PRENs are determined and what information this value provides.
What is Pitting Corrosion?
Before you can understand what PREN values might tell you, it’s important to know what they’re related to -- in this case pitting corrosion.
Pitting corrosion occurs on metals with protective films -- such as stainless steel.
Often initiated by poor aeration or chemical exposure within the environment, this type of corrosion creates small localized attacks which spread quickly from the protective surface layer to the metal itself.
While sources of pitting corrosion may vary, common causes include exposure to:
How is the PREN of an Alloy Determined?
Exact formulas might differ. For this guide, we’ll be using two of the most widely accepted modelling formulas to determine resistance to localized pitting corrosion by chlorides.
These formulas determine the rating based upon levels of chromium (%Cr), molybdenum(%Mo), nitrogen(%N), and tungsten (%W) present in the alloy.
PREN = %Cr + (3.3 x %Mo) + (16 x %N)
PREN = %Cr + 3.3 x (%Mo + 0.5%W) + 16 x %N
The first formula is commonly used for stainless and duplex alloys without tungsten while the second adjusts the model to account for the tungsten present in many super duplex stainless alloys.
For either formula, results typically start around 16 and can reach numbers greater than 40. You can see an example of typical results using data from the BSSA below.
|STEEL TYPE||CHROMIUM %||MOLYBDENUM %||NITROGEN %||PREN|
|316L (2.5% min Mo)||17.0-19.0||2.5-3.2||0.11 MAX||25.3-30.7|
|Sanicro 28||24.0-26.0||3.0-4.0||0.11 MAX||35.9-43.0|
|SAF 2507||24.0-26.0||3.0-4.0||0.24-0.35||> 40|
|Zeron 100||24.0-26.0||3.0-4.0||0.20-0.30||> 40|
|Ferrinox 255||24.0-26.0||3.0-4.0||0.20-0.30||> 40|
What Can PREN Values Tell You About A Material Choice?
A PREN should not be taken absolutely.
In other words, a 0.5 difference doesn’t necessarily indicate an exact difference in pitting corrosion resistance as characteristics for various alloys and grades vary. In fact, PRENs are only comparable within the same stainless steel family (austenitic, ferritic, duplex, etc.)
In most cases, components for use in the oil and gas industries require a PREN greater than 32. For hydrogen sulphide (H2S) environments and seawater service, a PREN of 40 or higher are typically used.
These numbers also vary depending on the use environment. Corrosive media outside chlorides, service temperatures, component surface finishes, system design, and acidity can all further impact pitting resistance.
As such, a PREN alone is only one facet of effective material choice. Other critical aspects include:
The information in this guide should not be used as a direct recommendation or reference for PREN values. Consulting with an engineer is recommended before making any material choices and to ensure both regulatory compliance and greater efficiency on your materials investment.
That said, PREN values are an excellent way to get a relative understanding of how the pitting corrosion resistance of various stainless steel alloys within the same family might compare.
While the information is not absolute and shouldn’t be used to a great level of precision, when combined with data related to costs, availability, and fabricability, PREN can be a useful metric to help to drive the decision making process and provide a level of confidence.
Unified Alloys has more than 4 decades of experience providing high-quality stainless steel products to industries across North America and Canada. As a leading provider for the oil, gas, and maritime industries in Western Canada, our expert sales analysts have an in-depth understanding of the impact pitting corrosion and other hazards of these tough environments have on the design and operation of piping systems of all sizes. Contact us today to speak with an expert about how we can help you achieve the goals of your next system design or operation.
British Stainless Steel Association: Calculation of pitting resistance equivalent numbers (PREN)
Corrosionpedia: Pitting Resistance Equivalent Number (PREN)
Wikipedia: Pitting resistance equivalent number
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