Overview
Have you ever given thought to how stainless steel fails, given these steels are stainless? According to the relevant statistics, pitting and crevice corrosion failures account for 1 in 4 corrosion types. One factor that contributes to thus high failure rate is that pitting corrosion can grow, which can lead to structural weaknesses in pitted stainless steel over time.
Pitting corrosion occurs when the protective layer of stainless steel (also known as the passive film) is damaged by corrosive medium, such as bromides, chlorides, fluorides, hypochlorite, iodides, sulphides and even water. This article will introduce stainless steel PREN in 3 dimensions:
- 1) What is PREN?
- 2) Why we need PREN?
- 3) How can PREN help us?
What is PREN?
PREN is abbreviated from the Pitting Resistance Equivalent Number, a theoretical method to evaluate and compare stainless steel pitting corrosion resistance performance, from their key stainless steel chemical components.
Those alloying elements such as chromium (Cr), molybdenum (Mo), N (nitrogen) and W (Tungsten) can say a lot in corrosion resistance ability.
Cr: the most important element that determines the 'stainless' characteristics of the alloy. Generally speaking, the alloy containing 10.5% above chromium can be regarded as stainless steel.
Mo: can significantly and comprehensively improve stainless steel corrosion resistance, especially when exposed to chloride solutions.
N: can effectively enhance stainless steel intergranular corrosion resistance, pitting corrosion resistance and crevice corrosion.
W: can improve stainless steel corrosion resistance under high temperatures.
PREN can be expressed in formulas. While there can be varied formulars, but the below two are most widely accepted.
① Without Tungsten:
PREN = %Cr + (3.3 x %Mo) + (16 x %N)
② With Tungsten:
PREN = %Cr + 3.3 x (%Mo + 0.5%W) + 16 x %N
Why we need PREN?
According to the formulas as above listed, we can understand the key elements that affect pitting corrosion resistance are all proportionally weighted and considered.
Many industries, such as chemical processing, petroleum, marine and desalination industries apply the formulas to calculate the PREN for comparison and assess the best possible stainless steel materials for their applications.
However, be mindful that PREN comparison can just be made within the same stainless steel family, i.e., austenitic, ferritic, duplex stainless steels.
Generally speaking, stainless steel PREN fall from 16 to 50, as below,
- • Ferrite PREN: 16~30. Within the low values, ferritic stainless steels are relatively inexpensive with wide applications.
- • Austenite PREN: 18~34. With higher values, austenitic stainless steels feature better pitting corrosion resistance overall.
- • Duplex stainless steel PREN: 30~40. Duplex stainless steel features not only more corrosion resistance, but also better strength for their metallurgical structure.
- • Super duplex stainless steel PREN: 40~50. As super duplex stainless steels possess extremely high PREN values, with better corrosion resistance comes much higher price.
It is generally accepted that a PREN ranging from 32 to 35 minimum is suitable for seawater projects. A PREN 40 or above is suitable for hydrogen sulfide services.
How can PREN help us?
PREN helps us to identify the suitable stainless steel for our projects in a straightforward and measurable way. With the calculated PREN, we can explore different stainless steel grades and choose the most appropriate material which makes the best trade-offs between corrosion resistance, mechanical properties and cost.
While PREN can be quick and effective in ranking and comparing different stainless steel grades, but they cannot be the only indicator to determine the suitable material. Other indicators such as working environment, stainless steel physical properties, mechanical properties, availability, and cost all should be considered for the final decision making.
Conclusion
In conclusion, PREN is a fast and effective way for us to compare and assess the pitting corrosion resistance among stainless steels of the same family. However, PREN cannot be interpreted as the absolute value. PREN can be affected by the below factors.
- • Alloying Elements – these elements form a protective oxide film on stainless steel finish to resist pitting and corrosive elements.
- • Temperature – Extreme temperature such as high temperature or extremely low temperature can break the protective film to hurt the underlaying stainless steel. Therefore, in real world, the PREN may decrease under extreme temperatures.
- • High-Concentration Corrosive Medium – Whether it is high-concentration acid, alkali, or chloride, the exposure to these corrosive media will undoubtedly affect the stability of the protective film and influence the speed with which the corrosion erodes the stainless steel.
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