Stainless Steel: What is a PREN Number?
Among the many types of metals and alloys used for all kinds of applications, stainless steel is one of the most resistant to corrosion. That said, stainless steel is not entirely immune to corrosion though some types and grades are more resistant than others. PREN stands for Pitting Resistance Equivalent Numbers, which are used to compare how well these different types and grades are likely to resist pitting corrosion.
Pitting corrosion can affect all kinds of metals from iron and copper to chromium and aluminium. It is widely considered to be the most dangerous form that corrosion can take because the real damage occurs deep within the structure of the metal, with very subtle visual appearance at the surface; this means that it can often go unnoticed until the damage is severe. Pitting corrosion can lead to the complete failure of a structure or engineering system.
The pits that give pitting corrosion its name form in small areas and are often hidden by small amounts of surface rust. They form when cathodic reactions in the large passive layer help to sustain anodic reactions in smaller internal areas; this allows oxidation to occur even without the presence of oxygen.
The most common reason that pitting corrosion occurs in stainless steel is damage to the 'passive layer' of chromium oxide which spontaneously forms on the alloy's surface. This can be caused by physical damage or chemical attack, particularly from chlorides, bromides, sulphides, iodides and fluorides.
Clearly, it is essential to establish how well a particular alloy of stainless steel is likely to perform where pitting corrosion may be an issue. This is where PREN values come into play. They are extremely useful when it comes to comparing grades but, as noted by the British Stainless Steel Association, are not absolute figures and cannot be used to predict with certainty whether any particular grade is suitable for given applications as other factors must be taken into account.
PREN values are based upon the composition of a given alloy and take into account its percentage level of chromium (Cr), molybdenum (Mo) and nitrogen (N). For duplex stainless steels, tungsten (W) is also included. The two most widely used formulas for determining the PREN value of stainless steel are as follows.
- PREN = Cr + 3.3Mo + 16N
- PREN = Cr + 3.3(Mo+0.5W) + 16N
Using these formulas provides a PREN value, which can be anywhere from around 15 to over 40. The higher the PREN value, the more resistant it is to pitting corrosion. PREN values of above 33 are considered to be seawater resistant while those over 40 are classified as 'super'.
BS Stainless employs a team of skilled, highly-trained experts who can offer authoritative advice about choosing the right grade and type of stainless steel products. Please do not hesitate to get in touch.