The corrosion resistance of any given material is heavily dependent upon the specific environmental conditions. Although the root cause of corrosion can originate from a number of different factors, it most commonly seen form of corrosion in stainless steels and nickel alloys is pitting corrosion.
As a first review, the Pitting Resistance Equivalent number (PREN) is a helpful theoretical way of comparing the pitting corrosion resistance of various types of metals based on their chemical compositions. The PREN numbers can be used to compare different grades, but cannot be used to predict absolute performance of a grade in a specific application.
The most widely accepted version of the PREN formula utilises the content of chromium, molybdenum and nitrogen as: PREN = %Cr + 3.3x %Mo + 16x %N. Due to the fact that most product specifications allow a range of compositions, certain end users will also specify a minimum PREN value to ensure adequate levels of corrosion resistance.
The widely used 316L stainless steel has a typical PREN value of 25.
The following alloys have a typical PREN greater than 40:
A family of super duplex stainless steels have been developed over time. As the name suggests, duplex grades combine the favourable properties of both austenitic and ferritic stainless steels. They were originally developed in the 1930’s with a 22% chromium content. A super duplex is typically based upon a 25% chromium content, helping to push up the PREN and improve the pitting corrosion resistance appreciably.
Ferralium 255 is the original super duplex stainless steel, invented by Langley Alloys in the 1960’s. One element not included in the PREN formula is copper, although it has been shown to improve resistance to corrosion in chloride-containing solutions and selected acids. The 2% copper addition reacts with absorbed sulphides at corrosion sites, forming an insoluble copper sulphide ‘protective layer’ that stifles ongoing corrosion.
Alloy 32760 is an alternative super duplex stainless steel, originally developed as Zeron 100 by Mather & Platt (UK). Its additional alloying elements are up to 1.0% copper and up to 1.0% tungsten.
Alloy 254 (S31254, F44, 6Mo, 254SMO)
Alloy 254 is a high-alloyed austenitic stainless steel, sometimes called a ‘super austenitic’. Its increased alloy content, significantly up to 6% molybdenum, raises its PREN albeit at a cost penalty to alloys of a similar PREN value. Alloy 254 would typically be used in applications where super duplex stainless steels cannot i.e. where operating temperatures exceed 250degC.
Alloy 625 is a highly-alloyed nickel-chromium alloy, which retains most of its useful properties from sub-zero temperatures up to 980degC. Its austenitic structure means that it is easy to machine and fabricate, and has good toughness, plus it is non-magnetic and spark resistant. As would be expected with such a highly-alloyed grade, it is not only resistant to pitting corrosion but also to stress corrosion cracking. Consequently, although high cost, it is widely used in the most demanding applications for the Oil and Gas market, such as wellhead components, downhole equipment, fasteners, motors and sheaths.