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 are 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 the 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 vfallow 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.
Duplex stainless steel is a natural step-up in performance from 316L. The increase in chromium content from c. 18% to c. 22% drives improved pitting corrosion resistance. It also offers approximately twice the strength.
For some applications, duplex stainless steel may not be suitable. A higher impact toughness may be required, sometimes combined with the need to operate at lower temperatures too. Austenitic stainless steel retains its mechanical properties down to cryogenic temperatures, has higher toughness, is non-magnetic and easier to fabricate. When increasing those elements that improve the PREN, other elements i.e. nickel, also need to be added to maintain an austenitic microstructure.
Alloy 718 (N07718, Inconel 718, 2.4668) PREN = 31
Alloy 718 is extremely widely used in Oil and Gas applications. It provides very high strength levels through precipitation strengthening, which are largely retained up to c. 700degC. General corrosion resistance is good, but its resistance to sulphide stress corrosion cracking is particularly appreciated. However, its high alloy content (Ni, Cr) makes it a relatively expensive alloy, which encourages substitution where possible.
Alloy 825 is the entry point for nickel alloys in terms of price and performance. It has a relatively modest yield strength, and pitting corrosion resistance less than super duplex stainless steels despite the greater cost. However, it provides a far wider operating temperature range, retaining its mechanical properties at lower temperatures and not being restricted to a maximum service temperature of 250degC.