Nitrogen improves the pitting corrosion resistance of stainless steels, working alongside chromium and molybdenum. It carries the highest weighting of the three in the PREN formula, where its effect is multiplied by 16, so even small additions make an important contribution.

During the early development of stainless steels, nitrogen was an unintentional alloying element. Modern steelmaking processes such as AOD (argon oxygen decarburisation) and VOD (vacuum oxygen decarburisation) allowed its level to be carefully controlled, and at around the same time it became known that nitrogen improves resistance to pitting corrosion.

The pitting resistance equivalent number (PREN) is a helpful theoretical way of comparing the pitting corrosion resistance of different metals based on their composition. The most widely accepted formula is:

PREN = %Cr + 3.3 x %Mo + 16 x %N

PREN can be used to compare grades, but cannot safely predict absolute performance in a specific application. Because most specifications allow a range of compositions, some end-users specify a minimum PREN value to ensure adequate corrosion resistance.

The precise role of nitrogen is still not fully understood. It is thought to improve surface passivation of stainless steels, and the rate of re-passivation after pitting has initiated.

Nitrogen is also an austenite stabiliser, making it attractive as a lower-cost replacement for nickel. Its solubility in steel is limited, so it cannot fully replace nickel or other elements, although the solubility limit rises with the addition of manganese.

Alloys containing nitrogen

Increased nitrogen is exploited in duplex and super duplex stainless steels, letting them reach PREN values over 34 and over 40 respectively on a cost-effective basis. The duplex grade 2205 (Alloy 2205, SAF 2205®, 1.4462, F51/F60, S31803/S32205) typically contains up to 0.2% nitrogen. The super duplex grades Ferralium® 255 (DIN 1.4507, S32550), S32750 (F53, DIN 1.4410, SAF2507) and S32760 (F55, DIN 1.4501) all contain up to 0.3% nitrogen.

The Nitronic® range of alloys, most notably Fermonic® 50 (Nitronic 50, XM-19, UNS S20910, DIN 1.3964), also uses elevated nitrogen additions to achieve a PREN over 34. This grade may contain up to 0.4% nitrogen.

More recently, there has been research into very high nitrogen steels, aiming to produce high-strength, highly corrosion-resistant grades at lower cost for medical applications, removing the nickel content that can be problematic for some people.