Molybdenum improves pitting corrosion resistance in stainless steels and nickel alloys by strengthening the passive film: it reduces point defects in the film, lowers the oxidising intensity needed to maintain passivity, and slows dissolution in acidified chloride solutions. It is a major term in the PREN (pitting resistance equivalent number) formula alongside chromium and nitrogen.
For a steel to be considered stainless it must contain at least 10.5% chromium. Above this threshold, a highly stable passive film forms immediately on exposure to an oxygen-containing environment. This very thin, self-healing layer protects against a wide range of harsh environments, from seawater to acids, across a range of temperatures. Increasing chromium generally increases corrosion resistance through a thicker passive film.
Because of this inherent resistance, general surface corrosion is rarely a problem. Pitting corrosion is far more likely, forming where there is a defect or weakness in the passive film.
The PREN calculation
Molybdenum has empirically been shown to improve pitting corrosion resistance, so it is a major component of the PREN value. PREN is a helpful theoretical way of comparing the pitting corrosion resistance of different metals based on their chemical composition. It can be used to compare grades, but cannot safely predict absolute performance in a specific application. The most widely accepted formula uses chromium, molybdenum and nitrogen:
PREN = %Cr + 3.3 x %Mo + 16 x %N
Because most product specifications allow a range of compositions, some end-users specify a minimum PREN value to ensure adequate corrosion resistance.
Increasing molybdenum gives a greater uplift in pitting corrosion resistance than chromium. In principle, simply adding more molybdenum would quickly improve performance, so why is that logic not followed further?
Raising molybdenum has a direct impact on cost. It is currently around 4 times the cost of chromium, offsetting its favourable factor of 3.3 in the PREN calculation. To keep the mechanical and physical properties of austenitic stainless steel, any increase in chromium or molybdenum must be balanced by nickel and other 'austenite formers'. This is best shown in the Schaeffler diagram. Molybdenum is itself a ferrite former, with a chromium-equivalent factor of about 1.0, so extra austenite formers such as nickel or nitrogen must be added to offset it and keep the structure austenitic. Adding molybdenum instead of chromium can still be cost-effective, as long as the improvement in pitting resistance is larger than the extra nickel or nitrogen content needed to stay austenitic.
Molybdenum increases pitting and repassivation potentials, enhancing the passive film by reducing the number of point defects in it. It lowers the oxidising intensity required to maintain passivity and reduces the tendency of formed passive films to break down. It also reduces the critical dissolution rate in acidified chloride solutions, so higher-molybdenum alloys stay passive in stronger acids. In short, more molybdenum enhances pitting resistance by making the passive film more protective and slowing pit growth.
Molybdenum content and PREN vary widely by grade family:
| Grade | Family | Molybdenum | PREN |
|---|---|---|---|
| Alloy 316L | Austenitic stainless | ~2% | ~25 |
| Fermonic® 50 | Austenitic stainless | 1.5-3.0% | 34 |
| Alloy 254 | Austenitic stainless (6Mo, not stocked by Langley) | 6% | 43 |
| Alloy 2304 (lean duplex) | Duplex | Negligible | - |
| Alloy 2205 | Duplex | 3.0-3.5% | 34 |
| Ferralium® 255 / SAF2507 / S32760 | Super duplex | ~3.0% | >40 |
| Alloy 718 / Alloy 825 / Alloy 925 | Nickel | ~3.0% | 31 |
| Alloy 625 / Alloy 725 | Nickel | ~10.0% | 45 |
Austenitic stainless steels
Alloy 316L (UNS S31603, 1.4404, 17-12-2) contains around 2% molybdenum, which is the main difference between it and Alloy 304L. This gives an appreciable improvement in pitting resistance, allowing use in more aggressive environments including pharmaceutical equipment, street furniture and hand railing, plus marine fixtures and fittings.
Fermonic® 50 (Nitronic® 50, XM-19, UNS S20910, 1.3964) is a much more highly-alloyed austenitic stainless steel. Against Alloy 316L with a PREN of around 25, this grade has a PREN of 34. It contains 1.5 - 3.0% molybdenum, balanced by higher nickel (11.5 - 13.5%) to retain a fully austenitic microstructure. The result is strong corrosion resistance with the benefits of austenitic stainless steel, namely excellent impact toughness and ease of fabrication. It is most widely used for valve stems, but also for downhole tooling thanks to its non-magnetic nature, and for marine components where Alloy 316L is not sufficient.
Alloy 254 (F44, UNS S31254, 1.4547, 254 SMO, 6Mo) is one of the most extreme applications of molybdenum, with a 6% content. This gives a PREN of 43, equal to or greater than most standard alloys, at a cost due to its more expensive additions. It is used in very aggressive applications where duplex and super duplex stainless steels may not be suitable, for instance maritime flue gas desulphurisation units where operating temperatures can exceed the maximum working temperature for a super duplex grade.
Duplex stainless steels
The industry-standard 2205 duplex (Alloy 2205, Sanmac® 2205, SAF 2205®, 1.4462, F51/F60, S31803/S32205) uses 3.0-3.5% molybdenum, pushing its PREN to 34. That is equivalent to a Fermonic® 50 / XM-19 grade but at a significantly lower price, which is why it is widely used where standard 3xx-series stainless steels are not corrosion resistant enough.
Lean duplex grades such as Alloy 2304 (SAF2304, 1.4362, UNS S32304) have negligible molybdenum to reduce cost, with manganese added for strength. These are more common in sheet and tube form, where the saving matters on larger projects. To avoid stock duplication, lean duplex grades are not commonly stocked as solid or hollow bar.
Super duplex grades
The most popular super duplex stainless steels, including Ferralium® 255, SAF2507 (S32750, F53, 1.4410) and S32760 (F55, 1.4501), all contain around 3.0% molybdenum, helping them achieve a PREN over 40.
These grades offer a cost-effective combination of high strength and corrosion resistance, mostly through raised chromium and nitrogen with more sparing use of the expensive additions nickel and molybdenum. The molybdenum present works in synergy with the other additions to improve pitting resistance.
Nickel-based alloys
Many nickel-based alloys contain around 3.0% molybdenum, for the same reason as the stainless steels above, to enhance pitting resistance. At this level, strong performance is achieved most cost-effectively. Alloys with around 3.0% molybdenum include Alloy 718 (Inconel® 718, UNS N07718, 2.4668), Alloy 825 (Incoloy® 825, UNS N08825, 2.4858) and Alloy 925 (Incoloy 925, UNS N09925).
Alloy 625 (Inconel 625, UNS N06625, 2.4856) and Alloy 725 (Inconel 725, N07725) contain closer to 10.0% molybdenum, raising their PREN to 45 against 31 for the nickel alloys above. Alloy 625 therefore tends to be used in the most aggressive conditions, in both environment and temperature, where its lower strength matters less.