The role of molybdenum in enhancing the corrosion resistance of stainless steel

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The role of molybdenum in enhancing the corrosion resistance of stainless steel. The most common use of molybdenum is in the production of stainless steel and nickel-based alloys. Molybdenum is widely recognized for its ability to improve the pitting resistance of stainless steel, making it a key alloying element in its composition. Molybdenum is used in the manufacturing process of duplex, superduplex, and austenitic stainless steels. What is stainless steel? For a steel considered to be stainless steel, it must contain at least 10.5% chromium. Above this threshold, it promotes the formation of a highly stable passivation film, which forms immediately upon exposure to an oxygen-containing environment. This extremely thin self-healing layer can provide protection at various temperatures against various harsh environments, from seawater to acids. Increasing the chromium content generally enhances corrosion resistance, which is associated with a thicker passive film. Due to the inherent corrosion resistance of stainless steel, surface corrosion generally poses little problem. Pitting is more likely to occur when there are defects or weaknesses in the passivation film. PREN calculation: Empirically, molybdenum can improve pitting resistance. Therefore, it is a major component in calculating the pitting resistance equivalent number (PREN value). This PREN value can be used to compare different grades, but it cannot be reliably used to predict the absolute performance of a grade in a specific application. It is a useful theoretical method for comparing the pitting resistance of different types of metals based on their chemical compositions. The most widely accepted PREN formula uses the contents of chromium, molybdenum, and nitrogen, namely PREN=%Cr+3.3x%Mo+16x%N. Since most product specifications allow for a range of components, some end-users also specify a minimum PREN value to ensure sufficient corrosion resistance. PREN calculation results Therefore, it can be seen that an increase in molybdenum content improves pitting resistance more significantly than chromium does. In principle, adding more and more molybdenum will rapidly improve the properties of the alloy. So, why isn’t this simple logic followed more widely? Increasing the molybdenum content has a direct impact on the cost of the alloy. Compared to chromium, its current cost is about 4 times higher, which offsets the positive factor (3.3) in the PREN calculation. However, in order to maintain the good mechanical and physical properties of austenitic stainless steels, the chromium or molybdenum added to improve corrosion resistance must be balanced by nickel and a series of other elements that act as \"austenite formers\". Compared to chromium, molybdenum has a coefficient of 1.5 for austenite formation. Therefore, as long as the improvement in pitting resistance (compared to chromium) is greater than the additional nickel content required to maintain austenite, it is more cost-effective to add molybdenum rather than chromium. Adding molybdenum to stainless steel has been shown to increase the pitting and repassivation potentials, thereby enhancing the resistance of the passivation film by reducing the number of point defects in it. Molybdenum reduces the intensity of the oxidation required to ensure passivation, and decreases the tendency of previously formed passive films to break down. Furthermore, molybdenum reduces the critical dissolution rate of the alloy in acidified chloride solutions, allowing alloys with higher molybdenum content to remain in a passive state in strong acids. Therefore, it has been proven that increasing molybdenum improves pitting resistance by enhancing the protective properties of the passivation film and reducing the pitting growth rate. Austenitic stainless steel 316L alloy (UNS S31603, 1.4404, 17-12-2) contains about 2% molybdenum, which is its main difference from 304L alloy. By adding molybdenum, pitting resistance can be significantly improved, which means it can be used in more corrosive environments, including pharmaceutical equipment, street furniture and handrails, as well as marine fixtures and accessories. Nitronic 50 (Fermonic 50, XM-19, UNS S20910, 1.3964) is an austenitic stainless steel with a much higher degree of alloying. Compared to 316L alloy with a PREN of around 25, this grade has a PREN of 34. It contains 1.5–3.0% molybdenum, which is balanced by a higher nickel content (11.5–13.5%) to maintain a fully austenitic microstructure. This means it has excellent corrosion resistance, and it possesses all the advantages of austenitic stainless steel, namely superior impact strength, toughness, and ease of fabrication. It is most widely used in the production of valve stems, but it is also used in downhole tools due to its non-magnetic properties, as well as in marine components where 316L alloy is insufficient. Alloy 254 (F44, UNS S31254, 1.4547, 254 SMO, 6Mo) is perhaps one of the most extreme applications of molybdenum additives, with a content of 6%. As a result, the PREN value is 43, which is equal to or greater than that of most standard alloys, although this comes at the cost of including more expensive alloying elements. It will be used in highly demanding applications where duplex and superduplex stainless steels may not be suitable, such as in offshore flue gas desulfurization units, where the operating temperatures can exceed the maximum operating temperature of superduplex grades. Double-phase stainless steel: The industry standard 2205 grade (alloys 2205, Sanmac 2205, SAF 2205, 1.4462, F51/F60, S31803/S32205) contains 3.0–3.5% molybdenum, which helps to raise its PREN value to 34. This is equivalent to Nitronic 50/XM19 stainless steel, but at a much lower price. This cost-benefit factor explains why it is widely used in applications where standard 3xx series stainless steels are not sufficiently corrosion-resistant. Super duplex stainless steels – The most popular grades of super duplex stainless steel include Ferralium 255, SAF2507 (S32750, F53, 1.4410) and S32760 (F55, 1.4501). All of these contain approximately 3.0% molybdenum, which helps to achieve a PREN value of >40. Super duplex stainless steel possesses incredibly high strength and corrosion resistance, primarily due to its increased chromium and nitrogen content, as well as a reduced amount of expensive nickel and molybdenum alloying elements. However, a certain amount of molybdenum helps improve pitting resistance by working in synergy with other alloying additives. Nickel-based alloys: Many nickel-based alloys contain about 3.0% molybdenum, for exactly the same reason as in austenitic, duplex, and superduplex stainless steels, in order to improve pitting resistance. At this level, achieving strong performance is the most cost-effective. Alloys with a molybdenum content of about 3.0% include 718 alloy (Inconel 718, UNS N07718, 2.4668), 825 alloy (Incoloy 825, UNS NO8825, 2.4858), and 925 alloy (Incoloy 925, UNS N09925). Alloy 625 (Inconel 625, UNS N06625, 2.4856) and alloy 725 (Inconel 725, N07725) contain approximately 10.0% molybdenum, which helps to raise their PREN value to 45, whereas the nickel alloys mentioned earlier have a PREN value of only 31. Therefore, in terms of environment and temperature, alloy 625 tends to be used under the most aggressive conditions, where its lower strength is less important.