How Mo Enhances Corrosion Resistance of Stainless Steel

Stainless steel is appreciated for its toughness, strength, and corrosion-resistant nature. While chromium is the standard go-to as the primary alloying element responsible for stainless steel's corrosion-resistant properties, there is one other element that goes about enhancing performance in harsh environments in the background: molybdenum (Mo).

The Role of Molybdenum in Stainless Steel

Molybdenum is a high-melting-point transition metal with good hardness and strengthening properties of alloys. Regardless of concentration, even at low levels, Mo added to stainless steel enhances resistance of the steel against localized corrosion in the presence of high concentrations of chlorides. Chloride ions are very aggressive to stainless steel, often leading to pitting or crevice attack. Molybdenum stabilizes the passive oxide layer on the steel surface, reducing the rate of initiating localized corrosion.

Apart from corrosion resistance, molybdenum is also credited with crevice corrosion resistance of stainless steel. Crevice corrosion occurs in narrow, enclosed spaces where the passive layer is likely to fail due to oxygen depletion. Mo increases the chemical stability of the steel in such confined locations, improving the material life in complex structures such as heat exchangers, chemical tanks for storage, and piping systems.

Molybdenum in Common Stainless Steel Grades

The molybdenum influence can be seen in different grades of stainless steel, which are designed for various uses:

• 304 Stainless Steel: The general-purpose austenitic stainless steel, consisting of approximately 18% chromium and 8% nickel. Basic 304 contains no molybdenum, limiting its application in chlorides. Though 304 performs well in air, water, and mild chemicals, it can be susceptible to pitting and crevice corrosion in marine or chemical process applications.
• 316 Stainless Steel: Another name for this is marine-grade stainless steel and contains 2–3% molybdenum. This apparently moderate addition provides substantial corrosion resistance where chlorides are present in the environment and, for instance, appears in applications such as boat fittings, medical implants, and chemical plant equipment. Molybdenum added makes the passive layer formed more stable, providing a greater resistance to localized corrosion than 304.
• 317 Stainless Steel: Austenitic in nature, this one contains approximately 3% molybdenum, slightly higher than 316, giving it still greater resistance to corrosive chemicals like sulfuric acid. 317 stainless steel is used in the kind of setting that causes 316 to deteriorate, such as chemical facilities processing more aggressive acids or very salty environments.
• Duplex Stainless Steels: Duplex grades such as 2205 have a balanced ferrite and austenite composition with 3% molybdenum. They are mechanically strong and have excellent resistance to pitting and stress corrosion cracking. They are used in the oil and gas sector, seawater desalination plants, and chemical storage tanks where high strength and corrosion resistance are required.
• Super Austenitic Stainless Steels: Types like 254 SMO contain a maximum of 6% molybdenum and are intended for applications in harsh conditions. They possess outstanding resistance to crevice and pitting corrosion even in very aggressive chloride-bearing environments. Applications are in seawater desalination, marine chemical processing, and special chemical storage.

Outside Corrosion Resistance

Besides improving corrosion resistance, molybdenum also improves the overall mechanical performance of stainless steel. Molybdenum improves the tensile and yield strength of the alloy, particularly at elevated temperatures, making Mo-bearing stainless steels suitable for elevated-temperature and elevated-pressure applications. Molybdenum further improves weldability and reduces intergranular corrosion susceptibility if appropriately treated, making welded structures suitable for long-term use.

Further reading: 6 Uses of Molybdenum

Applications Enabled by Molybdenum

The addition of molybdenum introduces application possibilities not available to common stainless steels. In the marine industry, Mo-alloyed steels resist corrosion from seawater, extending the life of ship components, offshore equipment, and desalination plants. In chemical processing, Mo-containing stainless steels may process aggressive acids, chlorides, and oxidizing conditions without loss of strength. The healthcare industry also benefits, with medical devices and surgical implants fabricated from 316L stainless steel due to its corrosion resistance and biocompatibility.

Conclusion

The addition of molybdenum to stainless steel significantly increases its chemical attack resistance, crevice corrosion, and pitting and enables it to be employed in marine to chemical processing aggressive environments. The transition from 304 to 316, 317, and the super austenitic grades illustrates the revolutionary role molybdenum plays in making stainless steel versatile, reliable, and indispensable. For more metal products, please check Advanced Refractory Metals (ARM).