Molybdenum is one of the most widely used elements in the manufacturing of stainless steel and nickel-based alloys to improve their corrosion resistance.
Molybdenum plays a key role in enhancing the pitting corrosion resistance in stainless steel.
Molybdenum improves the ductility, toughness, and wear resistance of steel.
Furthermore, molybdenum also increases the high-temperature strength of stainless steel.
It means that the higher the molybdenum content, the higher the strength of the steel, along with strength other properties also increased such as durability, stability, and creep strength.
The major role of molybdenum in stainless steel is to improve the steel’s corrosion resistance against reducing media, crevice, and pitting corrosion resistance.
In this article, we will learn more about molybdenum alloys such as austenitic stainless steel, nickel-based superalloys, duplex and super duplex stainless steel.
Stainless steel is the general name for a group of corrosion-resistant steels containing at least 10.5% chromium.
This minimum amount has the effect of forming a chromium oxide layer on the surface, which acts as a self-healing oxygen barrier, preventing further oxidation.
The oxide layer is not self-healing if it contains less than 10.5% chromium.
That very thin, self-healing coating protects against a wide range of hostile conditions, from seawater to acids, at various temperatures.
Stainless steels are utilized commonly in various industries, including construction, automotive, marine, aerospace, and consumer goods.
The material has exceptional strength, durability, and corrosion resistance, making it perfect for various attractive and open-air components.
Stainless steel is graded according to its alloy composition.
The PREN calculation
Molybdenum plays a vital role in improving the pitting corrosion of stainless steel.
Therefore, it is the major component in the calculation of the PREN value (pitting resistance equivalent number).
The different grades can be compared with the help of this PREN number, but the performance of an alloy is not calculated with this number in the specific application.
It is a useful theoretical method for assessing the pitting corrosion resistance of different metals based on their chemical compositions.
The most generally used version of the PREN formula uses the chromium, molybdenum, and nitrogen content as PREN =%Cr + 3.3x%Mo + 16x%N.
For example, grade 316 stainless steel contains 20% chromium, 4 % molybdenum, and 0.1% nitrogen, and the PREN calculation is = 20+3.3×4+16×0.1. As a result, the PREN value for grade 316 is 34.8.
Results of the PREN Calculation
As a result, increasing the amount of molybdenum in the alloy also increases the pitting corrosion resistance more than increasing the chromium level.
Now, the question is if the quantity of molybdenum in the alloy would quickly improve its performance. So, why isn’t that basic logic more commonly applied?
Increasing the molybdenum concentration has a direct impact on the cost of an alloy.
Molybdenum is currently around four times more expensive than chromium.
However, any increases in chromium or molybdenum to enhance corrosion resistance must be compensated by nickel and other alloying elements that operate as ‘austenite producers’ to maintain the favorable mechanical and physical qualities of austenitic stainless steel.
The addition of molybdenum is more cost-effective than chromium to improve the pitting corrosion resistance.
Molybdenum addition to stainless steel has been demonstrated to boost pitting corrosion resistance and also improve passive film resistance by reducing the overall amount of point defects in the film.
Molybdenum minimizes the oxidizing effect and also reduces the potential of previously created passive films.
Austenitic Stainless Steels
Some austenitic stainless steel grades with excellent corrosion resistance are given below:
The fundamental difference between alloy 316L (UNS S31603) and Alloy 304L is that it contains about 2% molybdenum. The addition of molybdenum improves resistance to pitting corrosion, allowing it to be utilized in more demanding situations such as pharmaceutical equipment, street furniture, naval fixtures, and fittings.
UNS S20910 Austenitic stainless steel has a significantly higher alloy content.
In comparison to Alloy 316L, which has a PREN value of about 25, this grade of stainless steel has a PREN of 34. To maintain a fully austenitic microstructure, it is formulated with 1.5 – 3.0% molybdenum, which is compensated by increased nickel content (11.5 – 13.5%). This means it has excellent corrosion resistance as well as all of the advantages of austenitic stainless steel. It is mostly utilized in the manufacturing of marine components and valve stems.
With a 6% molybdenum content, (UNS S31254, 1.4547) is one of the most extreme applications of molybdenum alloys. The PREN value of this grade is 43, which is equal to or more than most ordinary alloys; however, due to the presence of more expensive alloy additives, it is a very expensive alloy. It will be employed in extremely harsh applications where duplex and super duplex stainless steels may be unsuitable for use.
Duplex Stainless Steels
The 2205 duplex (Alloy 2205, S31803/S32205) stainless steel is known as industrial-standard and contains 3.0-3.5% molybdenum.
The PREN value of this grade is 34.
This is the same as a alloy 50 but at a significantly lower price. This low cost explains why it is often utilized in areas where typical 3xx-series stainless steels are insufficiently corrosion-resistant.
Super Duplex Stainless Steels
UNS S32750 (F53, 1.4410) and S32760 (F55, 1.4501) are the most widely used super duplex stainless steel grades; these grades have about 3.0% molybdenum, which contributes to a PREN >40.
Due to their higher ratios of chromium and nitrogen, super duplex stainless steels provide an extraordinarily cost-effective combination of high strength and corrosion resistance.
Nickel-based Superalloys
Many nickel-based superalloys contain roughly 3.0% molybdenum, which is the optimal ratio for improving pitting corrosion resistance.
Alloys with a molybdenum concentration of approximately 3.0% include Alloy 718, Alloy 825, and Alloy 925.
Alloys 625 and 725 contain more than 10% molybdenum, which contributes to their PREN value of 45.
As a result, these grades are utilized in extreme weather conditions, where their lower strength is less essential.
Find Out More About Corrosion Resistance
If you want to get more information about corrosion and molybdenum-based corrosion resistance alloys, please contact us and feel free to ask your questions.
We have experienced staff to guide you and sort out all of your problems regarding corrosion-resistant stainless steel projects.
Conclusion
The molybdenum is the best element for enhancing the corrosion resistance properties of stainless steel alloys.
These alloys are slightly expensive but have outstanding crevice and pitting corrosion resistance and high-temperature strength.
At SEATHER, we are experts in the field of stainless steels and nickel based superalloys manufacturing. As such, we offer a variety of services to ensure your project is formed to your exact requirements.
