Molybdenum grade stainless steels
The generic term “Stainless Steel” covers a large group of iron-base alloys that contain chromium. By definition, a stainless steel is an iron-base alloy that contains at least 10.5% chromium. As the term implies, stainless steels resist staining or rusting when exposed to atmospheric conditions. The chromium content in stainless steel promotes the formation of a thin, chromium-rich surface oxide that serves as a protective barrier to corrosion. The surface oxide is also called passive film. Without this minimum amount of chromium, iron-base alloys or steels corrode in moist air, forming the familiar red rust.
While a minimum chromium content of 10.5% provides corrosion resistance to moist air, more severe environments such as increased levels of chlorides, strong acids or bases will require increased levels of chromium and possibly additions of other elements to maintain corrosion resistance.
Molybdenum is one of the few elements that can be readily added to stainless steel and substantially improves the corrosion resistance. It has a particularly strong positive effect on pitting and crevice corrosion resistance in chloride-containing solutions and is very effective in increasing resistance to reducing acids such as dilute sulphuric acid.
Today, there are literally hundreds of different stainless steel alloys or “grades” and the choice of a grade for any specific application is based on the required level of corrosion resistance, strength, and toughness as well as other factors such as fabrication requirements, availability, and cost.
Stainless steels are divided into subgroups or families based on their crystal structure and strengthening mechanism. The five primary stainless steel families are:
- Austenitic
- Ferritic
- Duplex
- Martensitic
- Precipitation-hardened
The austenitic family accounts for almost 75% of all stainless steels used in the world; ferritic, about 25%; duplex, about 1%; and the martensitic and precipitation- hardened families about 1%.
The chemical composition is the primary determinant of stainless steel microstructure and it is the microstructure that determines the strength and physical properties of the alloy. Although stainless steels in these subgroups can have very different properties, molybdenum additions are used for improving corrosion resistance in all these subgroups.
To view entertaining and informational video presentations about stainless steel's recyclability, the nature of its protective passive film, and the role of alloying elements in stainless steel, see the Team Stainless videos:
Stainless Steel Videos
