Who invented stainless steel?
Answer:  Go to our History section for information on stainless steel’s development.

What is stainless steel?
Answer:  Stainless steel is the general name for the family of iron-based alloys that contain at least 10.5% chromium. When there is sufficient exposure to oxygen, an invisible protective passive chromium-rich oxide film forms on the surface. This invisible film forms automatically as long as the surface is clean and exposed to oxygen. Higher levels of chromium and the addition of other alloying elements such as nitrogen and molybdenum enhance this surface layer and improve the corrosion resistance of the stainless material. See the "Stainless Steel Overview" for more information.

What is the difference between 18/8 and 18/10 stainless steel?
Answer:  The first number is the amount of chromium that is contained in the stainless, so 18 is 18% chromium. The second number is the amount of nickel, so 8 stands for 8% nickel. So 18/8 means that this stainless steel contains 18% chromium and 8% nickel. 18/10 is 18% chromium and 10% nickel. Both 18/8 and 18/10 are often used to describe Type 304 or austenitic (300-series) stainless steels with similar chromium and nickel levels. The ASTM chemistry requirements for the stainless steel grades that are sometimes identified with these designations vary, but generally they contain 17 to 20% chromium and 8 to 10.5% nickel. Although a higher nickel content is implied, the term 18/10 is sometimes used strictly for marketing purposes when the stainless steel’s actual chemistry is closer to 18/8. These designations should never be used for buying raw materials.

What is the difference between Types 304 and 316 stainless steel?
Answer:  Both of these stainless steels contain chromium and nickel alloy additions, but Type 316 also has a molybdenum (Mo) alloying addition. Molybdenum improves corrosion resistance and is particularly helpful when there is exposure to chlorides (coastal or deicing salts) or corrosive pollutants. Type 304 contains 18 to 20% chromium and 8 to 10.5% nickel. Type 316 contains 16 to 18% chromium, 10 to 14% nickel and 2 to 3% molybdenum. Corrosion resistance increases with higher alloying additions of molybdenum.

What are AISI Specifications for stainless steel?
Answer:  The AISI (American Iron and Steel Institute) was the originator of the 300 and 400 series numbering system (i.e. Type 304 stainless steel). They also published a stainless steel products manual that listed these designations and the typical chemical analysis and mechanical and physical properties of each individual grade. These documents were NOT specifications or material standards. The AISI has not issued numbers for stainless steel for some time. Although AISI numbers are still commonly used to generically identify stainless steels, the UNS (Unified Numbering System) numbering system is preferred internationally to identify specific chemistries. AISI or UNS numbers only indicate the chemistry, so it is important to also use to the appropriate material standards. In North America, the ASTM International standards should be used. See "Specifications for Stainless Steel" for more information.

How do different alloying elements influence the properties and corrosion resistance of stainless steels?
Answer:  An overview of the alloying elements used in each stainless steel family and their role is provided on our website under Alloying Elements in Stainless Steel.

What are duplex stainless steels?
Answer:  The term "duplex" describes the microstructure that is characteristic of this family of stainless steels, which is typically equal parts austenite and ferrite. Duplex stainless steels are characterized by high strength levels and provide a range of corrosion resistance levels. They are used in applications ranging from moderately corrosive architectural applications to corrosive industrial and seawater environments. They have very good resistance the chloride stress corrosion cracking and provide better chloride crevice corrosion resistance than austenitic stainless steels with comparable pitting corrosion resistance.