What is stainless steel?

The most general definition of stainless steel is an alloy of iron and chromium, with at least 10.5 percent chromium. When exposed to oxygen or oxidizing solutions, such as nitric acid, stainless steels form a thin, protective "passive film." Although the passive film is commonly called a chromium oxide layer, it is not a true oxide film like rust; scraping the surface of stainless steel will not result in chromium oxide dust.

Stainless steel is divided into two general classes based upon the atomic crystalline structure. Ferritic steels are magnetic and the atoms are arranged in a body-centered cubic arrangement. In general, ferritic steels lack intentional additions of nickel, cannot be hardened by heat treating and are only slightly hardened by cold working. Most ferritic grades lack toughness and are typically used in light gauge applications, for example in automotive exhaust systems and kitchen surfaces. Some ferritic grades, for example Type 444, are exceptions to this generalization.

Austenitic steels are not magnetic and the atoms are arranged in face-centered cubic arrangement. Unlike ferritic grades, austenitic steels do contain nickel (with the exception of some special alloys that substitute manganese for nickel), have excellent workability and are hardened by cold working, imparting toughness. Austenitic is by far the most common type of stainless steel.

The common alloys and significance of carbon

Although there are hundreds of stainless steel alloys available for countless applications, breweries routinely use only a few general-purpose types. The most common stainless alloy is type 304 and contains at least 18 percent chromium, eight percent nickel and no more than 0.08 percent carbon, two percent manganese, one percent silicon, 0.045 percent phosphate and 0.03 percent sulfur. Nickel promotes the stability of austenite, aids the formation of passive film and increases toughness, ductility and ease of welding.

Type 316 contains at least 16 percent chromium, 10 percent nickel and two percent molybdenum and no more than 0.08 percent carbon, two percent manganese, one percent silicon, 0.045 percent phosphate and 0.03 percent sulfur. The most notable difference between type 304 and 316 is molybdenum. Type 316 is alloyed with molybdenum for improved pitting and crevice corrosion resistance.

Almost all austenitic steels are available in so-called "L grades." The L grades, for example 304L and 316L, contain no more than 0.03 percent carbon. The carbon content of stainless steel effects a phenomenon known as "sensitization." At certain high temperatures, chromium-rich carbides will form in carbon-containing stainless steels.

This sensitization depletes the chromium content of the surrounding steel making it susceptible to certain types of corrosion. This phenomenon can occur during steel production, welding or in steels used for high temperature service. When caused by welding, it is known as "weld decay" and the area around the weld is known as the "heat affected zone." Low carbon steels effectively prevent sensitization. Regular alloys affected by sensitization can be annealed to dissolve the carbide and then rapidly cooled.