What Is Stainless Steel and Why Is It Stainless ?
In 1913, English metallurgist Harry Brearly, working on a project to improve rifle barrels, accidentally
discovered that adding chromium to low carbon steel gives it stain resistance. In addition to iron, carbon,
and chromium, modern stainless steel may also contain other elements, such as nickel, niobium,
molybdenum, and titanium.
Nickel, molybdenum, niobium, and chromium enhance the corrosion resistance
of stainless steel. It is the addition of a minimum of 12% chromium to the steel that makes it resist rust, or
stain 'less' than other types of steel. The chromium in the steel combines with oxygen in the atmosphere to
form a thin, invisible layer of chrome-containing oxide, called the passive film. The sizes of chromium atoms
and their oxides are similar, so they pack neatly together on the surface of the metal, forming a stable
layer only a few atoms thick. If the metal is cut or scratched and the passive film is disrupted, more oxide
will quickly form and recover the exposed surface, protecting it from oxidative corrosion.
(Iron, on the other
hand, rusts quickly because atomic iron is much smaller than its oxide, so the oxide forms a loose rather
than tightly-packed layer and flakes away.) The passive film requires oxygen to self-repair, so stainless
steels have poor corrosion resistance in low-oxygen and poor circulation environments. In seawater,
chlorides from the salt will attack and destroy the passive film more quickly than it can be repaired in a low
oxygen environment.
Types of Stainless Steel
The three main types of stainless steels are austenitic, ferritic, and martensitic. These three types of steels are identified by their microstructure or predominant crystal phase.
Austenitic
Austenitic steels have austenite as their primary phase (face centered cubic crystal). These are alloys
containing chromium and nickel (sometimes manganese and nitrogen), structured around the Type
302 composition of iron, 18% chromium, and 8% nickel.
Ferritic
Ferritic steels have ferrite (body centered cubic crystal) as their main phase. These steels contain iron
and chromium, based on the Type 430 composition of 17% chromium. Ferritic steel is less ductile
than austenitic steel and is not hardenable by heat treatment.
Martensitic
The characteristic orthorhombic martensite microstructure was first observed by German microscopist
Adolf Martens around 1890. Martensitic steels are low carbon steels built around the Type 410 composition
of iron, 12% chromium, and 0.12% carbon.
There are also other grades of stainless steels, such as precipitation-hardened, duplex, and cast
stainless steels. Stainless steel can be produced in a variety of finishes and textures and can be tinted
over a broad spectrum of colors.
