In 1913, English metallurgist Harry Brearley, working on a project to improve rifle barrels, accidentally discovered that adding chromium to low carbon steel makes the steel stain resistant. In addition to iron, carbon and chromium, modern stainless steel can also contain other elements, such as nickel, niobium, molybdenum and titanium.
Nickel, molybdenum, niobium and chromium improve the corrosion resistance of stainless steel. It is the addition of at least 12 percent chromium to the steel that allows it to resist rust, or staining 'less' than other types of steel. The chromium in the steel combines with oxygen in the atmosphere to form a thin, invisible layer of chromium-containing oxide, often called the passive film.
The sizes of chromate atoms and their oxides are the same, so they pack nicely together on the surface of the metal and form a stable layer that is only a few atoms thick. If the metal is cut or scratched and the passive film is disturbed, more oxide will quickly form and restore the exposed surface, protecting it from oxidative corrosion.
The passive film requires oxygen to repair itself, so stainless steel has poor corrosion resistance in low oxygen and poor circulation environments. In seawater, chlorides from the salt will attack and destroy the passive film faster than it can be repaired in a low oxygen environment.
The three main types of stainless steel are austenitic, ferritic and martensitic. These three types of steel are identified by their microstructure or dominant crystal phase.
Austenitic: Austenitic steels have austenite as the 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 percent chromium and 8 percent nickel. Austenitic steels are not hardenable by heat treatment. The best known stainless steel is probably Type 304, sometimes called T304 or simply 304. Type 304 surgical stainless steel is austenitic steel containing 18-20 percent chromiumand 8-10 percent nickel.
Ferritic: Ferritic steels have ferrite (body-centered cubic crystal) as the main phase. These steels contain iron and chromium, based on the type 430 composition of 17 percent chromium. Ferritic steel is less ductile than austenitic steel and cannot be hardened by heat treatment.
Martensitic: The characteristic orthorhombic martensite microstructure was first observed by the German microscopist Adolf Martens around 1890. Martensitic steels are low carbon steels built around the type 410 composition of iron, 12 percent chromium and 0.12 percent carbon. They can be hardened and tempered. Martensite gives steel great hardness, but it also reduces toughness and makes it brittle, so few steels are fully hardened.
There are also other grades of stainless steel, such as precipitation hardened, duplex and cast stainless steel. Stainless steel can be produced in a variety of finishes and textures and can be colored over a wide range of colors.
There is disagreement among professionals as to whether the corrosion resistance of stainless steel can be improved by the passivation process. Essentially, passivation is the removal of free iron from the steel surface. This is done by immersing the steel in an oxidant, for example nitric acid or citric acid solution. Since the top layer of iron is removed, passivation reduces the discoloration of the surfaces.
Although passivation does not affect the thickness or efficiency of the passive layer, it is useful to produce a clean surface for further treatment, such as acid washing or painting. On the other hand, if the oxidant is incompletely removed from the steel, which sometimes occurs in pieces with tight joints or corners, crack corrosion can occur. Most studies indicate that reduced surface particle corrosion does not reduce susceptibility to local corrosion.
Acid-resistant stainless steel is actually neither acid-resistant nor stainless. It is a layman's term used colloquially for stainless steel that contains 2-3 percent molybdenum (Mo). As mentioned, it is a term used to describe steel that is alloyed with molybdenum and thus has improved corrosion resistance. The most common acid-resistant steel is ASTM 316. However, it is important to point out that "acid-resistant steel" does not necessarily mean that the material is immune to acid in corrosive environments.
