Stainless Steel

There are three main types of stainless steels which are identified by their micro-structure or predominant crystal phase.

• Austenitic:
  Austenitic steels have austenite as their primary phase. These are alloys containing primarily chromium and nickel structured around a composition of iron, 18% chromium, and 8% nickel. Austenitic steels are not harden-able by heat treatment. The most familiar of the austenitic family of stainless steel is probably Type 304, sometimes called T304 or simply 304.
• Ferritic:
  Ferritic steels have ferrite 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 harden-able by heat treatment.
• Martensitic:
  Martensite microstructure was first observed around 1890.  Martensitic steels are low carbon steels built around a Type 410 composition of iron, 12% chromium, and 0.12% carbon. They may be tempered and hardened. Martensite gives steel great hardness, but it also reduces its toughness and makes it brittle, so few steels are fully hardened.

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 even be tinted over a broad spectrum of colors.

Stainless steel is often perceived as being resistant to corrosion, relatively inert and requiring minimal treatment in fabrication, and little maintenance. The term passivity refers to the natural corrosion resistant property of many metals and alloys including chromium, titanium and stainless steels. Passivity is conferred on stainless steel by an invisible film of chrome oxide. The stability of this film depends largely on the corrosive environment in which the stainless steel is placed.

In a clean environment the passive film forms spontaneously following its removal over a period of at least 8 to 24 hours. This means that stainless steels are self passivating "under favorable conditions".

Disruption of the passive film by chemicals, mechanical action, embedded iron particles, or oxygen starvation can readily occur. Surface free iron particles, dust, grit and iron oxide contaminants arise from handling, fabrication/forming, welding, grinding, machining, paint and crayon marks, polishing, tumbling and workshop cross contamination.

These contaminants penetrate the passive film, absorb and generate chlorides, ferric chloride or produce inorganic chlorides by the decomposition of organic compounds.

It should be understood that this passive surface condition is not a static situation. The chrome oxide layer is constantly affected by the environment and is slowly lost, but at the same time it reacts with oxygen to reform. The process is in dynamic equilibrium. Only when the balance is brought towards loss of the passive film does corrosion occur.

The passive film restoration is dependent on the availability of oxygen for its formation.  Where the chromium content is in excess of 12.0%, the formation of a chromium oxide passive film on the surface is possible either through auto-passivation or enhanced passivation methods..

Passivation

There is some dispute over whether the corrosion resistance of stainless steel can be enhanced by the process of passivation. Essentially, passivation is the removal of free iron from the surface of the steel. This is performed by exposing the steel in an oxidant, such as nitric or citric acid solution. Since the top layer of iron is removed, passivation diminishes surface discoloration and contamination. While passivation does not affect the thickness or effectiveness of the passive layer, it is useful in producing a clean surface for a further treatment.