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What is Stainless Steel?

Date: 2013-11-29

Stainless steel is a general name for all stainless steels. Especially nickel and molib are used for making the steel stainless; but, the main element that makes the steel stainless is chrome.

The minimum chrome needed for steel to become stainless is 10,5 percent as a mass. The most of the produced steel is carbon and alloyed steel. In reference to carbon and alloyed steel, the market of the stainless steel is little but is inviting and developing.

Only gold and platinum are found in pure form in the nature; normal metals are combined with other elements. Thus, corrosion is a natural fact. Found in the nature in iron source, iron is thus unbalanced and wants to corrode. Corrosion is combining the pure iron with oxygen in water.

A rust layer forms on carbon steel that does not have any protective plating and protects the remaining parts of the steel. In taking the rust layer on the surface, a new rust layer forms. This is called corrosion. Dying, galvanizing, plating with epoxy resins stops the corrosion process. The stainless steel used in various usage fields competes with carbon steels, aluminum, brass and bronze. The competence power of the stainless steel comes from its surpassing and charismatic feature.

The chrome in stainless steel is very near to oxygen. When chrome meets oxygen, an chrome oxide film forms on the surface of steel in molecular level. The thickness of this film is 130 angstrom ( 1 angstrom = 10-6 cm. ). This is like protecting a big building with a roof plate in a thickness of a letter paper.

Chrome oxide layer is passive, strong and self-reforming. Passive means that it does not react with other elements. Strong means that is strictly cohesive to steel surface and unfixable. Self-reforming means that if chrome oxide layer is worn away or rubbed away from the surface by force, the chrome inside the steel forms a new chrome oxide layer reacting with air again.

If a jacknife made from steel is used for a long time, it rubs away and becomes blunt; if a whetstone sharpens, the chrome oxide layer is worn away. By this self reformation of chrome, it maintenances its nonrusting. The stainless steel has four main groups; austenitic, martensitic, ferritic and precipitation hardening. Austenitic stainless steel chrome is composite of nickel and manganese. This group of stainless steel are in 200 and 300 series. 304 stainless steel is famous in this group. Ferritic steel is in 400 series. The main element in this kind of steel is chrome. The process of this group steels is higher than 200 and 300 series. Martensitic stainless steel are in 400 series but they contain less chrome, more carbon than ferritic steel. They need heating to make them harder. Precipitation hardened stainless steel is a minor group of stainless steel, they contain high chrome and nickel. They harden due aging. They are similar to martensitic stainless steel in feature.

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