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In many situations galvanic protection or painting of a mild steel surface is impractical. This is where the special oxide film formed on chromium alloy stainless steels becomes so useful. Many types of stainless steel have been developed to resist different corrosive environments and working conditions, thereby ensuring that factories are safe, structures last longer and food is hygienic. Stainless steel is even used in systems to clean up the exhaust gases from cars and power stations. A recyclable material, it can also be melted down to become new material.
Stainless steels are chromium containing steel alloys. The minimum chromium content of standardized stainless steel is 10.5%. Chromium makes the steel 'stainless', which means improved corrosion resistance, as can be seen in the chart provided. The better corrosion resistance is due to a chromium oxide film that is formed on the steel surface. This extremely thin layer, under the right conditions, is also self-repairing. Besides chromium, typical alloying elements are molybdenum, nickel and nitrogen. Nickel is mostly alloyed to improve the formability and ductility of stainless steel. Alloying these elements brings out different crystal structures to enable different properties in machining, forming, welding etc.
Four major types of stainless steel are:
Austenitic is the most widely used type of stainless steel. It has a nickel content of at least of 7%, which makes the steel structure fully austenitic and gives it ductility, a large scale of service temperature, non-magnetic properties and good weldability. The range of applications of austenitic stainless steel includes housewares, containers, industrial piping and vessels, architectural facades and constructional structures.
Ferritic stainless steel has properties similar to mild steel but with the better corrosion resistance. The most common of these steels are 12% and 17% chromium containing steels, with 12% used mostly in structural applications and 17% in housewares, boilers, washing machines and indoor architecture.
Ferritic-Austenitic (Duplex) stainless steel has both ferritic and austenitic lattice structures - hence common name: duplex stainless steel. This steel has some nickel content for a partially austenitic lattice structure. The duplex structure delivers both strength and ductility. Duplex steels are mostly used in petrochemical, paper, pulp and shipbuilding industries.
Martensitic stainless steel contains mostly 11 to 13% chromium and is both strong and hard with moderate corrosion resistance. This steel is mostly used in turbine blades and in knives.
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