Thursday, August 24, 2006

Identify Genuine Stainless Steel

Stainless steel is a ferrous alloy that has higher resistance to oxidation (rust) and corrosion in the present of chromium and nickel.

Chromium forms a passivation layer of chromium(III) oxide (Cr2O3) when exposed to oxygen. The layer is too thin to be visible, shiny, and impervious to water and air to protect the metal beneath. The Cr2O3 reforms quickly when the surface is scratched, a phenomenon called passivation by materials scientists, and is seen in other metals such as aluminium.

Nickel stabilize the austenite structure of iron to prevent stress corrosion cracking. Stress corrosion cracking is a rapid and severe form of stainless steel corrosion that forms when the material is subjected to tensile stress and chloride-rich environment (e.g. sea water) at higher temperature (e.g. 50°C). Nickel is the most expensive element in stainless steel. Nickel is also the material that cause a stainless steel non-magnetic and less brittle at low temperature.

The 400 series of stainless steel, which is usually used for kitchenware are magnetic positive. The 400 series is generally cheapest in price as there is no nickel material in this alloy.

The 300 series stainless steel that is used in chemical plant generally non magnetic, except the cold-rolled stainless steel. The better stainless steel is of grade 304 or 18-8 (18% chromium and 8% nickel) stainless steel. Stainless steel of 18-8 grade offers better corrosion resistance than the 400 series. Most architectural stainless work are done with the 300 series.

Cold-rolled 300 series stainless steel has similar physical composition (18% chromium and 8% nickel) and corrosion resistance of type 304. These are all nominally austenitic alloy (nonmagnetic), but some of the austenite transforms to martensite (magnetic) when the material is deformed, as in cold rolling. This martensite transformation causes the 300-series to be magnetic when cold formed. The cold-rolled material is much harder than annealed material. An annealed 304 stainless steel panel could dent easily.

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