Steel is considered stainless when it does not corrode when in contact with humid air or water.
Stainless steel is mainly composed of iron. By adding chromium, nickel and other elements in certain percentages, its molecular structure is altered and it is given different characteristics, making it resist to corrosion. Adding a minimum of 11% chromium is enough to guarantee that steel will be resistant to corrosion. This resistance is designated as “Passiveness”. The elements in stainless steel react easily with the environment (especially with the humidity that condenses on the cold metal) and one in particular, the chromium, helps to form a thin and adherent film that protects the metal from corrosive attacks. This cover is designated as “passive film”. Due to phenomena of chemical reactions, the passive film is strengthened with time. This thin, invisible and stable film is very adherent to the metal. Its resistance is strengthened as more chromium is added to the metal.
We can categorize steel considered stainless in three groups: Ferritic, austenitic and martensitic. In our activity we are only interest in the following groups:
– Martensitic: the chromium level in this steel is of 11% to 18%. Other elements might be added in smaller quantities (like nickel), but never more than 2,5%. As mentioned before, by adding chrome the surface of the steel is protected with the passive film, improving the resistance to oxidation. The steel maintains its magnetic feature.
– Austenitic: In this steel, the chromium level can vary from 17% to 26% and with the addition of nickel (7% to 22%), the steel loses its magnetic feature.
Corrosion resistance increases considerably. An addition of 3% of molybdenum improves the resistance against acids and adverse environmental conditions.
The SAE (Society of Automotive Engineers) together with the AISI (American Iron and Steel Institute), developed a table to classify stainless steel in its standard way according to their composition:
Brass is an alloy composed of copper and zinc, this last component being of 3% to 45%. Occasionally, other elements may be added in smaller quantities (like aluminum, tin, or arsenic) to enhance the alloy.