High resistance to atmospheric oxidation
It is an iron and chromium alloy that may also contain nickel, molybdenum and other elements, presenting higher physico-chemical properties than ordinary steels, and its main characteristic is high resistance to atmospheric oxidation.
These alloying elements, in particular chromium, impart excellent corrosion resistance when compared to carbon steels. They are, in fact, oxidizable steels. That is, the chromium present in the alloy oxidizes in contact with the oxygen in the air, forming a very thin and stable film of chromium oxide – Cr 2 O 3 – which forms on the surface exposed to the medium. This is called a passive layer and its function is to protect the surface of the steel against corrosive processes. For this a minimum amount of chromium of about 11% by mass is required.
This film is adherent and impermeable by insulating the metal below it from the aggressive medium. Thus, care must be taken not to locally reduce the chromium content of stainless steels during processing. This process is known in metallurgy as passivation.
Passivation is based on electrochemical kinetics, which results from the formation of protective films on the metal surface by imposing currents. This process serves to ensure the corrosion resistance of the component or part and hence increase its durability.
Nitric acid is one of the most commonly used reagents for this purpose in commercially available passivation treatments for stainless steels. Weaker acids, such as citrus, may also aid in the formation of the passive layer.