AISI 304 Stainless Steel

It is a carbon steel alloyed with percentages of chromium, nickel, manganese and other elements in a lower quantity. Contrary to the general opinion also this steel oxidizes, but its life, if properly used, is generally much longer than any other type of protective coating.
Its specific behaviour is due to the fact that, in an oxidizing environment, chromium and nickel self-generate a chromium oxide fi lm that prevents from the subsequent oxidation. This fi lm is very delicate and it can break for hits, workings, weldings, etc. The stainless steel easily loses its property to be stainless if in the working tools made of other types of steel are used or, for example, when it is welded. In this case it is necessary to regenerate the protective fi lm through a chemical aggression process (decontamination or passivation). This treatment is specifi cally coded by the letter “J” instead “I” in the article code. It is recommended for installations in fresh water, industrial environment, marine environment, normal chemical industry, food industry (except for high temperatures), textile industry, pharmaceutical industry, paper industry and in the zootechnical breedings.

Decontamination or passivation - Pickling

The decontamination according to ASTM A/380-88 or passivation is a chemical treatment used to restore the natural passive state of stainless steel surfaces, removing traces of less noble metals or of various deposits, possible causes of corrosion starting. It is made by dipping the manufactured article in a nitric acid bath thinned for some minutes. Pickling is made instead when it is necessary to remove the fl akes of oxides resulting from welding or from high thermic treatments. This treatment, much more aggressive than the previous one, can be made mechanically (siliceous sand, steel balls, etc...) or chemically (thinned sulphuric acid or thinned nitric plus hydrofl uoric acid).

 

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