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Definition of Stainless Steels


Definition of Stainless Steels
The Stainless Steels are alloys iron, chrome and carbon based, enriched with other elements such as nickel, molybdenum, silicon, titanium and more.

Their main characteristic is the high resi stance to the corrosion of the atmosphere.

This characteristic is due to the spontaneous formation of a thin layer of chrome oxides (Pict.1) that protects the metal against the corrosive attacks.



FIG. 1: protective self-passivating layer

This layer, very stable and resistant, avoids the direct touch between the surrounding atmosphere and the inner steel and, unlike the common coating treatments (zinc-plating, coating and so on), can reform after an accidental break, making the very material self resistant to corrosion.

In simple words, we could state that stainless steels are indeed “prone to stains”, in the sense that have a high capability to self-passivation, so they cover themselves with an invisible layer of oxides (we are talking about molecular dimensions) that protects from corrosive attacks the metal below. As already stated, this phenomenon “repeat itself” even in the case the protective layer is teared (for instance through abrasion or removal).

In these alloys the chrome has never to be less than the 11%, that because it needs to be prone to the formation of oxides, as you can see clearly in fig.2, in which is showed the loss of weight through the oxidation of Steel (corrosion) depending on the percentage of chrome present in the alloy (pict. 2)



FIG. 2: loss of weight compared to the Chrome %

The chrome oxides protective layer displays a high resistance and anchorage to the material, depending on the chrome concentration in the alloy and on the possible presence of other elements, like nickel, molybdenum, titanium and so on.

There are different degrees in which a metal can be Stainless or, in other words, a scale of nobility of the different types of Stainless Steels, due to the presence in the alloy of the elements named before.

All these characteristic allow the use of materials in environment which are particularly aggressive, such as chemical facilities or sea environment, but it is recommended in the common architectural and outdoor application as well, to obtain long lasting handiworks.