Durability and resistance of stainless steel

Contrarily to what people generally believe, chrome is a very chemically reactive metal that oxidizes easily, but it’s oxide forms a transparent and protective layer. Alloyed to iron and nickel, it causes the formation of an oxided surface component capable of reducing or stopping corrosion.

Chrome and nickel oxide this way :

4 Cr + 3 O2 -> 2 Cr2O3
2 Ni + O2 -> 2 NiO

Stainless is a misleading and inappropriate term

There is a variety of stainless steels, and the type may be difficult to choose. Each stainless steel reacts differently to its environment. The type is often represented by the mass percentage of nickel and chrome. An inox 18\10, used in cutlery and cooking in general, contains 18% of its mass in chrome and 10% in nickel. This designation is insufficient since it does not refer to its metallurgical structure.

The chrome content is always at least 12%. Other alloying elements, for most “noble” metals such as nickel, molybdenum, copper or brass, improve its chemical resistance mostly in non oxidant environments.

The characteristics of the resistance of these alloys were discovered in 1913 when it was noticed that some polished samples for laboratory exams wouldn’t oxidize. In fact, we could say that:

• Stainless Steels can be corroded in cold temperatures only in the presence of humidity. Stainless steels resist to chlorine, very corrosive gasses, as long as it is perfectly dry.
• In the presence of aqueous solutions, electrochemical corrosion takes over direct chemical corrosion ; The resistance of the material depends, as said earlier, on the electrochemical potentials of the surface and its distribution.
• Like aluminium which is very likely to oxidize and covers itself with a protective oxide; stainless steels actively behave when just manufactured, stripped, or polished, and passively when exterior attacks allows it to form its protective “skin”
• A good use of stainless steels requires a very homogeneous metal to avoid local corrosion and a transition from the active to the passive state everywhere on the exposed surface.

Compared to a referenced hydrogen electrode, stainless steel’s potential is set between molybdenum and mercury, close to silver and platinum.

Ferrous deposits on stainless steel surfaces are very dangerous in humid environments since rust is used as a catalyst and the surface ends up being punctured.