Metals, excluding precious metals such as gold or platinum, found in their natural state, are always extracted from minerals (oxides, sulphides, various chemical compounds). Most metals tend to deteriorate in contact with the atmosphere, water or other natural or industrial environments and gradually return to their original compound state. Metal corrosion is equivalent to a chemical dissolution or electrochemical dissolution of a metal or an alloy, causing material losses dependent on the type of material and the nature of the environment (chemical composition, temperature, etc). In the case of steels, the term corrosion is applied to phenomena developed in the liquid phase (aqueous or organic solutions, salts or molten metals), as opposed to oxidation (or "dry" corrosion) which occurs at high temperature.
Corrosion can be limited, even avoided, if the chemical reactions that take place do not lead to the formation of a soluble type of oxide, but rather to the appearance of a stable, protective oxide layer or compound. If these compounds form a thick enough compact layer (called a "passive film" or "passive layer") on the surface of the metal, the risk of corrosion is considerably reduced by the phenomenon of passivation.
Stainless steels are a modern solution to the problem of corrosion. The addition of highly oxidisable elements such as chromium increases the tendency to form stable oxides. Paradoxically, it can be stated that the corrosion resistance of stainless steels is due to the particularly oxidisable nature of one of its elements, chromium, provided that it is present in the proportion of at least 10,5%. Corrosion resistance is obtained by the spontaneous formation in the open air of a very thin passive film, no thicker than about ten nanometers (the equivalent of a protection given by a sheet of paper placed on a 20-storey building). The stability of the passive film is the factor determining the corrosion resistance of stainless steels and it depends on various factors, the main ones being: nature of the corrosive environment, composition of the steel, surface condition, any contact with other materials. When stainless steel is used correctly and the appropriate grade chosen to suit the surrounding medium, the passive film is spontaneously regenerated after accidental damage.
Wrong choices and the prevalence of various conditions, may lead to failure of stainless steels and the emergence of corrosion. In their case, corrosion may not appear with the obvious rust visible in common steels. The effects of corrosion often propagate with fast rate and destructive results. Different media can cause different types of corrosion attack that may vary in nature and appearance, and several forms of corrosion can occur on stainless steels. Below, follows a short presentation of the most common forms of corrosion that can attack stainless steels.
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