Passivation
The passivation or passivity represents a state of metals or alloys in which them speed of corrosion is notably slowed down by the presence of a natural or artificial passive film, compared to what it would be in the absence of this film.
In the majority of the cases (aluminum, steel, stainless steel, titanium…), this passive film appears spontaneously by oxidation, because the oxide formed on surface is insoluble and constitutes an obstacle which slows down the later processes. The formation of this film is related to a field of electrochemical potential as to a field of pH in which the oxide is stable.
Figure 1 (curve intensity-potential to come) represents the current of dissolution of metal according to the electrochemical potential. This one increases exponentially in the absence of lorque film the potential increases, then starting from a certain potential known as " of passivation" , one observes a drastic fall of the current of dissolution, generally of several orders of magnitude. This phenomenon expresses the appearance of passive film, very generally caused by the formation of a nonporous protective oxide. This film slows down one of the key stages of the process of corrosion, either the transport of matter until the metal interface, or the transport of electrons necessary to oxidation because the film is not very conducting. By increasing the potential further, one observes a zone or the current varies little with the potential, called " field of passivité". Then, beyond by a certain value, the current increases again quickly. The passive film became unstable with this potential and its protection diparait. The " field was reached; transpassif".
The presence of passive film is also related to a field of stability in pH. Generally, the film becomes unstable towards the acid media, in on this side pH which depends on material, like worms the very basic mediums. Thermodynamic diagrams potential-pH of metal make it possible to envisage with a sufficient approximation these field of stability (figure 2, to come)
One can also obtain passivation in an artificial way, by the treatment with a suitable product which makes it possible to obtain a barrier layer (usually, a product Alcalin) of a Métal deoxidized beforehand with a Acide to prevent it quickly réoxyder.
To deoxidize a piece of metal, one proceeds to a Dissolution of the Oxyde using an attack with a Acide adapted. It results a Ionization H+ from it from the surface of this part (pH <7: acid).
In contact with the air, the Ions H+ combine with the Ions OH- and thus support a reoxidation of the part.
One thus carries out a treatment, for example with a product Alcalin (H), to eliminate this Ionization and thus to prevent this recombination. This treatment is named passivation.
In the case of the Stainless steel, passivation is an operation which consists in eliminating the foreign compounds of Fer on the surface from this metal by chemical dissolution so that iron, sensitive to oxidation, is eliminated from the surface of the part.
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