Passivation
Introduction
Passivation is the formation of a relatively inert film on the surface of a material (frequently a metal), which masks it against the action of external agents. Although the reaction between the metal and the external agent is thermodynamically feasible at a macroscopic level, the passivating layer or film does not allow them to interact, in such a way that the chemical or electrochemical reaction is reduced or completely prevented.
Passivation should not be confused with immunity, in which the base metal is itself resistant to the action of corrosive media, for example gold and platinum, which do not oxidize easily and are therefore called noble metals.
In many cases, the formation of this passivating film is spontaneous when the metal comes into contact with the external agent. A classic example is aluminum. When a surface of this metal comes into contact with ambient air, the outermost part of the object spontaneously oxidizes to form a transparent and impermeable layer of ceramic-type AlO alumina, very congruent and adherent. For this reason, although aluminum is thermodynamically very reactive, the passivating layer protects it very effectively against corrosion under ordinary conditions. To achieve corrosion of this metal, mineral acids or a certain electrochemical overpotential are required. Another typical case is stainless steel. As a result of its chromium content, this alloy naturally forms an oxide layer a few angstroms thick, and is thus protected against many corrosive agents, finding wide use in industry and daily life.
On the other hand, the formation of a passivating film is not limited to the oxidation of a base metal. There are also cases where the passivating film is formed by reduction. In this case it may be the product of the electrochemical reduction of some oxide or sulfide. For example, direct electro-refining of copper matte (copper sulfide) has been attempted without going through the metallurgical converter stage. However, the reduction of sulfide forms a passivating film of elemental sulfur that hinders the process, which is why this alternative is still under investigation, which shows that the formation of this passivating layer is not always desirable.
Passivation as a manufacturing process
There are many techniques to promote, strengthen or even artificially create a passivating film on metals, such as parkerization") or phosphating, bluing, anodizing, etc. However, the name or is usually reserved for the process of forming an external layer to the metal, in order to isolate it from the outside. This external layer is formed by an association of the metal itself (or another metal added to the alloy for this purpose, such as chromium) with oxygen, forming an M-O-M chain.