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The Passivation Layer
The passivation layer on metallic surfaces is a thin layer of oxide that forms to varying degrees (depending on the magnitude of the free energy of formation of the metallic oxide and the availability of oxygen or other species at the surface). This oxide layer can serve to greatly reduce the transport of corrosive species to the underlying metal's surface. Freshly exposed metallic surfaces will adsorb and react with oxygen present in the atmosphere almost instantaneously. One exception to this phenomenon is gold. Gold's oxide is unstable, as indicated by the negative value for its oxide's heat of formation. The following table provides a list of the heats of formation of some metal oxides in decreasing order. (reference)
Heats of Formation of Some Metal Oxides
|
Metal |
Heat of Formation
of Lowest Oxide (kcal/mol) |
|
Tantalum |
500.1
|
|
Aluminum |
389.5
|
|
Chromium |
267.4
|
|
Uranium |
256.6
|
|
Titanium |
217.4
|
|
Vanadium |
209.0
|
|
Iron |
64.0
|
|
Gold |
-12.0
|
Metals at the top of the table form oxides quite readily while those at the bottom form passive layers much more slowly (or, in the case of gold, not at all). The tenacity of the passive layer is also higher for the metals at the top of the table. However, in the presence of mechanical abrasion, even the most tenacious passive film can be breached. Once this occurs, the surrounding chemistry (i.e., the availability of free oxygen) and the nature of the metal-oxide bond will determine the rate at which the protective layer is repaired. If an insufficient amount of oxygen is available, the layer may remain damaged and corrosion is likely to occur.
Numerous pages of the Corrosion Doctors Web site discuss passivation related topics: Beer, Biomaterials, Blocking, Calcareous deposits, Electrochemical noise, Electrode passivation, Galvanized, Inhibitors, Iron, Nickel aluminum bronze, Oxidizers, Passivation layer, Passive curve, Passivity, pH, Pickling, Pitting, Potentiodynamic polarization, Rouging, Stainless steels, Steel, Stress corrosion cracking, Surface contaminants
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