It will be used here to demonstrate how such diagrams are constructed from basic principles. In the following discussion, only four species containing the aluminum element will be considered: reference two solid species Al and Al2O3. Since there is no change in valence of the aluminum present in the two ionic species considered, the associated equilibrium is independent of the potential and the expression of that equilibrium can be derived in the following expression for standard conditions. By using the standard thermodynamic data from the literature, it is possible to calculate that the free energy of reaction is in fact equal to Equation then becomes equation.
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On either side of the line, one form of the species will instead be said to be predominant. Usually, the activity of a species is approximated as equal to the concentration for soluble species or partial pressure for gases.
The same values should be used for all species present in the system. Sometimes additional lines are drawn for other concentrations. For instance, carbonate has a great effect upon the diagram for uranium. See diagrams at right. The presence of trace amounts of certain species such as chloride ions can also greatly affect the stability of certain species by destroying passivating layers.
In addition, changes in temperature and concentration of solvated ions in solution will shift the equilibrium lines in accordance with the Nernst equation. The diagrams also do not take kinetic effects into account, meaning that species shown as unstable might not react to any significant degree in practice.
A simplified Pourbaix diagram indicates regions of "immunity", "corrosion" and "passivity", instead of the stable species. They thus give a guide to the stability of a particular metal in a specific environment. Immunity means that the metal is not attacked, while corrosion shows that general attack will occur.
Passivation occurs when the metal forms a stable coating of an oxide or other salt on its surface, the best example being the relative stability of aluminium because of the alumina layer formed on its surface when exposed to air.
Analisis Diagramas de Pourbaix
La presencia de agentes complejantes, como por ejemplo, cloruros, bromuros, cianuros, amoniaco, carbonatos, etc. Como consecuencia, en los diagramas aparecen nuevos dominios para las especies inicas en disolucin que ocupan espacios ms amplios que los correspondientes a los sistemas metal-agua. Tambin se puede observar que las lneas aparecen representadas en el diagrama con dos tipos de trazado: continuo y discontinuo fino. Si aparecen con trazado continuo indican un equilibrio bien entre dos especies slidas o bien entre una especie slida y una especie soluble.
Diagrama de Pourbaix