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.
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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.
E-pH diagram showing the soluble species of aluminum in water at 25oC The next phase for constructing the aluminum E-pH diagram is to consider all possible reactions between the four chemical species containing aluminum retained for this exercise, i. Al, Al2O3. These reactions are summarized in the following Table.
Possible reactions in the Al-H2O system A computer program would compare all possible interactions and rank the chemical species involved in terms of their thermodynamic stability for all conditions of pH and potential would typically carry out this work.
The following Figure illustrates the results of such computation for aluminum in the presence of water at 25oC when the activities of all species considered were set at value unity. E-pH diagram of solid species of aluminum when the soluble species are at one molar concentration 25oC However, an additional consideration is necessary to make such diagrams useful for corrosion situations for which the presence of soluble species in the environment never reaches values of the order of one molar.
The following Figure illustrates the results that were computed by setting the concentrations of soluble species at decreasing values of one , one hundredth , one in ten thousand , and one in a million The apparent stability of the solid species considered gradually recedes as lower values of soluble species are used in the calculations. E-pH diagram of aluminum with four concentrations of soluble species 25oC It is customary to use the lowest boundary as a practical indication of the corrosion stability of a metal and its solid products Figure 4.
E-pH corrosion diagram of aluminum at 25oC Immunity region: In the conditions of potential and pH of that region a metal is considered to be totally immune from corrosion attack and safe to use.
Cathodic protection may be used to bring the potential of a metal closer to that region by forcing a cathodic shift, as shown for aluminum by the domain specified in the previous Figure CCSRE Passive region: In such region a metal tends to become coated with an oxide or hydroxide that may form on the metal either as a compact and adherent film practically preventing all direct contact between the metal itself and the environment, or as a porous deposit which only partially prevents contact between the metal and the environment; Corrosion region: Thermodynamic calculations indicate that, in such region of an E-pH diagram, a metal is stable as an ionic soluble product and therefore susceptible to corrosion attack.
Experience is required to find out the extent and form of the corrosion attack that may occur in the corrosion region s of a Pourbaix diagram.
Diagrama 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. Si aparecen con trazado discontinuo fino indican un equilibrio entre dos especies solubles. Las dos lneas discontinuas gruesas sealadas como a y b, que representan el equilibrio de descomposicin del agua con desprendimiento de oxgeno e hidrgeno, respectivamente. La regin entre las dos lneas representa una zona donde el agua es estable con respecto al oxgeno y al hidrgeno.
Diagramme de Pourbaix
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