Download/Embed scientific diagram | Diagrama de Pourbaix del hierro  y voltaje en función del pH (línea azul). from publication: Síntesis de nanopartículas. and radionuclide transport in groundwater. The most well-known studies on comprehensive Eh-pH diagrams are those of Pourbaix () and. Brookins ( ). Aluminum E-pH (Pourbaix) Diagram. The E-pH diagram of aluminum is one of the simplest E-pH diagrams. It will be used here to demonstrate how such.
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It will be used here to demonstrate how such diagrams are constructed from basic principles.
Diagrama de Pourbaix – Wikipédia, a enciclopédia livre
In the following discussion, only four species containing the aluminum element will be considered: 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 25 o C. 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. These reactions are summarized in the following Table. Possible reactions in the Al-H 2 O 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 25 o C 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 25 o C.
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 10 0one hundredth 10 -2one in ten thousand 10 -4and one in a million 10 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 25 o C.
It is customary to use the lowest boundary 10 -6 as a practical indication of the corrosion stability of a metal and its solid products Figure 4. E-pH corrosion diagram of aluminum at 25 o C. The usefulness of this graphical representation of thermodynamic data for corrosion studies was discussed by Pourbaix who showed three possible states of a metallic material: In the conditions of potential and pH of that region a metal is considered to be totally immune from corrosion attack and safe to dkagrama.
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 In such region a metal tends to pourbalx coated with an oxide or hydroxide that may pouraix on the diaframa 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.
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 pourbaox of the corrosion attack that may occur in the corrosion region s of a Pourbaix diagram.
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