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 of such computation for aluminum in the presence of water at 25 oC when the activities of all species considered were set at value unity.Į-pH diagram of solid species of aluminum when the soluble species are at one molar concentration (25 o C) Possible reactions in the Al-H 2 O systemĪ 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. These reactions are summarized in the following Table. 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.e. This is represented, in the E-pH diagram shown below, by a dotted vertical line separating the dominant presence of Al 3+ at low pH from the dominant presence of AlO 2 - at the higher end of the pH scale.Į-pH diagram showing the soluble species of aluminum in water at 25 oC
#Activity diagrams thermodynamics free
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.Īssuming that the activity of H 2O is unity and that the activities of the two ionic species are equal, one can obtain a simpler expression of the equilibrium in equation based purely on the activity of H +, equation and its logarithmic form, equation.Īnd if G 0 is expressed in Joules and the temperature is 25 oC or 298 K equation is even further simplified.īy using the standard thermodynamic data from the literature, it is possible to calculate that the free energy of reaction is in fact equal to 120.44 kJ mol -1 when both and are equal. The first equilibrium to consider examines the possible presence of either Al 3+ or AlO 2 - expressed in equation. In the following discussion, only four species containing the aluminum element will be considered: ( reference) It will be used here to demonstrate how such diagrams are constructed from basic principles. The E-pH diagram of aluminum is one of the simplest E-pH diagrams. Corrosion: Impact, Principles, and Practical Solutions
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