Pourbaix diagram, of aluminum

Fig. 15 Potential-pFI diagram, the so-called Pourbaix diagram of aluminum in water at 25 °C for hydrargillite as the most stable form of hydrated aluminum oxides.

The Pourbaix diagram of aluminum, presented in Fig. 4.3, indicates that hydrargil-lite, A1203-3H20, is the stable phase between about pH 4 and 9 [3]. Indeed, this film is considered to be responsible for the successful use of aluminum in many structural applications. This diagram also predicts the amphoteric nature... 

The E-pH diagram of aluminum and zinc are quite similar and surely amongst the simplest E-pH diagrams of all metals. The Pourbaix diagram of aluminum 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... 

Figure 2. The potential-pH (Pourbaix) diagram for aluminum in aqueous medium, defining regions of thermodynamic stability of the different species.

Figure 9.15 The Pourbaix diagram for aluminum and water illustrates the stable phases for the different potentials and pHs. (Reproduced by kind permission of NACE Int., Houston, USA)...

B g 9 Pourbaix diagram for aluminum showing Ihe condilioni of corrosion, im-muniiy, and passivation of aluminum at 25 °C (77 °F), assuming protection by a Rim of barite, AI203-3H20. Source Ref 3... 

Aluminum is an active metal and its resistance to corrosion depends on the formation of the protective oxide film. According to the Pourbaix diagram the metal is passive in the pH range —4-9. The protective oxide film formed in water and atmospheres at ambient temperatures is amorphous and a few nanometres in thickness. The stability of the oxide film and its disruption results in corrosion. 

Example 2.14 Calculate and draw the equilibrium lines in the aluminum Pourbaix diagram (in aqueous media) for a total dissolved aluminum concentration of 10-6 M. Assume that the main species existing in equilibrium and their corresponding values of A are those shown in the table. 

Fig. J5M Potential/pH diagram for aluminum. The solid phase is assumed to be hydrargillite (Al O -3H O). Filled areas represent regions where soluble species are stable and therefore corrosion can thermodynamically occur. Data from Pourbaix in Atlas of Electrochemical Equilibria in Aqueous Solutions", Pergamon Press, 1966.

The corrosion resistance of aluminum and its alloys has been well documented [129-132]. The Pourbaix diagram, accepting its limitations, shows that aluminum remains passive in the pH range 4-9. Outside this range, active dissolution occurs according to the following reactions ... 

Figure 4.3. Pourbaix diagram for the aluminum-water system at 25°C [2]. (M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd English edition, p. 171, copyright NACE International 1974 and CEBELCOR.)...

The potential-pH diagram (Pourbaix diagram) for the aluminum-water system at 298 K is shown in Fig. 3-1 (Pourbaix, 1966). The equilibrium diagram must be applied to realistic cases in particular it is valid in the absence of substances with which aluminum can form soluble complexes or insoluble salts. The aluminum complexes are those formed with the organic anions such as acetate, citrate, tartrate, oxalate, etc., and fluorine complexes. 

Another reason for protective ability of chromium oxide and hydroxide film over aluminum surfaces is their stability over a wider range of pH. Based on Pourbaix-diagrams, the approximate stability limit of the A1 oxide is at pH 9, while it is up to pH 15 for Cr(lII) oxide. ... 

Pourbaix potential-pH diagrams can be used to predict the regions of oxide stability and of uniform corrosion in atmospheric types as shown in Figure 4.5. Note that aluminum is thermodynamically stable only at low potentials. Because of its property of developing a protective coating of... 

The conditions for thermodynamic stability of the oxide film are expressed by the Pourbaix ( tential versus pH) diagram shown in Fig. 2. As shown by this diagram, aluminum is passive (is protected by its oxide film) in the pH range of about 4 to 8.5. The limits of this range, however, vary somewhat with temperature, with the specific form of oxide film present, and with... 

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