Aluminum oxides, stability diagram

Aluminum hydroxy species, 65,69,160 Stability constants, 69 Stability diagrams, 78 pH of minimum solubility, 65, 71, 72 Ammonium, 326, 331 Volatilization, 330 Oxidation, 334-336,472 Nitrate, 334-336,472 Adsorption, 336,465-466 Metal-ammine complexes, 460—461, 465... 

The stability diagrams of oxide/hydroxide of aluminum and silicon minerals/ aqueous solution systems are illustrated in Figures 1.6 and 1.7. In these diagrams, the following reactions were taken into account. 

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... 

Corrosion of A1 is an electrochemical process that involves the dissolution of metal atoms, only taking place once the oxide him has been dissolved or damaged. A1 is amphoteric in nature, its oxide film being stable in neutral conditions, but soluble in acidic and alkaline environments. The thermodynamic stability of aluminum oxide films is expressed by the potenhal versus pH diagram seen in Figure 16.3 [35]. 

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. 

For plotting the potential-pH diagram, characteristic limits of the domains are calculated from eqns. 39, 41, 43, 47, 49. For this purpose, concentrations of 10 , 10 and 10 mol 1 are taken into consideration. Additionally, the stability domain of water is plotted in the same diagram by dotted Knes. Aluminum is seen to be a very base metal as it has no domain of stability in common with water. Consequently it dissolves under hydrogen evolution (26) in aU solutions. In acids, the trivalent aluminum cation is formed, whereas aluminate ions are formed in alkaline solutions. In neutral solutions, however, the solubility of the oxide film formed is small enough to passivate the metal. 

In order to draw the potential-pH diagram for aluminum, it is neeessary to define concentrations of the dissolved species (Al " and Al(OH)4) at equilibrium. For these purposes, we will use here the value of c = 10" mol/l, but similar diagrams could be drawn for any eoneentration. Figure 2.18 shows the resulting potential-pH diagram. Because the eoneentration of the dissolved ionic species used to draw the diagram are the same as those used to define the protection potential (2.74), the diagram shown allows the stability domains of the metal, of the oxide and of the Al and Al(OH)4 ions to be identified. 

Here is a solvated ion, e is an electrom, and n represents the ion state of charge. The electrons, liberated by the oxidation, must flow through the material M to be consumed in an appropriate cathodic reaction. Beyond a solubility limit, precipitates of hydroxide or hydrated oxide are formed, and this surface film can provide a barrier to further dissolution. In fact, there are two film formation mechanisms the dissolution-precipitation mechattism addressed before and also the solid-state oxidation proeess M + H2O MO + 2H+ + 2e. Some films are termed passive, for stainless steels or aluminum alloys, for instance. These films can play an important role in environment-sensitive erack mitiation and fracture. Under thermodynamic equilibrium conditions, the film stability may be inferred from E =y(pH) diagrams, where E is the electrical potential related to the chemical free energy G by G = -nEF, and F is Faraday s ntrmber. At eqirilibrium, one can define the electrode potential (related to AG) and the eurrent density 1(1 ... 

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. ... 

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... 

However, the Pouibaix potential-pH diagram for aluminum shown in Kg. 2, which is based soldy on theoretical thermodynamic considerations and does not provide infimation on conosion rates, predicts oxide film stability and thus resistance to gen dissolution in the pH range 4 to 9. Aluminum alloys have become a standard mataial of construction for storage... 

---