Gallium passivation

Al, Ga, In and T1 differ sharply from boron. They have greater chemical reactivity at lower temperatures, well-defined cationic chemistry in aqueous solutions they do not form numerous volatile hydrides and cluster compounds as boron. Aluminium readily oxidizes in air, but bulk samples of the metal form a coherent protective oxide film preventing appreciable reaction aluminium dissolves in dilute mineral acids, but it is passivated by concentrated HN03. It reacts with aqueous NaOH, while gallium, indium and thallium dissolve in most acids. 

Figure 8. Theoretical conditions of corrosion, immunity, and passivation of gallium, at 25°C, assuming passivation by a film of a-Ga203 (16)...

Gallium (10 % of the earth s crust) is present in zinc blende, which may contain up to 0.5%, and in certain e.g. British) coal ash. The metal, which is deposited by electrolysis from alkaline solutions of its salts, is silvery-white, hard and brittle. The orthorhombic crystal has a complex lattice. The co-ordination is strictly one-fold, a given gallium atom having one atom situated at 2.43 A and six others at distances varying from 2.70 to 2.79 A. This co-ordination is denoted by 1 + 6 the notation applies only if the six atoms are separated from the reference atom by a distance not greater than 1.2 times that of the nearest one. The metal is stable in dry air and does not decompose water. It dissolves in caustic alkalis and in mineral acids, other than nitric which renders it passive. Its chemistry (Fig. 151) is, in fact, very similar to that of aluminium. 

Gallium is only slightly more noble than Zn. However, it dissolves in mineral acids slowly due to surface passivity phenomena. Hot, concentrated nitric acid is the most effective, dissolving 5 g. of Ga in 10 hours. Sebba and Pugh report achieving rapid solution of Ga In concentrated nitric acid if the metal, which disperses in tiny spheres due to the action of hot acid, is alternately cooled to a powdery acid-metal mixture and then reheated. 

Magnesium and zinc are the predominantly used galvanic anodes for the cathodic protection of pipelines [13—16]. The corrosion potential difference of magnesium with respect to steel is 1 V, which Umits the length of the pipeline that can be protected by one anode. Economic considerations have led to the use of aluminum and its alloys as anodes. However, aluminum passivates easily, decreasing current output. To avoid passivation, aluminum is alloyed with tin, indium, mercury, or gallium. The electrochemical properties of these alloys, such as theoretical and actual output, consumption rate, efficiency, and open circuit (corrosion) potential, are given in Table 15.1. 

J. S. Herman, E. L. Terry, Hydrogen Sulfide Plasma Passivation of Gallium Arsenide, Appl. Phys. Lett. 1992, 60,716-717. 

M. Tabib-Azar, A. S. Dewa, W. H. Ko, Langmuir-Blodgett Film Passivation of Unpinned -Type Gallium Arsenide Surfaces, Appl. Phys. Lett. 1988, 52, 206-208. 

Phases formed on semiconductor surfaces can change the electrical properties in an uncontrolled, deleterious fashion. Oxide passivation layers on compound semiconductors (e.g., mercury cadmium telluride IR detectors or gallium arsenide solar cells) can be grown to impart protection to the surfaces and to stabilize electrical properties by preventing uncontrolled reactions. 

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