Thin oxide film formation, metal copper

Nonactive/slightly reactive electrode materials include metals whose reactivity toward the solution components is much lower compared with active metals, and thus there are no spontaneous reactions between them and the solution species. On the other hand, they are not noble, and hence their anodic dissolution may be the positive limit of the electrochemical windows of many nonaqueous solutions. Typical examples are mercury, silver, nickel, copper, etc. It is possible to add to this list both aluminum and iron, which by themselves may react spontaneously with nonaqueous solvent molecules or salt anions containing atoms of high oxidation states. However, they are not reactive due to passivation of the metal which, indeed, results from the formation of stable, thin anodic films that protect the metal at a wide range of potentials, and thus the electrochemical window is determined by the electroreactions of the solution components [51,52],... 

Dissolution of steel or zinc in sulfuric or hydrochloric acid is a typical example of uniform electrochemical attack. Uniform corrosion often results from exposure to polluted industrial environments, exposure to fresh, brackish, and salt waters, or exposure to soils and chemicals. Some examples of uniform or general corrosion are the rusting of steel, the green patina on copper, tarnishing silver, and white rust on zinc on atmospheric exposure. Tarnishing of silver in air, oxidation of aluminum in air, attack of lead in sulfate-containing environments results in the formation of thin protective films and the metal surface remains smooth. Oxidation, sulfidation. 

An oxide layer is readily formed on many metals when they are made anodic in aqueous solutions. In the case of aluminum, this process is called anodization. It is also referred to as a passive film which reduces the corrosion rate. Such passive films can be thin, from 0.01 pm, and fragile and easily broken. Thus, when steel is immersed in nitric acid or chromic acid and then washed, the steel does not immediately tarnish nor will it displace copper from aqueous CUSO4. The steel has become passive due to the formation of an adhering oxide film which can be readily destroyed by HCl which forms the strong acid FeCU-. 

It is argued (12) that on typical transition metals (Ni, W, Cr, Ti, Ta) the formation of such a layer (l.e., M-O-O-) proceeds with more favorable free energy of formation than tne oxide formation, as they have unfilled d electron energy levels leading to the formation of strong chemical bonds between oxygen and the metal. These are the metals that typically exhibit passivity. For nontransition metals with filled d levels, such as copper or zinc, the heats of oxygen adsorption are expected to be lower, and the formation of oxides is less favorable. Such metals do not exhibit thin-film passivity. 

This technique was used by researchers at BASF [147,148] for the preparation of thin films of HKUST-1 on copper substrate. For the electrochemical synthesis of powdered MOF-5, a copper electrode was immersed into a solution of the organic MOF building block, benzenedicarboxylic acid (bdc) in the case of MOF-5. By applying an appropriately biased electrical voltage, the electrochemical oxidation of Cu atoms led to the dissolution of Cu + metal ions and subsequently to the formation of crystallites in the vicinity of the electrode surface. The continuous supply of more Cu " " leads to a continuous growth of the crystallites. 

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