Magnesium anodic oxidation

The electrochemical effects of slowly and erratically thickening oxide films on iron cathodes are, of course, eliminated when the film is destroyed by reductive dissolution and the iron is maintained in the film-free condition. Such conditions are obtained when iron is coupled to uncontrolled magnesium anodes in high-conductivity electrolytes and when iron is coupled to aluminium in high-conductivity solutions of pH less than 4-0 or more than 12 0 . In these cases, the primary cathodic reaction (after reduction of the oxide film) is the evolution of hydrogen. 

The anodic oxidation of magnesium does not normally produce a film that has sufficient corrosion resistance to withstand exposure without further protection by painting, and the solutions used are complex mixtures containing phosphates, fluorides and chromates. In the case of aluminium, a relatively simple treatment produces a hard, compact, strongly adherent film of oxide, which affords considerably increased protection against corrosive attack . 

A large number of electrolytic treatments of magnesium, anodic or a.c., have been developed, in which adherent white or grey films consisting of fluoride, oxide, hydroxide, aluminate or basic carbonate are deposited from alkaline solutions containing caustic alkali, alkali carbonates, phosphates, pyrophosphates, cyanides, aluminates, oxalates, silicates, borates, etc. Some films are thin, and some are relatively thick. All are more or less absorbent and act as good bases for paint, though none contributes appreciable inhibition. All can, however, absorb chromates with consequent improvement of protective efficiency. 

Other materials such as gold (< = 4.9 eV), aluminum (< = 4.2 eV), indium-doped zinc oxide, magnesium indium oxide, nickel tungsten oxide, or other transparent conductive oxide materials, have been studied as anodes in OLEDs. Furthermore, the WF of ITO can be varied by surface treatments such as application of a very thin layer of Au, Pt, Pd, or C, acid or base treatments, self-assembly of active surface molecules, or plasma treatment. 

The effect of passivating films on aluminium and magnesium has been the subject of much research. By incorporating chromate/dichromate mixtures and other substances in the electrolyte, a coherent insoluble oxide film is formed which effectively inhibits further corrosion. Sealed cells with aluminium or magnesium anodes may therefore be successfully stored for several years, even at high temperatures. However, once current has been drawn from the cell, the film is broken down and rapid attack on the metal follows due to reactions such as... 

The XPS data were obtained with an extensively modified AEI ES-100 photoelectron spectrometer. The samples were analyzed at a pressure typically < 10 g Torr. A magnesium anode (1253.6 eV) was used as the excitation source. The analyzed sample area was of the order of 5 mm2. Survey scans from 0 to 1000 eV were first obtained for each sample to confirm that only the expected elements were present on the fiber surfaces. Subsequently, high resolution spectra were obtained by slowly scanning — 20 eV binding energy windows that included the Si 2p, Al 2p, Ca 2p, B Is, O Is, and C Is photoelectrons, respectively. Integrated peak areas of the photoelectron spectra were determined. The sensitivity factors, which were independently obtained on this spectrometer with oxide standards, were then utilized in the determination of the surface atomic percent compositions of the fibers. 

The situation became complicated when a divided cell was employed. Although unsuitable for effective preparative electrolyses because of resistance losses, this control experiment yielded an unexpected reduced product, an a-halosilane, from the anode (oxidizing) compartment containing the sacrificial magnesium electrode. This suggested the involvement of a sonochemically induced reaction, without... 

Electrogenerated radicals can also react with other active electrode materials. Reduction of allyl bromide at a tin cathode, or ethyl bromide at lead affords the corresponding organometallic tin or lead compounds in 75% or 90% yield, respectively [177]. The anodic oxidation of ethyl magnesium bromide also generates ethyl radicals that can be trapped at a lead anode. This reaction is the basis of the NALCO process, which until recently has been used to produce tetraethyllead in a technical scale [178]. 

Among the most stable cation radical salts are those of metallo-porphyrins. Most commonly the zinc, and magnesium metallo-porphyrins are used (M = Zn, Mg), but others, such as the cadmium porphyrin, are also used (Dolphin et al., 1973 Fajer et al., 1973 Dolphin and Felton, 1974). Tetraarylporphyrins [6, R = H Rj = aryl] or octaalkylporphyrins (R = alkyl, R = H) are the most common parent molecules. Recently, some zinc tetraalkylporphyrin cation radicals were also reported (Fajer et al., 1974). The metallo-porphyrins have low oxidation potentials (Fuhrhop and Mauzerall, 1969 Fuhrhop et al., 1973 Stanienda and Biebl, 1967), and their salts (e.g. perchlorates) can be made quite easily by anodic oxidation in the presence of an appropriate electrolyte. Chemical oxidation, by bromine or ferric ion for example, is also easily achieved, while... 

Phenomena of this type are well known and exploited typically when we protect by means of passivating and anodic oxidation treatments, that is, when we precorrode" in controlled environments materials such as magnesium, aluminum, zinc, copper, and stainless steel in order to create a layer of protective corrosion products able to reduce, if not to cancel, their corrosion rate in the different environments. Obviously, any heterogeneity in the topography of the layer of corrosion products, even if in the presence of homogeneous environments and homogeneous materials, may involve the localization of the cathodic and anodic processes and the consequent localized corrosion. 

Anodized films are most often applied to protect aluminum alloys. However there are processes for other metals such as titanium, zinc, and magnesium. Anodized titanium is used in dental implants and sometimes in art and costume jewelry because it generates various colors without dyes. Each color depends on a specific thickness of the oxide [9]. To ensure the preparation of a consistent oxide layer, one must control conditions such as electrolytic concentrations, acidity, and current. Also a sealing process is often needed to achieve corrosion resistance because thick coatings (oxide layers) are generally porous. 

Anodizing is an electrolytic passivation process that increases the thickness of natural oxide layers on the surface of metals [13]. It basically forms an anodic oxide finish on a metal s surface to increase corrosion resistance. For the anodizing process, the metal to be treated serves as the anode (positive electrode, where electrons are lost) of an electrical circuit. Anodized films are most often applied to protect aluminum alloys. An aluminum alloy is seen on the front bicycle wheel in Fig. 2 [14]. For these alloys, aluminum is the predominant metal. It typically forms an alloy with the following elements copper, magnesium, manganese, silicon, tin, and zinc [15]. Two main classifications for these alloys are casting alloys and wrought alloys, both of which can be either heat treatable or non-heat treatable. 

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