Anodic processes, aluminum

Porous anodic alumina attracts an attention of a scientific community due to an ordering nanostmcture resulting from self-regulated electrochemical anodic process. Aluminum is a unique material, which forms a regular porous oxide, composed of a packed array of columnar hexagonal cells, each having a cylindrical pore in the center [1], The cell size is known to be determined by the electrolyte and anodization regimes [1,2]. We have found that the cell size can be also tuned by Ti in Al-Ti alloys. The results obtained are presented in this paper. 

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

Boeing Process Specification, BAC 5555 Issue M. Phosphoric acid anodizing of aluminum for structural bonding. Boeing Airplane Company, 1995. 

Despite the progress outlined in this chapter, much work remains to be done in the metal surface preparation arena. For example, there is still no ideal surface preparation method that does for steel what anodization processes do for aluminum and titanium. The plasma spray process looks encouraging but because it is slow for large areas and requires rather expensive robot controlled plasma spray equipment, its use will probably be limited to some rather special applications. For more general use, the sol-gel process has potential if future studies confirm recently reported results. 

Aluminum has a low density it is a strong metal and an excellent electrical conductor. Although it is strongly reducing and therefore easily oxidized, aluminum is resistant to corrosion because its surface is passivated in air by a stable oxide film. The thickness of the oxide layer can be increased by making aluminum the anode of an electrolytic cell the result is called anodized aluminum. Dyes may be added to the dilute sulfuric acid electrolyte used in the anodizing process to produce surface layers with different colors. 

FIGURE 14.24 In the Hall process, aluminum oxide is dissolved in molten cryolite and the mixture is electrolyzed in a cell with carbon anodes and a steel cathode. The molten aluminum flows out of the bottom ot the cell. 

The catalyst is not necessary either for the electrocarboxylation of aryl halides or various benzylic compounds when conducted in undivided cells and in the presence of a sacrificial anode of aluminum [105] or magnesium [8,106], Nevertheless both methods, i.e., catalysis and sacrificial anode, can be eventually associated in order to perform the electrocarboxylation of organic halides having functional groups which are not compatible with a direct electroreductive process. 

ANODIZE. This term means to place a protective film on a metal surface by electrolytic or chemical action in winch the metal surface is made the anode in an electrochemical process. Aluminum and magnesium parts of electronics equipment are frequently anodized. 

Another type of reaction is the oxidation of complex ions, for example in the case of aluminum electrolysis. These reactions are rather complicated and occur in several steps. During the first step, the discharge of oxygen ions takes place the oxygen atoms formed are adsorbed on the surface of the carbon anode and molecules of C02 are then obtained. These molecules of C02 can react with the anodic carbon, and a certain proportion of CO may appear. All these gases form bubbles which escape. Usually, the anodic processes have a high overvoltage. 

Aluminum anodization in basic A1C13-MEIC melts was studied by Carlin and Osteryoung [462], and two different anodization processes were observed. The first step occurred in the catholic region, at a potential of -1.1 V, versus the aluminum electrode, and it was controlled by diffusion of chloride to the electrode surface. The authors found that the number of chlorides required to produce one A1C14 anion for each Al being anodized was 4. The second anodization which occurs on the anodic side of 0 V was not diffusion limited. It has not been possible to reduce the A1C14 anion in basic melts. 

Passivity — An active metal is one that undergoes oxidation (-> corrosion) when exposed to electrolyte containing an oxidant such as O2 or H+, common examples being iron, aluminum, and their alloys. The metal becomes passive (i.e., exhibits passivity) if it resists corrosion under conditions in which the bare metal should react significantly. This behavior is due to the formation of an oxide or hydroxide film of limited ionic conductivity (a passive film) that separates the metal from the corrosive environment. Such films often form spontaneously from the metal itself and from components of the environment (e.g., oxygen or water) or may be formed by an anodization process in which the anodic current is supplied by a power supply (see -> passivation). For example, A1 forms a passive oxide film by the reaction... 

Eagen and Weinbeig120 conducted a life-cycle assessment on two different anodizing processes, differing in the mixture of boric and sulfuric acid or chromic acid used. Boric and sulfuric acid are shown to be a better choice than a mixture of boric and chromic acid. Tan et al.121 have conducted a cradle-to-gate life-cycle assessment of an aluminum billet, which included the mining of bauxite, the processing of the alumina, and the final... 

Galv moaluminum layers precipitated from electrolytes containing alkyl aluminum possess a much lower microhardness (21 HV) than other electrolytically deposited metals or aluminum layers deposited from other electrolyte systems. The soft galvanoaluminum deposits can be hardened considerably by a subsequent anodizing process. Because of the high purity of the aluminum layer, a transparent oxide layer is produced which can be colored as desired for decorative purposes. The obtainable hardness values are dependent on the selected anodizing technique. 

Then the anodic alumina layer formed was removed chemically in the selective etchant composed of phosphoric (6 wt.%) and chromic (1.8 wt.%) acids at 60 C. Hemispheric etching pits - replica of the alumina cell bottoms - remain on the surface of the aluminum foil. The second porous anodization of aluminum was made. At this stage, the pores on the aluminum foil surface arise not in random way but at the sites of primary alumina cell Imprints to repeat the cell size. The pore diameter and spacing are dictated by the parameters of the anodization process, specifically by the electrolyte composition and the anodization voltage. The alumina film thickness is defined by the anodization time and the anodization current density. The second stage provides a continuous development of the alumina film. Total etching process takes 10-20h to get pores of approximately 100 pm lengths. 

In the modern version of this process, aluminum metal is obtained by electrolysis of aluminum oxide, which is refined from bauxite ore (AI2O3 2H2O). The aluminum oxide is dissolved at 1000°C in molten synthetic cryolite (Na3AlFg), another aluminum compound. The cell is lined with graphite, which forms the cathode for the reaction. Another set of graphite rods is immersed in the molten solution as an anode. The following half-reaction occurs at the cathode. 

The industrial manufacture of aluminum is ba.sed on the Hall-Heroult process developed in 1886. In this process aluminum oxide (see Section 3.2.4.2 for the production of aluminum oxide from bauxite) is dissolved in a cryolite (Na3AlF5) melt and electrolyzed at 940 to 980°C with direct current. Molten metallic aluminum is deposited at the carbon electrode (cladding of the bottom) and taken off as a liquid. Oxygen is formed at the anode, also of carbon (presintered or Soederberg-electrode), with which it reacts forming carbon dioxide and carbon monoxide. 

The anodization process uses high purity A1 foils. Before anodization the aluminum foil is de-greased with acetone or trichloroethylene for some time followed by a sodium carbonate wash at around 80 °C and vacuum annealed (typically 10 Pa) at around 500 °C for a few hours. This is followed by an electrochemical polish in acidic solution (like perchloric acid and ethanol mixture or H3PO4, H2SO4 and Cr03 mixture). The anodization process is a two-step process as shown in Figure 21.5. In the first step the oxidation is carried out in 0.3 M-0.5 M acid medium at... 

Available forms Structural shapes of all types, plates, rods, wire foil flakes, powder (technical and USP). Aluminum can be electrolytically coated and dyed by the anodizing process (see anodic coating) it can be foamed by incorporating zirconium hydride in molten aluminum, and it is often alloyed with other metals or mechanically combined (fused or bonded) with boron and sapphire fibers or whiskers. Strengths up to 55,000 psi at 500C have been obtained in such composites. A vapor-deposition technique is used to form a tightly adherent coating from 0.2 to 1 mil thick on titanium and steel. 

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