Aluminum anodizing process

It has long been known that the anodization of certain metals leads to the production of porous metal oxides and hydroxides. These metals include A1 [14-16], Ti [17,18], Ta [19], Cd [20], Nb [21], Mg alloys [22], W [23], Sn [24], Fe [12,25], Ag [26], and Si [27]. Only recently conditions have been identified that allow the formation of well-controlled, uniform structures. For example, in 1995 an aluminum anodization process to develop hexagonally packed pores with ordered domains was developed [16]. In 2001, it was discovered that anodization of titanium foils led to the production of Ti02 nanotube arrays [18]. More recently, a anodization process to form nanoporous [25] and nanotubular [28] iron oxides was developed. Examples of these materials are shown in Fig. 9.6. Pore sizes in the range of 20-100 nm are... 

The electrolysis protection process using impressed current aluminum anodes allows uncoated and hot-dipped galvanized ferrous materials in domestic installations to be protected from corrosion. If impressed current aluminum anodes are installed in water tanks, the pipework is protected by the formation of a film without affecting the potability of the water. With domestic galvanized steel pipes, a marked retardation of the cathodic partial reaction occurs [15]. Electrolytic treatment alters the electrolytic characteristics of the water, as well as internal cathodic protection of the tank and its inserts (e.g., heating elements). The pipe protection relies on colloidal chemical processes and is applied only to new installations and not to old ones already attacked by corrosion. 

A higher content of AI2O3 and SiOj is critical for the composition of the protective films in the tubing, assuming the water contains silicates or silicic acid. The protective films have a maximum thickness of 1.5 mm and cannot grow further. The corrosion process can be stopped even in copper pipe networks with type I pitting [21] by providing a reaction tank with impressed current aluminum anodes. 

Pure aluminum is used in the electrolysis protection process, which does not passivate in the presence of chloride and sulfate ions. In water very low in salt with a conductivity of x < 40 yUS cm" the polarization can increase greatly, so that the necessary protection current density can no longer be reached. Further limits to its application exist at pH values < 6.0 and >8.5 because there the solubility of Al(OH)3 becomes too high and its film-forming action is lost [19]. The aluminum anodes are designed for a life of 2 to 3 years. After that they must be renewed. The protection currents are indicated by means of an ammeter and/or a current-operated light diode. In addition to the normal monitoring by service personnel, a qualified firm should inspect the rectifier equipment annually. 

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. 

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. 

In the future, further studies should be addressed to improve the chemose-lectivity and diastereoselectivity of the reductive coupling process, especially searching for novel reagents and milder experimental conditions. As a matter of fact, a few novel reductive couphng procedures which showed improved efficiency and/or stereoselectivity have not been further apphed to optically active imines. For example, a new electrochemical procedure which makes use of the spatially addressable electrolysis platform with a stainless steel cathode and a sacrificial aluminum anode has been developed for imines derived from aromatic aldehydes, and the use of the N-benzhydryl substituent allowed 1,2-diamines to be obtained with good yields and dl-to-meso ratios... 

In the aluminum electrowinning process a phenomenon called the anode effect is normally encountered when the alumina content in the electrolyte falls below 2%. The anode gets partially covered with a gas blanket and as a consequence, sparking occurs and the cell voltage fluctuates considerably due to frequent breaking and reestablishment of local contact between the anode and the electrolyte. A heavy current passes through the anode area... 

P, y-Unsaturated esters (184) have been synthesized by a one-step electrochemical procedure from a-chloroesters (183) and aryl or vinyl halides (Scheme 73b) [294, 295]. This novel electroreductive cross-coupling method is based on the use of a Ni(II)(bpy) catalyst and a sacrificial aluminum anode in a one-compartment cell (Scheme 73). The whole cathodic process progresses at —1.2 V (SCE) (Scheme 73c),... 

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. 

Early workers [103] detected benzilic acid formed during the reduction of benzophenone in dimethylformamide in the presence of carbon dioxide. The carbon dioxide radical anion system is known to have E" = —2.2V (vs. SCE) [104] and will thus not be formed in preference to the ketone radical anion. Reaction occurs through trapping of aromatic carbonyl radical anions by carbon dioxide, and this has been developed into a convenient synthesis of aryllactic acids. The modern technological process uses constant current conditions. On a small scale, a divided cell with mercury cathode has been used to obtain benzilic acids from substituted benzophenones and carbon dioxide in 70-90% yields [105] and to convert 4-isopropylacetophenone to the corresponding phenyllactic acid in 85% yield [106]. On a technical scale, these reactions are best carried out in an undivided cell using a lead cathode and a sacrificial aluminum anode with dimethylformamide as solvent... 

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. 

While the metal or alloy electroless deposition reactions can be considered as cathodic processes, formation of oxides at the metallic surfaces without an external current source can be analyzed as anodic processes. This type of deposition can be illustrated in the example of chemical oxidation of aluminum in chromic acid solutions.9... 

The anodizing process consists of 1) trim forming, 2) cleaning, 3) anodic electrolysis usually in sulfuric acid and, 4) sealing in an aqueous bath. Each of these steps are important, for example, the anodized oxide layer must be controlled to provide an optimum thickness. Sealing the anodized layer is also very important and must be complete. Corrosion problems associated with anodized aluminum trim include etching of the oxide film (known as "blush and bloom ) and pitting of the aluminum. 

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