Anodising composition

Alloys are generally of the Al-Mg-Si type with additions of copper and chromium or manganese. Colour varies with the particular alloy and the film thickness. For optimum control of colour, the alloy must be carefully produced with strict attention to composition, homogenisation and heat-treatment, where appropriate, and the anodising conditions must be maintained within narrow limits. It is usual to arrange matters, preferably with automatic control, such that current density is held constant with rising... 

The simplest method of chromate sealing involves immersion in a dilute alkali chromate or dichromate solution followed by washing retained chromate imparts a yellow colour to the film. More substantial amounts of slightly soluble chromate can be deposited in the thicker type of absorbent anodic film by a method developed by Dr. L. Whitby at High Duty Alloys Ltd. In this, anodised parts are immersed first in a boiling 30% solution of sodium chromate and then in a boiling 2% solution of zinc nitrate. Residues of the first solution in the film react with the second solution to give a substantial yellow deposit of a basic zinc chromate, probably similar in composition to zinc yellow. 

Those experiments that did involve anodisation at more reasonable potentials, i.e. below oxygen evolution, suffered from an inability to characterise the initial PtOH species formed at coverages below a monolayer. Dickinson et ai (1975) systematically investigated the surface composition of a large number of Pt electrodes, polarised at various potentials in sulphuric acid, using XPS via the emersion approach. Figure 3.24 shows XPS spectra obtained from a Pt electrode after polarisation in sulphuric acid at 1.00 V and 1.5 V vs. SCE. 

Atomic oxygen - which is present up to 800 km altitude - proved to be one of the most aggressive agents in space the impinging atoms eroded many polymers, including the composite matrices, destroyed the Kapton films exposed on the experiment trays and ate deeply into the polytetrafiuoroethylene thermal protections. Luckily, silicones and anodised aluminium confirmed their invulnerability. 

In particular, for the synthesis of optically pure chemicals, several immobilization techniques have been shown to give stable and active chiral heterogeneous catalysts. A step further has been carried out by Choi et al. [342] who immobilized chiral Co(III) complexes on ZSM-5/Anodisc membranes for the hydrolytic kinetic resolution of terminal epoxides. The salen catalyst, loaded into the macroporous matrix of Anodise by impregnation under vacuum, must exit near the interface of ZSM-5 film to contact with both biphasic reactants such as epoxides and water. Furthermore, the loading of chiral catalyst remains constant during reaction because it cannot diffuse into the pore channel of ZSM-5 crystals and is insoluble in water. The catalytic zeolite composite membrane obtained acts as liquid-liquid contactor, which combines the chemical reaction with the continuous extraction of products simultaneously (see Figure 11.28) the... 

In this procedure the first step, once again, is to pickle the aluminium substrates in accordance with the specifications quoted above. Under certain circumstances it may be sufficient to use only an alkaline degrease before anodising. The etched panels are then clamped to the anode of a standard anodising bath and are immersed in a solution of phosphoric acid at 25°C. Anodising then takes place in accordance with one or other of several European and American specifications [29,30]. A typical bath composition is ... 

The substrates are then attached to the anode of a standard anodising bath (anode to cathode ratio of about 3 1) and are immersed, at 40 C, in chromic acid of the following composition ... 

As has been indicated, the production of the tail rotors generally follows a similar pattern. The exception is the Sea King. Here there is a difference between the 6-hub and the 5-hub versions. In the former case, the outer skin is of anodised aluminium, which has been adhesively primed and then bonded to the inner aluminium spar and honeycomb structure using 3M s Scotch-Weld AF-6 and AF-10 nitrile-phenolic 170°C curing film adhesives, and the erosion shield is of stainless steel which has been chemically etched and primed before bonding with AF-30 (another 170°C curing nitrile-phenolic film adhesive). The 5-hub version, however, is more conventional. This uses an epoxy film adhesive to bond a comoulded composite spar and skin structure to a titanium erosion shield. 

In all cases, the aluminium substrates are pretreated using conventional chromic acid anodising techniques, often this is augmented with the application of a pretreatment protection primer. The carbon composites use peel ply techniques to prepare the surfaces for bonding. 

In general, the suitability for anodising depends on the composition and the microstructure of the aluminium, and may vary firom one alloy series to another. The resulting anodic coatings will not necessarily have the same properties (Table B.5.1). 

Within the composition limits of contemporary alloys, the iron and silicon content does not have a major influence on the corrosion resistance of semi-products of this family. Resistance to atmospheric corrosion is good. Pure alloys 1199 and 1190 are anodised when used outdoors. 

The widespread use of aluminium alloys in applications ranging from transport to architecture demands surface treatments for their corrosion protection [1,2], Anodising of aluminium is a widely studied and accepted method that consists of electrochemically forming a protective aluminium oxide layer [2], The growth, morphology, composition and performance of anodic films have been considered in the literature, for both pure aluminium and aluminium alloys [2 7], Furthermore, sealing mechanisms of the porous aluminium oxide layers have also been reported [6-9]. 

Figure 3.28 Schematic diagram of a three-electrode electrochemical cell in which the Ti samples are anodised. Fabrication variables include temperature, voltage, pH and electrolyte composition. Reprinted from Mor etal., 2006. Copyright (2006) Elsevier...

Other strategies for producing hydrophobic membranes for MD are the modification of hydrophilic polymers or ceramic materials. Qtaishat et al. [144] produced two different types of hydrophobic surface-modifying macromolecules (SMMs) and prepared hydrophobic/hydrophilic polyetherimide composite membranes. The SMMs blended PEI membranes achieved better DCMD fluxes than those of a commercial PTFE membrane tested under the same conditions. Similarly fluorinated SMMs were used to modify hydrophilic poly(sulfone) [145]. Krajewski et al. [146] used lH,lH,2H,2H-perfluorodecyltriethoxysilane to create a hydrophobic active layer on commercial tubular zirconia membranes supported on alumina. The produced membranes were tested in air-gap MD (AGMD). Hendren et al. [147] used 1H,1H,2H,2H-perfluorodecyltriethoxysilane, trichloromethylsilane, and trimethylchlorosilane to modify, by surface grafting, two types of alumina Anodise ceramic membranes. The authors demonstrated that this surface treatment was effective and tested the produced membranes in DCMD. 

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