Aluminium coatings heat-resistant

Blends with alkyd or other organic resins have, however, been preptu ed and these show heat resistance intermediate between those of the organic resins and the silicones. Of ptu ticular interest is the use of silicone-organic resin blends filled with aluminium powder for the coating of metal chimneys and furnace doors. At the operating temperatures the resins are destroyed, leaving a layer of aluminium film. 

Other metals used are chromium, lead, and aluminum. Electro-chromium coated sheet is used for the tops of steel drinks cans. Aluminium coated sheet provides a combination of corrosion and heat resistance ideal for car exhaust pipes. 

Aluminium is particularly resistant to sulphur-polluted atmospheres, and sprayed coatings are used in sulphuric-acid plants for the main convectors for hot, intermediate and cold heat exchangers and for the internal surfaces of interconnecting ducting. Coatings of 0 -15 mm thickness have given good service. 

Schmitt, R. J. and Rigo, J. H., Corrosion and Heat Resistance of Aluminium-coated Steel , Materials Protection, 5, No. 4, 46-52, April (1966)... 

The bath is normally at a temperature in the range 620-710°C, depending on whether the coating material is an aluminium-silicon alloy (for use in high-temperature conditions) or pure aluminium (for corrosion prevention). It is heated by inductors, by resistance heaters or by an external flame. The pot will usually be refractory lined unless cast-iron pots are needed to ensure adequate heat transfer from an external flame. As molten aluminium is extremely aggressive towards ferrous metals, replacement of cast-iron pots is fairly frequent. Refractory-lined pots obviously do not have this drawback, although the bath hardware, in particular the sinker roll and support mechanism, will still be attacked and need replacement at intervals. 

These are produced by several conversion processes (evaporation, sputtering, chemical plasma deposition). Evaporation is the same method as that used to create metallisation using aluminium. A material is heated in a crucible by either a resistive heat or an electron beam gun (hence the name electron beam deposition), whereby the material evaporates and subsequently condenses on a chilled film in a vacuum chamber. In the case of Sit), coatings, the aluminium used in metallisation is replaced by SiO/Si02. 

Volatile metal halides, usually chlorides and fluorides, also form the heart of several processes used to produce surface layers, rich in aluminium, chromium, or silicon, or combinations of these. In these processes, the workpiece to be coated is buried in a powder bed and heated to reaction temperature. The bed consists of a mixture of inert alumina filler, a master alloy powder that contains the aluminium, etc., and an activator such as ammonium chloride. Basically, at about 630°C, the activator volatilizes and the aluminium chloride vapour reacts with the master alloy to produce a volatile aluminium chloride, which then reacts with the workpiece surface to deposit aluminium. The deposited aluminium proceeds to diffuse into the surface layers of the workpiece to produce a diffusion coating. The process is driven basically by the difference in aluminium activity between the master alloy and the worlqtiece. These processes are well documented in principle, but their execution to provide reproducible and reliable results still involves considerable experience, or rule of thumb. These processes will be described in detail in Chapter 10. Finally, a chlorination treatment is used to remove tin from tin-plated steel. This uses a normally deleterious reaction to advantage and profit in the recovery of both tin and steel for recycling. Fluorination is used in the manufacture of polymers and fluorocarbon consequently, materials suitable for construction of these plants must be resistant to fluorine attack. 

The smallest metal particle is a single atom. In recent years atoms of metallic elements have attracted a great deal of interest. First, their formation by evaporation of the bulk element has been studied extensively. It is possible to evaporate metals that vaporize under vacuum at temperatures below 2000°C by using electrical resistive heating of ceramic crucibles (such as aluminium oxide coating on a tungsten wire basket). The metals listed below all evaporate nicely using this method. All but Sn, As,... 

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