For some applications there is no single material that fulfils all the requirements in equal measure. For example, aluminium is ideally suited for use in the hydraulic cylinders of flight control systems because of the weight savings, but it is not sufficiently hard. In such cases, coating with a suitable material can be the solution, for instance with hard chromium on the aluminium of the hydraulic cylinder. With coatings, the beneficial properties of the coating are transferred to the base material and open up fields of application for the latter for which it otherwise could not have been used. One only talks about "coating", a surface treatment, however, when the coating is merely applied to the surface as a layer in much the same way as in painting (it can also be applied as a gas or vapour, liquid or paste or out of a chemical solution); "cladding" is the term used to describe the application of flat, solid materials, for example steel sheet, either by casting, soldering or brazing, extrusion or rolling.
Coating of aluminium
Aluminium can be coated with metals, ceramics or plastics in order to improve its good corrosion resistance and/or its surface hardness or to change its appearance. The most important processes and available techniques:
- Galvanising (named after the Italian doctor Luigi Galvani, 1737-1798): the galvanising bath contains a salt of the coating metal and current is passed via an electrode (electrical conductor) from this metal to and over the object to be coated as the second electrode. After removing the oxide layer, aluminium can be surface treated with all common metals, such as chromium (for hard surfaces, for example on the piston rings of internal combustion engines), copper (as preparation for soft soldering) or nickel (decorative purposes) up to a thickness of several tenths of a millimetre, even with unalloyed aluminium to enhance corrosion resistance.
- Dispersion coating: hard materials such as metal carbides or diamond are added to the galvanising bath, they distribute themselves uniformly in the metal coating and thus increase its wear resistance.
- Enamelling, which is coating with a glass-like material.
- Painting using suitable paints and processes (for example by spraying or immersion) for corrosion protection and decoration.
- Lamination of aluminium strip and foil by adhesive bonding with plastic film or paper, for flexible laminates.
- Thermal spraying: molten metals or plastics (heated in a flame, arc or plasma) are impinged onto the workpiece using compressed air.
- Arc spraying: two metal wires to which an electric voltage is applied are moved towards each other in a jet so that they short circuit and melt. The jet transfers the hot droplets at a temperature of 4000 °C to the metal surface where they produce a strongly adherent layer.
Coating with aluminium
Aluminium is used as a coating on different base materials and for all kinds of purpose:
- With vapour deposition on glass, ceramic, plastic, metals (for optical purposes), paper and plastic film (for packaging) and textiles, unalloyed aluminium is vaporised with an electron beam in a vacuum; it condenses on the cold substrate as a uniform layer.
Aluminium plating is used, for example, for screws made of high-strength steels for road vehicles to make them corrosion resistant.