Aluminium coating

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. 

Filiform corrosion is characterised by the formation of a network of threadlike filaments of corrosion products on the surface of a metal coated with a transparent lacquer or a paint him, as a result of exposure to a humid atmosphere. This phenomenon first attracted attention because of its formation on lacquered steel, and for this reason it is sometimes referred to as underfilm corrosion, but although it is most readily observed under a transparent lacquer it can also occur under an opaque paint film or on a bare metal surface. Filiform corrosion has been observed on steel, zinc, magnesium and aluminium coated with lacquers and paints, and with aluminium foil coated with paper. Surface treatment of the metal by phosphating or chromating lessens the tendency for filiform corrosion to occur, but it is not completely... 

Fischer, K. P., Thomason, W. H. and Finnegan, J. E., Electrochemical Performance of Flame Sprayed Aluminium Coatings of Steel in Water , Paper No. 360, Corrosion/87, San Fransisco, USA, March (1987)... 

Aluminium coatings exposed as sprayed show remarkable protective properties and in many cases the only evidence of age is the appearance of small hard nodules of aluminium oxide, and these do not appear to be deleterious. However, the dead white of the sprayed surface soon becomes discoloured by dirt and so a paint covering is usual. A chromate inhibitor is not essential and two coats of a simple vinyl-based sealing paint have given remarkable results in the tests of the American Welding Society over periods of 12 and 15 years. 

A Plain Guide to BS 5493 1977, Assn. Metal Sprayers, Birmingham (1978) Inspection of Sprayed Aluminium Coatings, data sheets and reprints of various symposia papers... 

Klinge, R., Sprayed Zinc and Aluminium Coatings for the Protection of Structural Steel in Scandinavia . Proc. 8th Ini. Thermal Spray. Conf., American Welding Society, pp. 203-213 (1976)... 

The aluminium coatings are highly corrosion resistant, and are less liable to contact corrosion than cadmium when in contact with light alloys. They can also be used at temperatures of up to 496°C as against 232°C for cadmium. The aluminium coating is also unaffected by aviation fuels, unlike cadmium. 

Aluminium may be applied as coatings by metal spraying, cladding, hot-dipping and electrodeposition, though the last-named process is difficult to apply and by far the largest proportion of aluminium-coated metals are produced by the first two methods. 

Sprayed, vacuum-deposited and plated coatings can be applied to most metals and to many non-metals, e.g. vacuum deposition is applied to many substrates including plastics spray application can be used for coating fabric, plastic and paper. Hot dipping and other diffusion processes are dependent on the nature of the substrate for the properties of the coating. Most commercial applications of aluminium coatings are on iron and steel with smaller quantities applied to aluminium alloys and plastics. 

Steel with up to 0 075 mm thick aluminium coatings can be gas-cut by both hand and machine methods, and welded, without removal of the aluminium. No fume problems arise in welding aluminium-coated material in practice, however, it is usual to leave the edges bare and to spray the joint with aluminium after welding. 

The corrosion resistance of aluminium coatings is generally related to that of solid aluminium of similar thickness. Additional factors arise with sprayed coatings associated with texture, and with aluminised and other coatings when diffusion from the substrate can occur. 

Ordinary temperatures The protection of steel by aluminium coatings... 

Other investigations indicate that half-tide conditions give results similar to fully immersed conditions, and that in sea-water the 99-5% aluminium coating is preferred to aluminium-zinc or aluminium-magnesium alloy coatings. 

Tests by Clark for the Corrosion Sub-committee of the American Welding Society were carried out at severe marine and industrial sites. After four years, the greatest protection to steel was given by sprayed aluminium coatings combined with aluminium vinyl paint in the following environments id) sea air, ib) sea-water immersion, (c) alternate sea-water immersion and exposure to air (tidal conditions) and id) industrial atmospheres contaminated with sulphur compounds. 

Exposure tests of 5-years duration in the atmosphere and during immersion in fresh water or sea-water by Welding Institute and by Stanners and Watkins have indicated the best paint systems for aluminium-coated steel. For immersed conditions two systems giving good performance are ... 

Up to 550°C, aluminium coatings may be used in the as applied condition, and the hot-dipped aluminium-silicon alloy may be used up to at least 680°C, but for service at higher temperatures additional diffusion treatment is recommended for all except Calorised coatings. 

Up to 750°C, the performance of all aluminium diffusion coatings is considered to be very good, but above this temperature the results appear to be dependent on the coating thickness, diffusion treatment and the specific service environment. Sprayed aluminium coatings can be used up to 900°C after diffusion treatment. Hot-dipped coatings also benefit from additional diffusion treatment, and omission of silicon from the coating alloy improves performance at the elevated temperatures. 

Above 900°C, the life of aluminium-coated steel is more limited although the coating can still provide significant protection to steel, e.g. at 900-980°C the life is said to be increased 20 times, and at 980-1 000°C the life is said to be increased 5 times. 

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