Sprayed coatings aluminium

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)... 

Nagaska el ai., Weathering Test on Zinc, Aluminium and Zinc-Aluminium Alloy Sprayed Coating for 5000 Days, Proc. lOth Int. Thermal Spray. Corf., DVS, Diisseldorf, Poster report (1983)... 

Shaw, B. A. and Moran, P. J., Characterization of the Corrosion Behaviour of Zinc-Aluminium Thermal Spray Coatings , Corrosion 85, Boston, March 1985, NACE, Houston. Paper 212 (1985)... 

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. 

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. 

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. 

Sprayed aluminium coatings were applied over twenty years ago for a number of buildings in steelworks, notably at Margam, Trostre and Velindre in South Wales. At Margam all steelwork above crane girder level has been aluminium sprayed to 0-10 mm thickness and painted with two coats of... 

Ca(OH)2, respectively. The mechanical properties of the deposits such as critical load, scratch hardness and friction coefficient were determined by scratch testing under loads of 3-9 N. The coatings sprayed onto titanium alloy indicated critical load of 12.3 N and the ones onto aluminium substrate were lower and equal to 10.5 N. 

Klinge R. Sprayed zinc and aluminium coatings for the protection of stractural steel in Scandinavia. Proceedings 8th International Thermal Spray Conferance, American Welding Society, 1976 203-213. 

Hudson reported lives of about 4y years for 38 /tm thick metal-sprayed aluminium coatings on steel exposed at Sheffield, and more than 1 ly years for coatings 75/tm thick. Sprayed aluminium coatings (approximately 125/tm thick) have also provided complete protection against exfoliation and stress corrosion to aluminium-copper-magnesium (HE 15) and aluminium-zinc-magnesium (DTD 683) alloys in tests lasting up to 10 years in industrial and marine environments . 

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