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coating thickness, the weight is less than half that of zinc. One of the advantages of hot dip alu minizing is that a thinner coating is possible. The relative costs of aluminizing and galvaniz ing are governed, of course, by the prices of the A Comparison of Hot-dip Coating with Aluminium and Hot Galvanizing two metals. With the price of aluminium on a weight basis twice that of zinc, there is a slight saving when metal is coated with aluminium if By A. G. Thomson the coatings are of the same thickness. From the manufacturer's point of view the process offers several advantages, notably the fact that anneal ing and coating may be combined in a single INCE 1949 the British Iron and Steel Research observed that clean cold-reduced strip could be operation with obvious economic gain. It also Association have been studying the properties coated without any preliminary treatment, and S has certain disadvantages arising mainly from the of hot-dip aluminized coatings. This work advantage was taken of this fact when designing relatively high temperature of operation, such as has led to the development of a simplified process the pilot plant. softening of cold-worked high tensile material, for coating steel or cast iron with aluminium This investigation is of considerable interest to slight hardening of thin material, and a greater which appears to offer interesting possibilities users of hot dip aluminized coatings such as the tendency to warping when dipping fabricated for the protection of aircraft components. aircraft industry. In the first place, it is resulting articles containing residual stresses. The high The value of aluminium as a protective coating in a better understanding of the advantages and temperature also results in higher fuel cost and is now widely recognized, largely because of the limitations of a method of protection which holds in more discomfort for the operator than in good results obtained with sprayed coatings. out considerable promise. The resistance of these galvanizing. Doubtless the choice between hot- Hot-dipped aluminium coatings also have valu coatings to humidity is greater than that of zinc dip aluminizing and hot-dip galvanizing will be able properties, the most important being re coatings, but bare spots on aluminium test panels influenced by such factors as the annealing action were found to be subject to localized attack, sistance to corrosion and heat, but commercial of aluminizing or the danger of warping, but each which is likely to lead to perforation. The steel development has been restricted by difficulties in type of protection will probably have its own applying them. The outstanding method of ap is anodic and pitting occurs. distinct field of applications. plication is the continuous aluminizing of cold- It is well known that aluminium's resistance reduced mild steel strip in Sendzimir hot galvaniz In 1950, the United States used 1,000 tons of to corrosion is due to a surface film of oxide or ing units to produce Armco-Aluminized sheet. aluminium in hot-dip aluminizing and consump hydroxide which is very tenacious. Hence the tion is probably very much greater now. The This process has been in commercial production electrochemical protection which might other processes developed by B.I.S.R.A. are technically in the U.S. since about 1939, but the equipment wise be expected from such an electro-negative capable of providing Britain with a new method required is expensive and would be suitable only metal as aluminium is largely lost. A bare spot of protecting steel which is now well established for continuous processing on a fairly large scale. in an aluminized coating is therefore more serious The smaller manufacturer would require either in the United States. Their practical significance than one in a galvanized coating. It is also evident a simpler and cheaper continuous unit, or a can only be determined by full-scale trials, which that holes could not be drilled through an alu method of producing single sheets. are outside the scope of a Research Association. minium coating to expose bare steel or iron Apart from the potential value of hot-dip alumi Another method of protection which has given without causing rusting. nizing to aircraft manufacturers, the markets very satisfactory results is the Al-fin process for Cut edges are not protected by hot dip alu awaiting development seem large enough to bonding aluminium to steel or cast iron intro minizing, but it is noteworthy that the Aero justify the capital expenditure involved in laying duced during the war by the Fairchild Aviation nautical Material Specification for aluminized down a commercial plant. and Engineering Co. in the United States. It was sheet and strip, drawn up by the American Society developed originally for the purpose of coating of Automotive Engineers, states that 'corrosion the fins of air-cooled cylinders of aircraft engines of basis steel at cut edges shall be disregarded.' with aluminium in order to give them better Preliminary experiments were carried out by cooling properties. This method is now being B.I.S.R.A. to determine the heat resistance of used in the United States and Britain for the aluminized sheet and wire. Specimens of plain protection of aircraft cylinder barrels, brake steel, and of the same steel coated (a) with com linings, turbine blades, heat exchangers and other mercial quality aluminium and (b) with aluminium fabricated articles. It is entirely different from containing 5 per cent silicon, were heated for COLLEGE OF AERONAUTICS hot dipping, however, being essentially a casting 2,000 hours in a vertical position in a muffle at process involving the use of moulds. 500 deg. C. The plain steel scaled freely and lost DIPLOMAS 1·24 g. in weight; the specimen coated with Hot dipped aluminium coatings have not yet aluminium developed a characteristic dark colour been tested in Britain in actual commercial opera The following awards have been made to the stu tion, but samples received in 1944 by the Corro due to the growth of alloy to the surface, but dents of the two-year post-graduate Diploma course sion Committee of the Iron and Steel Institute the change in weight was merely a gain of 8 • 7 mg. of the College of Aeronautics ending on July 9, 1954. (Atmospheric Corrosion Sub-Committee) have The silicon-alloyed coating suffered little visible Specialization is denoted in brackets. been subjected to corrosion tests in several change and gained only 6·6 mg. Similar samples Diploma with Distinction laboratories. Published information indicates were tested at 600 deg. C. and after 1,500 hours J. B. Gilder, R. D. Milne, J. Sandford, F. G. Willox that coatings of this type are highly resistant to both aluminized coatings were in excellent con (Aerodynamics); La V. W. Brown, Jnr., A. E. Heiba, atmospheric corrosion, this property being parti dition. C. Preston (Aircraft Design); E. T. Curran, H. J. M. Londeau, I. T. A. Murchie, R. G. Price (Aircraft cularly marked in the humid and sulphurous Aluminium coatings appear to fail early under Propulsion); C. J. Norbury (Aircraft Economics and atmospheres in which zinc is liable to greatly compression, but under tension they are better Production); F. Mobey (Aircraft Electrical Engi increased attack. than hot-dipped zinc. For many applications the neering). In order to obtain further information on the coating must withstand some deformation. Diploma variables of the process, the British Iron and Sheet samples prepared both in the laboratory G. A. Cropper, G. T. Downer, R. B. Erb, R. C. Steel Research Association examined the results and in the continuous unit were therefore sub Hastings, G. W. Haynes, G. D. O. Humphreys, obtained when cold-reduced mild steel sheet jected to bending tests, samples of aluminized G. A. C. Searle, D. G. Turnbull (Aerodynamics); was dipped into molten aluminium after various tube were pressed flat, and wire was subjected J. A. H. Bailie, N. A. Barfield, W. Brookes, G. R. D. preliminary treatments. Laboratory studies on to wrapping tests over various radii. It was found Calder, S. Chennakeshu, G. Davies, D. A. Drane, sheet samples were followed by batch aluminizing that performance depends on the coating thick P. G. Hardie-Bick, F. E. Jarlett, E. J. Phillips (Air on a larger scale, using samples of steel tube, ness and its structure, while the latter is in turn craft Design); P. N. Dent, C. T. Ebel, P. W. Hill, F. G. Maccabee, J. Rousseau, J. H. Sargeant, J. P. angle iron and other rolled sections, up to 15 in. dependent on the composition of the bath. The Spillane, W. R. Stephens, T. V. Vareed, D. C. Withers long. It was concluded that 'general aluminizing', alloy layer is very hard and brittle when pure (Aircraft Propulsion); A. A. A. Abdin, D. H. Blyther, on lines similar to 'general galvanizing', should aluminium is used in the bath. T o obtain coatings B. Butterfield, D. Fletcher, J. T. D. Holt, J. Jagaciak, not be too difficult a process to operate, and that with good bending properties the time of immer R. N. Kashyap, H. S. Kennedy, K. G. Lane (Aircraft because of its technical merits, it was likely to sion must be short and the temperature low. If, Economics and Production); P. R. Graham, P. A. find its own field of applications. in addition, the outer layer is thin as the result Harper, J. W. Tuson (Aircraft Electrical Engineering). of slow withdrawal, the coating behaves very well In view of the obvious advantages of con The Governors' Prize has been awarded to 1. T. A. in the alternating bend test, though not as well as tinuous processing, B.I.S.R.A. decided that the Murchie. a corresponding thin zinc coating. The addition The Principal's Prize has been awarded to C. J. scope of the investigation should be further ex Norbury. of silicon to the aluminizing bath improves the tended by the erection of a small pilot plant in The Senate Prize has been awarded to L. W. behaviour of the coating in cupping and bend its laboratories at Swansea, suitable for the con Brown, Jnr. tests. tinuous aluminizing of strip up to 2 in. width, The Woods of Colchester Prize for Aerodynamics or of wire. The main objective was to reduce the The appearance of the coated articles is usually has been awarded to R. D. Milne. process to the simplest form consistent with bright and attractive and it can be improved by The Woods of Colchester Prize for Aircraft Electri satisfactory coating. It had previously been cold rolling or drawing after coating. For a given city has been awarded to F. Mobey. 266 Aircraft Engineering
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Aug 1, 1954
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