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Aviation History
1940
1940 - 1978.PDF
JULY II, 1940 ANODISING An Outline of the Processes Available for the Anodic Oxidation of Aluminium and Its Alloys Illustrated by " Flight" photos SO man r new firms havecome into the aircraftindustry that we have thought the following notes, prepared by an ex- pert in the subject, useful in giving those of them which are not too clear in their own minds about the basic nature of anodising an idea of '' what it is all about." The following article is intended purely as a guide to newcomers to the industry, and has not been written with the idea of appealing to those al- ready familiar with the three main processes in use in this country. Thus the author confines himself to the general aspects of the processes, and points to some of the difficulties which may beset beginners.—ED. All the processes for the anodic oxidation of aluminium and its alloys work basically in the same manner, although the details vary considerably. In all cases the aluminium or alloy to be treated is made the anode in an electrolytic cell, and a current is passed through the cell for a certain period. The water in the electrolyte is split up into hydro- gen and oxygen in the normal way. The hydrogen is evolved at the cathode (usually the sides of the cell or bath used), and the oxygen makes its appearance at the anode. Some very small quantities of oxygen are evolved there and pass away as bubbles, but the bulk of it combines with the surface of the aluminium to form a dense, microscopic- ally porous and highly adherent layer of aluminium oxide. This layer constitutes the anodic film, and protects the surface of the metal from further attack by air or other atmospheric influences. As it is formed from the metal itself and, as it were, grown into the crystal lattice of the metal, it is so adherent that it is impossible to remove the film intact from the surface of the metal. Films have been detached and examined, but only by destructifen of the base metal, usually by treatment at a high temperature with dry hydrogen chloride, when the aluminium combines to produce aluminium chloride which, at the high tempera- tures used, is in the form of vapour and may be distilled away and sublimed. Examination of such films shows them to vary in thickness from .0003m. to .00075m., and to be principally composed of aluminium oxide, together with salts derived from the electrolyte used. There are three main groups of processes: — The Bengough'Stuart Process This was the first process to be commercially applied, and was invented by Messrs. Bengough and Stuart, work- ing in the N.P.L. Laboratories. It was patented in 1923, and for a considerable time was specified for anti-corrosive yreatment of all aircraft parts. The value of the pioneer Mr. j. M. Perfect, the author of this article, in his laboratory. work done by Bengough and Stuart is shown by the fact that the process has barely been modified in any way from their original specifications, despite numerous attempts at improvement and simplification; and even to-day the majority of aircraft parts are treated by this process. Use is made, in this process, of a 3 per cent, solution of chromic acid at 40 deg. C. ± 2 deg. C, as electrolyte, and produces good results on most alloys, although those alloys containing high percentages of copper, magnesium or zinc tend to show pitting after the process. In alloys which have been subjected to heat-treatment without sufficient care, segregation may take place of the alloying con- stituents, and in such cases dangerous weakening may well occur owing to the attack of the chromic acid producing porosity. The process takes 60 minutes, during which time thfe voltage across the electrodes of the vat is varied in a some- what complicated manner. It is raised in stages of 2 to 3 volts from 0-40 volts during the first quarter-hoiir of treatment; it is then maintained at 40 volts for 35 minutes, raised during the next five minutes to 50 volts, and main- tained at this voltage for a further five minutes. Because of this complicated changing of the potential the process must be worked in batches. This is a disadvantage in preventing a smooth flow of work, although it ensures that all parts do receive full treatment. ; The film produced varies from opaque grey-white to dark grey in colour, and is highly absorbent of dyes or greases when newly formed. The property of dye absorp- tion is used as a test of the film in the well-known indelible pencil test. Owing to the absorptive properties, the film should be kept scrupulously clean after formation, or it will acquire unsightly blemishes and stains due to absorp- tion of dirty fluids or greases with which it may come in contact. The colour of the film varies with the alloy treated. On pure aluminium in a newly made bath a beautiful pearly grey-white colour is produced, but this is rarely seen in commercial practice, as slight impuritie.s in the metal and
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