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Aviation History
1940
1940 - 1982.PDF
34 JULY II, 1940 large percentages of copper, magnesium or zinc) the degree of metal solution during treatment is less than by any other process. Hence., such alloys as those which pit in other processes may be treated by the Sheppard process to produce a fine smooth finish. This is particularly useful when parts with fine-cut threads must be treated, and such parts made, for example, from R.R.66 alloy should be specified for sulphuric acid treatment. The film produced on most alloys and on pure aluminium is of an almost colourless glassy nature and does not greatly alter the appearance of the metal from that before treat- ment. Polished metal retains almost all its lustre, pro- vided its surface is of a homogeneous nature. An excep- tion is the group of silicon alloys. These progressively darken with increase in percentage of silicon, until L.33 anodises to a dark purple grey. Even 1-2 per cent, of silicon will produce a dullisn grey effect somewhat similar to pewter. The film when first produced is very highly absorbent and of very even texture, and it is on films produced by sulphuric acid treatment that the greatest success has been obtained in the production of coloured finishes by dyeing. One interesting application of the sulphuric acid process is the treatment of pistons of internal-combustion engines. The pistons, after anodising, are sealed in hot lubricating oil; absorption of the oil takes place, and the oil film so produced has very great resistance to removal. Even extraction with hot organic solvents will not remove it completely. Such anodising and impregnation has been shown to prevent dry starting, as the oil film on the piston operates until the lubricating system comes into play. Other improvements noticed are a reduction of scoring on the piston and cylinder wall, and a reduction of ring groove wear. The anodic film as formed has a high insulation value, and on water sealed sulphuric acid films will often with- stand an applied potential of 500 v. Oxalic Acid Processes These can only be briefly touched on here, as they are not operated in this country, and so are only of academic interest. The principal process is the Eloxal process, used exten- Packing rivets into containers for submersion in the anodisingbath. On removal from the bath the rivets are stained for identification. Small components are wired together to provide the necessaryelectrical contact. sively in Germany. It employs a 5 per cent, solution of oxalic acid at a temperature of from 20-35 deg. C, and an applied voltage of 40-60 volts. Either D.C. or A.C. may be used, and according to one process a D.C. current is superimposed on an A.C. current. This effect is obtained by a very complicated electrical circuit, but especially flexible films are claimed to be produced suitable for insu- lation of wires, or for the anodising of sheets prior to shallow pressing or drawing processes. These films can be coloured and sealed in a similar way to those produced by sulphuric acid processes, and those obtained by the use of A.C. at low temperatures are of a yellow to bronze shade when produced. Some notes on the limitations of anodising may also be of value. It is impossible to anodise any other metal than aluminium or its alloys ; most other metals in contact with aluminium undergoing anodic oxidation are dissolved. Hence, it is impossible to treat composite parts embodying other metals than aluminium. For example, an aluminium wheel with a cast-in bronze bush cannot be anodised as the bush would be destroyed. For decorative finishes the metal used must be as perfect as possible, as all defects will be accentuated by anodising. Castings for the purpose are best gravity die cast, as the close-grained structure due to the rapid cooling of the sur- face produces a much more even crystal structure which, even if revealed by the anodising, is not unpleasant in appearance. Pressure die castings anodise well, but usually contain a large percentage of silicon, and in consequence can only be dyed black. Alloys containing more than 10 per cent, of copper, 5 per cent, of zinc or 5 per cent, of magnesium are very prone to pit during anodising and should be avoided where possible. Secondary alloys produced from remelted scrap are also to be avoided, as quite large quantities of brass frequently find their way into the remelted scrap and produce un- wanted and unknown amounts of zinc and copper in the alloys. Deep crevices should be avoided, as it is very difficult to wash out the electrolyte from such places, and for the same reason it is preferable to anodise assemblies in their component parts wherever possible.
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