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Aviation History
1926
1926 - 0423.PDF
JUNE 24, 1926 63 THE AIRCRAFT ENGINEER StJl PLEMENT TO FLIGHT the attack is observed no particular harm will be done, except that the surface of the metal will have been slightly pitted and roughened. If the surface eorrosion is not removed, then the attack may proceed to some depth into the material, and at certain pits may progress to a comparatively considerable depth without giving any marked evidence of its increase (by the formation of surface deposits of corrosion products). The further corrosion of the metal may take place by the enlarge- ment of the surface pits, in which case the depth of any one pit may not be very great. On the other hand, the attack may be concentrated in a direction at right angles to the exposed surface of the metal, thereby producing a few rela- tively deep pits. Furthermore, the corrosion may proceed along the crystal boundaries, and thereby affect the metal for quite a considerable distance without producing any intense quantity of the tell-talo white surface deposit. In this last case it is quite possible to clean the surface of the material entirely from the corrosion product, and also to remove the obvious pits without removing entirely the metal which has been affected by the corrosive attack. Cleaning the surface of the metal removes the greater part of the corrosion and also removes the seat of the reaction. It therein' causes the corrosion to come to an end. It does not neces- sarily, however, remove all the Duralumin which has been affected by the corrosion and such metal may suffer consider- ably from the presence of internal lesions. Such material is usually weak and relatively brittle, and is thereby very readily detected. All the evidence goes to show that the formation of this type of corrosion is the result of a prolonged exposure to corrosive influences, and there is no evidence at all to indicate that the material develops an intercrystalline type of corrosion, or any severe attack at right angles to the surface, except when it has been exposed for a lengthy period to conditions that actively promote corrosion. It has been suggested that the corrosion of Duralumin may go on inside the material without any apparent attack on the surface. There is no evidence whatever that this opinion is correct, and there is multitudinous evidence to the contrary. Corrosion takes place from the surface of the metal, and, therefore, if the surface is protected against attack, the material throughout will be free from the effects of corrosion. If the corrosive agents are kept away from the surface, there can be no question whatever about corrosion occurring in the metal. It simply does not take place. What may occur is that on metal which is not protected on the surface the corrosive action may be confined to a relatively small number of surface pits, which allow the metal to be attacked at right angles to the surface, and not parallel to it, thereby producing an apparently large effect within the body of the metal, whilst giving comparatively small evidence of the existence of the attack on the surface of the metal. Duralumin which has been left undisturbed during the process of corrosive attack will always show upon its surface evidence of any corrosion that has occurred, and in the absence of such surface evidence it is entirely safe to assume that the interior of the metal is free from the effects of corrosion. If, however, the material has been allowed to corrode and then been cleaned, the evidences of corrosion may have been re- moved without the whole of the effects having been eliminated. It is for this reason that it is desirable to take precautions to avoid the occurrence of any corrosion over a lengthy period. As has been stated above, Duralumin does not corrode readily. It resists corrosion far more than steel does, and is much less attacked than the ordinary yellow metals. It is either from prolonged exposure to ordinary corrosive influences, such as saline solutions, or from shorter attacks by exceedingly violent re-agents, that a serious attack is likely to be met. It is foolish, however, to believe that Duralumin is entirely immune from corrosion. Its resistance to corrosion is certainly great, but by no means so great that the possibility of an attack can be overlooked, and it is decidedly desirable to protect the surface of the material at all times after manufacture so as to minimise the possibility of the production of pitting in such •a form as cannot readily be removed when once it has taken place. With regard to the ways in which the corrosion of Duralu- min may be accelerated, it appears only necessary to indicate those circumstances which tend to produce this result. The actual conditions to which Duralumin may be exposed in aeronautical work are sufficiently well-known to make it unnecessary to detail their effects separately. Provided that it is recognised that saline solutions, of which the most typical is sea water, exercise a corrosive influence upon Duralumin similar in effect to, but smaller in magnitude than, that produced upon ordinary iron and steel, little further need be said respecting the agents which actually produce the undesirable effects. Duralumin in all its different conditions does not, however, resist corrosion with the same facility. The most resistant form of Duralumin is the normal metal, that is the material that has been heat-treated and fully aged. Duralumin that has been annealed is not so resistant to corro- sion by a long way, and it is decidedly desirable not to use the metal in the annealed condition when it is likely tu be exposed to severely corrosive influences. Duralumin which has been cold-worked after heat-treatment is very nearly as resistant to corrosion as is the normal metal, and it is far and away better in this respect than the annealed metal. The difference between the normal and the cold-worked material is comparatively small, but the difference between annealed and heat-treated Duralumin is sufficient to make it possible to induce galvanic effects ly placing metal in those two conditions together. A further incentive to accelerated corrosion is produced by putting Duralumin in contact with other metals, and particularly the yellow metals. When copper or brass or bronze is in close contact with Duralumin, and the combina- tion is exposed to corrosive influences, the Duralumin is affected adversely. Of course, the two metals may be placed in contact and then protected adequately against corrosive influences. In such a case no detrimental result will follow. On the other hand, if steel and Duralumin are placed together corrosion occurs at a definitely accelerated rate but in this instance the more affected materal is usually the steel. Whilst discussing those points which affect the corrosion of Duralumin it may not be out of place to mention that dilute salt solutions are more active in their effect than con- centrated ones. If sodium chloride be taken as a typical salt solution it is found that the greatest attack takes place at a concentration of salt very much lower than that present in sea water. With a concentration about one-sixth as great as that present in sea water common salt solutions attack Duralumin the most intensely. It is desirable that this point should be borne in mind, particularly when the method of removing salt, e.g., from salt baths, is in question, and also when Duralumin may be exposed to an atmosphere that is essentially wet, and at the same time liable to be contami- nated with salt spray. The third question is that of the methods to be employed for the prevention of corrosion on Duralumin. These methods naturally form into two groups. In the first place it is quite possible to avoid corrosion completely by protecting the surface of the metal with paint or varnish or enamel, whilst in the second place it is possible by electro-chemical means to deposit on the surface of the metal a film of a protective nature. Duralumin lends itself fairly readily to protection by organic means, and this aspect of the problem, therefore, becomes simply one of providing a suitable paint, varnish or enamel. This is scarcely a metallurgical matter. It is quite well established that certain types of varnish do provide adequate protection against corrosion for Duralumin even under quite adverse conditions. There is always a tendency in aeronautical work to skimp the varnish with the intention of saving weight, but it appears that some of the better classes of varnish give adequate protection without any undue thickness of coating being rendered necessary. If stove enamels are being employed with Duralumin it is important to utilise an enamel that stoves at a reasonably low tem- perature. The majority of such stoving enamels need not be heated to a temperature greater than 101V C, and such a temperature has no deleterious effect upon the metal. If Duralumin is hsated to a temperature as high as 200' C. for any appreciable time, there is little danger of a lowering of the'maximum stress, but the normal metal tends to become 362 g
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