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Aviation History
1938
1938 - 2159.PDF
ULY 28, 1938 49 SUPPLEMENT TO FLIGHT THE AIRCRAFT ENGINEER ' CORROSION-RESISTING MATERIALS A Review of Those Relevant to Aircraft Construction By FRANCIS A. FOX, M.Sc* THE corrosive media acting, or liable to act, on the metallic parts of aircraft are numerous. Moisture, oxygen and carbon dioxide exist almost universally in any atmosphere. In addition to these, chlorine com pounds are present over and near the sea, and sulphur compounds are important factors in atmospheric corrosion in smoky manufacturing areas. Seaplanes are acted upon by sea-water itself, while cylinder walls may be liable to rapid corrosion at high temperature and humidity. Special cases of atmospheric pollution add further corrosive agents to this formidable list. There are two classes of metals which are inherently capable of resisting corrosion to a greater or smaller extent. These are :— 1. Those metals with little tendency to pass into solu tion—i.e. those with low solution pressures—the " noble " metals. Copper, for example, has less tendency to combine or go into solution than iron. The electro chemical series is an arrangement of the elements in the order of their solution pressures, and from this a general idea of the tendency of metals to corrode may be formed. In specific cases, however, which may be complex, a con sideration of the electro-chemical series may not in itself be sufficient to enable predictions as to behaviour to be made. 2. Those metals which form protective oxide films. Such a film, itself the result of tarnish or corrosion, may prevent further access of the corroding agent, may be sufficiently strong and flexible to maintain its continuity under conditions of stress, and may be self-repairing in the event of temporary damage. A film of this sort may confer outstanding corrosion-resistant properties on the basic material. A material which owes its immunity to an oxide film must, however, only be used under conditions in which the film can maintain itself chemically. For example, in oxidising conditions, chromium (and chromium alloys), though it has a much higher combining power than silver, will preserve a metallic surface much more readily than silver. Under reducing conditions, however, where the oxide film cannot persist, chromium will decompose, while silver will be unchanged. As distinct from these two classes, there is another category into which materials suitable for use under corrosive conditions may fall, and this is : materials which are not themselves corrosion-resistant, but which may be used when suitably protected from the form of attack likely to be encountered. Such protection is plating, spraying, lacquering, " sherardising," etc. In the case of metallic protection the relative positions of protecting and protected metals in the electrochemical series is of import ance. For example, chromium and nickel have high corrosion resistance, and when plated on iron confer this quality on the combined product so long as the plate forms an unbroken layer. If the iron is exposed, however, even at pores or small cracks, the presence of the " protecting " nickel or chromium accelerates corrosidn of the iron at these points, and rust " bleeds " out from the pores. Cadmium and zinc, on the other hand, do not behave in this way towards iron when exposed at discontinuities ; they actually tend to prevent corrosion of the iron, and they themselves are " sacrificially " corroded. Before proceeding to a review of those materials revelant to aircraft construction which are best able to resist corrosion, some reference may be made to the different types of corrosion which may be encountered. These can be indicated as follows :— a) Bi-metal " galvanic " attack. (b) Pitting. The author is a lecturer at the Birmingham Central Technical College. (c) Concentration or solution cells. (d) Corrosion-cracking (" season " cracking). (e) Corrosion-fatigue. (a) Bi-metal " Qalvanic" Attack This type of corrosive attack is well known as it is connected with the simplest form of electric cells ; it is due to the corroded metal having been electrically con nected to another metal under corrosive conditions. Because it is so well known, it is generally thought that this action must necessarily be severe, and that two dis similar metals in contact must inevitably corrode rapidly. In practice, however, there are various factors which modify this viewpoint, and under many sets of conditions no accelerated corrosion is produced at all. For example, the circuit must be complete through the metals and also the corrosive solutions : metals are good conductors, but there may be considerable resistance in the solution and in metal contacts ; accumulation of corrosive product also may have an insulating effect which will stop the action. This effect can, of course, be intensified by the deliberate insulation of the dissimilar metals (if the design permits) by a non-conductor such as pitch, shellac, red-lead paint, marine glue, or by varnished fabric. The electrochemical relation of the two metals is, of course, of great importance, and if one is much more "noble" than, the other, the galvanic tendency will be enhanced. Where the metals are near to each other in the electrochemical series, however, the effect is not great. For example, Magnesium in contact with copper will readily form a galvanic couple ; Magnesium in contact with Aluminium, however, forms a combination the galvanic action of which is not likely to be so severe. " Polarisation " effects due to the evolution of gases at the surface of one of the metals may also tend to slow down galvanic action. Where there is considerable difference in the areas of the two metals in contact, the action will be influenced; a larger area of " noble " metal produces more rapid corrosion on a small area of " baser " metal. For example, a steel rivet in copper sheet is rapidly corroded ; but a copper rivet in a steel sheet will produce little corrosion except at the contact. (b) Pitting Pitting is a common form ot corrosion, in which the attack is localised in a number of sharply defined areas ; it is usually associated with the formation of solid corrosion products which are located at or near the site ot each pit. Pitting may be due to a variety of causes, one ol the most important of which is lack of homogeneity in the metal. The heterogeneous particles or inclusions, under favourable conditions, may cause the formation ot local galvanic couples ; if there are inclusions which are permeable to the corroding fluid (even to moisture) there may form a pocket which can contain the fluid different in concentra tion or even in composition from that of the general body of the rest of the fluid ; this will give rise to " concentration- cell " pitting which is highly destructive because it tends to continue its action indefinitely, and not to stop itself, as i* often the case in other forms of corrosion. Heterogeneity which gives rise to pitting need not necessarily arise from the presence of inclusions. Com position gradients within individual metal crystals, or unevenly distributed strains may be sufficient to give rise to local differences in solution pressure which in turn may lead to pitting : this cause of pitting is not easy to discern and may be a dangerous one. Pitting is, of course, highly objectionable because of its stress-raising action, and particularly as its presence is not always immediately obvious.
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