1 General Survey

The aluminium bronzes are a family of copper-base alloys containing approximately 5% to 11% aluminium, some having additions of iron, nickel, manganese or silicon. They include alloys suitable for sand casting, gravity diecasting and for the production of forgings, plate, sheet, tube, strip, wire and extruded rods and sections. Compared with other copper alloys, the higher strength of the aluminium bronzes is combined with excellent corrosion resistance under a wide range of service conditions. Maximum corrosion resistance is provided by control of the composition and manufacturing history of the material as discussed in detail in Section 2(i).

Aluminium bronzes are the most tarnish-resistant copper alloys and show no serious deterioration in appearance and no significant loss of mechanical properties on exposure to most atmospheric conditions. Their resistance to atmospheric corrosion combined with high strength is exploited, for example, in their use for bearing bushes in aircraft frames. Aluminium bronzes also show low rates of oxidation at high temperatures and excellent resistance to sulphuric acid, sulphur dioxide and other combustion products and are, therefore, used for the construction of items exposed to either or both these conditions. For example, aluminium bronzes are used very successfully for inert gas fans in oil tankers. These operate under highly stressed conditions in a variable but very corrosive atmosphere containing salt-laden water vapour, sulphurous gases and carbon.

No engineering alloy is immune to corrosion. Corrosion resistance depends upon the formation of a thin protective film or layer of corrosion products which prevents or substantially slows down the rate of attack. The aluminium content of aluminium bronzes imparts the ability to form, very rapidly, an alumina-rich protective film which is highly protective and is not susceptible to localised breakdown and consequent pitting in the presence of chlorides. Aluminium bronzes are, therefore, very resistant to corrosion by sea water and probably find more use in sea water service than in any other environment.

Virtually all metals and alloys in common use are susceptible to some extent to crevice corrosion, i.e. accelerated attack within or just at the edge of areas shielded by close proximity to other components or by deposits on the surface. Crevice corrosion in service is particularly objectionable when it takes the form of pitting or severe surface roughening on shafts or valve spindles in the way of bearings or seals. Any crevice corrosion of aluminium bronzes, however, takes the form of minor selective phase dealloying as described in Section 3(iii), Section 3(iv) and Section 3(v) which results in little reduction of strength and practically no impairment of surface finish. Aluminium bronzes are, therefore, very widely used for pump shafts and for valve spindles - situations where pitting corrosion in crevices makes stainless steels, for example, unsuitable.

A form of selective phase dealloying of aluminium bronzes commonly known as 'dealuminification' which caused some concern some years ago is no longer a significant problem. This type of attack, similar to the dezincification of duplex brasses, results in selective dissolution of the principal alloying element (in this case aluminium) from one phase of the alloy leaving a residue of porous copper which retains the original shape and dimensions of the component but has little strength. By controlling the composition and, for the alloys of high aluminium content, the cooling rate from casting or working temperature, metallurgical structures are ensured that will not suffer dealuminification to any significant extent under any normal conditions of use.

Metal failures in service are often the result of the combined influence of corrosion and mechanical factors, the most common being stress corrosion, which occurs under the simultaneous action of high tensile stress and an appropriate corrosive environment, and corrosion fatigue which occurs under cyclic stressing in a corrosive environment. Brasses, for example, show high susceptibility to stress corrosion in the presence of even small quantities of ammonia, and austenitic stainless steels suffer stress corrosion cracking in hot chloride solutions. High resistance to stress corrosion cracking is an important reason for the use of aluminium bronzes by the British Navy for underwater fastenings. High tensile brasses, formerly used for this service, were very liable to fail by stress corrosion but stress corrosion failures of aluminium bronze fasteners have proved extremely rare.

High resistance to corrosion fatigue is essential for marine propellers and it is principally for that reason that most large propellers are made from nickel aluminium bronze. This material is quite outstanding in resistance to corrosion fatigue in sea water, being much superior to high tensile brass or to stainless steels. Manganese aluminium bronze, which is also used for large propellers, also has high corrosion fatigue strength though somewhat inferior to nickel aluminium bronze.

Turbulent water flow conditions can cause local erosion of the protective films on which alloys depend for their corrosion resistance and result in localised deep attack by a combination of corrosive and erosive action. The corrosion/erosion resistance of the aluminium bronzes is substantially higher than that of the brasses and similar to that of 70/30 copper-nickel which is generally recognised to be one of the alloys most resistant to this type of attack.

At higher water flow rates, such as exist in pumps and on some areas of marine propellers, formation and collapse of vapour cavities in the water can produce very high local stresses leading to cavitation damage. The resistance of alloys to cavitation damage generally increases with their resistance to corrosion fatigue and with their ability to reform protective films rapidly on the metal freshly exposed by cavitation erosion. The advantages of aluminium bronze over most other alloys in these respects have already been mentioned and it will be no surprise, therefore, that aluminium bronzes show exceptionally high resistance to cavitation damage. This is an important feature in their use for marine propellers and the principal reason for their use for impellers in high duty pumps.

However, the soundness of the casting has a very significant bearing on resistance to cavitation erosion and impingement attack, and maximum resistance cannot be expected from a casting produced by bad foundry practice.

One further property of aluminium bronzes should be mentioned in this general survey of their corrosion resistance. In most practical engineering situations different metals or alloys are used in contact with each other in the presence of an electrolyte such as sea water or fresh water. In these circumstances the possibility of galvanic action, causing accelerated attack on the less noble metal, can be very important. Aluminium bronzes are slightly more noble than most other copper alloys and slightly less noble than the copper-nickel alloys but the differences are too small to cause significant galvanic effects. Monel, stainless steel and titanium are all considerably more noble than aluminium bronze but it is found in practice that, providing the exposed area of the more noble metal does not greatly exceed that of the aluminium bronze, very little acceleration of corrosion of the aluminium bronze occurs. It is for this reason that aluminium bronze tubeplates are used in condensers with titanium tubes.

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