Between Aluminium Alloys

The dissolution potentials of alloys of the 5000 and 6000 series as well as those of magnesium and silicon-containing casting alloys of the series 40000 and 50000 are very close to each other, and very similar to that of unalloyed aluminium of the 1000 series (Table B.1.4). Therefore, there is no risk of galvanic corrosion between these materials. 

In aluminium ship building, planking and bulkheads are always in alloys of the 5000 series (5083,5086,5754, etc.), while stiffeners, handrails and floors are in extruded profiles 

However, galvanic corrosion may occur between alloys of the 1000 and 2000 series, as well as between 7072 and alloys of the 3000 series. The difference in potential between these alloys can be used for cathodic protection by cladding layers (see Section B.1.5) 2024 may thus be protected by 1050A, and 3003 by 7072. 

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From its ranking on the scale of potentials (Table B.1.1), aluminium is more electronegative than most common elements steel, stainless steel, cuprous alloys, etc. Whether in mechanical applications, building, electrical engineering, ship building, heat exchangers or circuits of aqueous liquids (fresh water or seawater), it is common to find heterogeneous assemblies in which there is contact between aluminium alloy components and components made of other metals. 

While contact between aluminium alloys and copper and cuprous alloys (bronze, brass) causes no appreciable galvanic corrosion of aluminium under atmospheric conditions, it is nevertheless advisable to provide insulation between the two metals to localise surface corrosion of the aluminium. 

Fig. I.I2 Curves showing the relationship between strength, stress-corrosion susceptibility and heat treatment for a high-strength precipitation-hardening aluminium alloy...

In more recent work embrittlement in water vapour-saturated air and in various aqueous solutions has been systematically examined together with the influence of strain rate, alloy composition and loading mode, all in conjunction with various metallographic techniques. The general conclusion is that stress-corrosion crack propagation in aluminium alloys under open circuit conditions is mainly caused by hydrogen embrittlement, but that there is a component of the fracture process that is caused by dissolution. The relative importance of these two processes may well vary between alloys of different composition or even between specimens of an alloy that have been heat treated differently. 

Another simplification would be to use fewer sections, say between 5 and 10, the length-to-diameter ratio L/D could be just 2 and the mould could be made of a material which is easier to machine like brass or aluminium alloy. 

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