The Corrosion Resistance of Aluminium Alloys

The corrosion resistance of aluminium mainly depends on the alloying elements, i.e. the series to which they belong. The types of corrosion they may undergo are listed in Table B.6.3. 

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It is well known that the corrosion resistance of aluminium alloys can be improved by adding inhibitors to the aqueous environment. Consequently it would be worth evaluating the corrosion behaviour of aluminium alloys in the presence of inhibitors such as phosphates and chromates. 

The first approaches proposed in the literature [54,55] consisted of the addition of alumina or silica particles to improve the mechanical properties of the silane coating. The addition of these particles increased the impact, scratch and wear resistance. The corrosion resistance of aluminium alloys also seemed to increase with controlled amounts of particles [54]. This effect was attributed to the formation of silicate species that delayed corrosion activity. Silane films containing silica and formed under applied potential also revealed improved anti-corrosion behaviour when applied to aluminium substrates. In this case, critical silica contents were proposed [55]. 

A common method of maintaining high corrosion resistance of aluminium alloys is to clad the alloy with pure aluminium. Subsequent to cladding, the alloy cannot be heat treated as diffusion of alloying metals into the pure aluminium cladding will again reduce corrosion resistance. 

Although aluminium and its alloys have attractive nuclear properties, they have limited strength, poor compatibility with uranium at high temperatures and low corrosion resistance in water or steam at temperatures above 523 K. Hence their use is restricted to core components in research reactors, where temperatures do not exceed 423 K. However, various parameters, such as water quality, structural design (crevices, galvanic contact with other materials), alloy composition and irradiation, have significant influence on the corrosion resistance of aluminium in research reactors. 

Several smdies have been undertaken since 1965 to investigate the corrosion resistance of aluminium up to a depth of 2000 m in the Pacific Ocean [14]. These tests have shown that the corrosion resistance in deep waters is comparable to that observed at the surface. The forms of corrosion are the same. Pitting depth on 5000 series alloys is of the same order of magnitude whatever the depth [15]. On other alloys (3000 and 6000 series), it can be higher. The role of oxygen has been put forward as an explanation for these differences. 

Christian Vargel is renowned as one of the Aluminium industry s leading experts in aluminium corrosion. During his long and successful career within the Pechiney group, his expertise in corrosion was valuable in the product development of key markets such as automotive, marine and other transport applications. He has also given many presentations on the corrosion resistance of aluminium, and has contributed to many of Pechiney s technical documents and brochures, such as Aluminium and the Sea and Aluminium in Industrial Vehicles . His first book Le comportement de Valuminium et de ses alliages (The behaviour of aluminium arul its alloys) was published by Dunod in 1979. 

As in my first book, I have chosen a practical and realistic approach to the corrosion of aluminium a practitioner s approach. The corrosion resistance of an alloy depends not only on its chemical composition and metallurgical temper, but also on the joining method and the conditions of service. 

Metal dusting usually occurs in high carbon activity environments combined with a low oxygen partial pressure where carburisation and graphi-tisation occur. Usually pits develop which contain a mixture of carbon, carbides, oxide and metal (Fig. 7.52). Hochmann" proposed that dusting occurs as the result of metastable carbide formation in the high carbon activity gas mixture which subsequently breaks down into metal plus free carbon. The dependence of the corrosion resistance of these nickel alloys on the protective oxide him has been described accelerated or internal oxidation occurs only under conditions that either prevent the formation, or lead to the disruption, of this him. In many petrochemical applications the pO is too low to permit chromia formation (ethylene furnaces for example) so that additions of silicon" or aluminium are commonly made to alloys to improve carburisation resistance (Fig. 7.53). 

Applications Ion implantation is widely employed to improve the life of tools. Thus press tools, dies and gear cutters can be treated to increase their durability by three times or more. Nitrogen-implanted tungsten carbide drawing dies for copper and iron wire can be improved up to fivefold. By implanting chromium, aluminium or silicon a considerable increase in the corrosion resistance of steel can be obtained. Implantation of chromium into aircraft bearing alloys has improved their durability in marine environments . 

The zinc-aluminium alloys are most important. The zinc-55%-aluminium-1.5 -silicon alloy hot-dip coating was initiated over 20 years ago by the steel industry and has recently become of major worldwide importance (known as Galvalume, Zincalume, Alugalva, Aluzink, Aluzinc, Zincalit or Zalutite). The coating usually has 1(X)-4(X)<% more corrosion resistance than galvanising in the atmosphere, but less cathodic protection and also has the inherent problem of aluminium alloys when in contact with alkalis. 

Borbe, P. C., Erdmann-Jesnitzer, F., and Jun, E. J. (1978). Verbesserung des Korrosionswiderstandes von Zink-Aluminium-Legierungen dutch Stickstoff (Improvement of the corrosion resistance of zinc-aluminum alloys by nitrogen). Metall, 32(12), 1231-1234 (in German). 

Methods of avoiding pitting failures in copper cold-water tubes have been further studied . Many hot-forged brass water fittings are now made from modified alloys that have an a0 structure during forging and are then heat-treated to a dezincification-resistant a structure . The corrosion resistance of 0 aluminium brasses (shape memory effect alloys) has been studied . ... 

Secondly, rare earths (eerium or neodymium) have been used in treatments of the aluminium before or after anodizing for the purpose of improving the corrosion resistance of the anodized substrate. These treatments involve immersion of the alloy in a rare earth-eontaining solution either to ineorporate the rare earth into oxides or hydroxides produced on the alloy surface, or to introduce the rare earth into pores within the oxide as a modified sealing proeess. 

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