THE CORROSION BEHAVIOUR OF ALUMINIUM ALLOYS

Corrosion is a complex phenomenon that depends on many parameters that are related to the environment or to the metal. The occurrence of corrosion is sometimes difficult to explain because the cause of corrosion has not been identified, or because the theoretical foundations are sometimes insufficient to give a satisfactory answer. 

For users as well as for corrosion experts, corrosion has a relative aspect. In principle, corrosion is defined as the degradation of a metal by its environment. At which point is this then called corrosion This question is neither simple nor innocent. Appropriate electrochemical methods measure corrosion currents as low as 1 p,A, which corresponds to a dissolution rate of less than 1 p,m per year. 

In practice, corrosion needs to be associated with a risk that is deemed either acceptable or not acceptable. As an example, for roofing sheet 0.80 mm thick, corrosion means perforation rather than a uniform decrease in ffiickness of a few micrometers per year. Water staining is not acceptable for luxury goods, is only slightly acceptable for kitchen utensils, but will not even be taken into consideration for many components for which it is a part of the natural surface aspect. 

It is the task of corrosion experts to predict what will happen, to highlight possible limitations in lifetime or in acceptable stress for the material under consideration, to the extent allowed by theory and experience. Many grey and uncertain areas still exist. These will incite corrosion experts to adopt a cautious attitude, but also promote a certain boldness, because in many cases, predictions can be made based on experience with existing applications, without taking an inconsiderate risk. 

Corrosion is a complex phenomenon that is sometimes difficult to explain it is not just corrosion but several possible types of corrosion for a given metal. No metal or alloy is capable of resisting all possible aqueous media, even at room temperature. The corrosion resistance of a metal or alloy depends on many factors that are inherent to the metal itself, the environment in which it is placed, and the conditions of use. 

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N.E. Idenyi, C.E. Ekuma (2006). The effects of zinc additions on the corrosion behaviour of aluminium alloy in saline environments. Journal of the Nigerian Environmental Society 3, p. 109. 

Some understanding of the corrosion of aluminium alloys used as cladding on research and test reactor fuel has been obtained from the CRP. Aluminium corrosion is extremely complex and the variables affecting localized corrosion (pitting and crevice corrosion) act both independently and synergistically. Additional information about the effects of deposited particle composition on the corrosion behaviour of aluminium alloys is needed. Surface finish affects the corrosion of aluminium alloys, and more information is required with respect to this parameter. Additional data on the effects of certain impurity ions in basin water on localized corrosion behaviour are necessary to better identify the ions that cause corrosion. A goal would be to develop an equation for pitting as a function of water chemistry parameters. 

The corrosion research programme could usefully be extended to investigate the influence of various surface treatments, for instance anodizing and preliminary autoclaving, on the corrosion behaviour of aluminium alloys. 

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. 

It can be concluded that the corrosion parameters result obtained from PP, LPR and EIS measurements show the inhibitive effect on the corrosion behaviour of aluminium alloy in seawater by the studied inhibitors. The performance of natural products as corrosion inhibitors was evaluated by inhibition mechanism The results reveal that inhibition efficiency increases with the increase in concentration of the studied inhibitors. Similar to the findings reported previously (Yurt et al., 2006 Bhrara and Singh, 2006 El-Etre and Abdallah, 2000) the adsorption corrosion inhibitor mechanism is related to the presence of heteroatom such as nitrogen, oxygen, phosphorous, sulphur and long carbon chain length, as well as triple bond or aromatic ring in their molecular structures. 

CORROSION BEHAVIOUR OF ALUMINIUM ALLOYS IN THE SPENT FUEL STORAGE SECTION OF THE lEA-Rl RESEARCH REACTOR,... 

CORROSION BEHAVIOUR OF ALUMINIUM ALLOY TEST COUPONS IN THE SPENT FUEL BASIN OF THE CHINA INSTITUTE OF ATOMIC ENERGY, BEIJING... 

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]. 

The main objectives of the programme were to provide comparative information about the corrosion behaviour of different types of aluminium alloy coupon in the IR-8 storage basin water. The following results were obtained ... 

From the three sets of data cited above, it would seem that the effects of R additions on the corrosion behaviour of an Al-Fe alloy are far from clear. The very low corrosion rates (Khobaib and Kirchoff 1985, Langenbeck et al. 1986) for the Al-Fe alloys relative to other structural aluminium alloys such as 7075 and 2024 make it difficult to identify any significant effects of R additions. 

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 copper is not an inherently reactive element, it is not surprising that the rate of corrosion, even if unhindered by films of insoluble corrosion products, is usually low. Nevertheless, although the breakdown of a protective oxide film on copper is not likely to lead to such rapid attack as with a more reactive metal such as, say, aluminium, in practice the good behaviour of copper (and more particularly of some of its alloys) often depends to a considerable extent on the maintenance of a protective film of oxide or other insoluble corrosion product. 

While additions of mischmetal have a huge impact on the oxidation behaviour of iron-aluminium-chromium alloys, changes in the concentration of aluminium or chromium do not effect the corrosion behaviour significantly. The differences between the... 

The high temperature corrosion behaviour of different iron aluminides and iron-aluminium-chromium alloys containing 6-17 wt% aluminium, 2-10 wt% chromium and additions of mischmetal has been investigated in both air and hot process gases. 

In air, carburising atmospheres and sulphidising/oxidising atmospheres all alloys showed excellent corrosion resistance due to the formation of protective oxide scales. It was found that the concentration of aluminium or chromium does not significantly effect the corrosion behaviour in any of these environments. 10 wt% aluminium, probably even less, are sufficient to enable the formation of protective Al203-scales even at temperatures as low as 650°C. Overdoping with reactive elements (mischmetal), however, causes high oxidation rates in air and should be avoided. 

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