Aluminium alloys aqueous corrosion

Amott DR, Hinton BRW, Ryan NE (1989), Cationic fihn-forming inhibitors for the protection of AA 7075 aluminium alloy against corrosion in aqueous chloride solution . Corrosion, 45,1,12-18. 

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. 

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. 

Table 2. Dissolution current densities and corrosion potentials ( ,) for aluminium alloy uncoated and coated with TEOS, PTES and PTMS, after immersion for 24 h in aqueous solution 0.05 mol/1 NaCl.

The review of rare earth conversion coatings presented in this chapter delineates the wide, and ongoing, interest in the rare earths for corrosion protection. The aqueous inhibition qualities of the simple rare earths salts for a range of metals have been known for nearly 30 years. The focus of the rare earths has been primarily on cerium, with a significant amount of research also performed on lanthanum systems. The evolution of the development of the immersion process to viable commercial processes has, however, been slow. Much work has focused on aluminium alloys and a commercial process was developed for non-aerospace applications for aluminium alloys. The development of conversion coatings for other metals has been slower for a range of reasons. 

Lanthanides, especially cerium, fulfil the basic requirements for alternative corrosion inhibitors the ions form insoluble hydroxides, which enable them to be used as cathodic inhibitors they have a low toxicity and are relatively abundant in nature. Cerium has a high afimity for oxygen and the bond between cerium and oxygen is unlikely to be broken under the potentials applied. For some aluminium alloys, cerium precipitation from aqueous solutions of cerium salts was observed on cathodic intermetalhc compounds and in some instances, the oxide covered the entire specimen surface [14-19]. 

Resistance to stress-corrosion cracking Commercially pure titanium is very resistant to stress-corrosion cracking in those aqueous environments that usually constitute a hazard for this form of failure, and with one or two exceptions, detailed below, the hazard only becomes significant when titanium is alloyed, for example, with aluminium. This latter aspect is discussed in Section 8.5 under titanium alloys. 

Although aluminium is a base metal, it spontaneously forms a highly protective oxide film in most aqueous environments, i.e. it passivates. In consequence, it has a relatively noble corrosion potential and is then unable to act as an anode to steel. Low level mercury, indium or tin additions have been shown to be effective in lowering (i.e. making more negative) the potential of the aluminium they act as activators (depassivators). Each element has been shown to be more effective with the simultaneous addition of zinc . Zinc additions of up to 5% lower the anode operating potential, but above this level no benefit is gained . Below 0 9 7o zinc there is little influence on the performance of aluminium anodes . Table 10.10 lists a number of the more common commercial alloys. 

Electrodeposition of PANI derivatives was achieved by Shah and Iroh [108] in the case of Al alloy (2024) and N-ethylaniline in aqueous oxalic acid solution. The coating was electrodeposited by cyclic voltammetry by application of an unusually large potential range (-1V to -I-3V/SCE). Corrosion current was measured in chloride solutions from Tafel plots and was found to be significantly lower (about one order of magnitude) than that of bare Al-2024. Multifunctional coatings (PPy/PANI) were also achieved on aluminium by coelectropolymerization of aniline and pyrrole (in a feed ratio of 3 7) in oxalic acid solution, but protection performances were not given [109,110]. 

RA3 spent fuel elements are normally maintained in a decay pool for a limited time before being stored in the Central Storage Facility (CSF) in Ezeiza. In the case of RA6, the bundles that are not in use are kept in this reactor s decay pool (DP). In all cases, the fuel has to sustain long periods of immersion in water. In aqueous environments, aluminium and its alloys are known to be subject to corrosion processes, which are strongly dependent on the water quality. In the RA6 reactor pool (RP) the water is continuously monitored for conductivity. Whenever this value reaches a level of about 0.8 pS/cm, the purification procedure is initiated and is continued until the value has decreased to 0.4 pS/cm. In the RA3 and RA6 DPs, however, the monitoring is not on-line. 

However, the pH is not sufficient in order to predict the corrosion resistance of a metal or an alloy in aqueous solution. The nature of the acid (and thus of the anion associated with the proton H ) and of the base (and thus of the cation associated with OH ) also need to be taken into account. For example, mineral acids such as hydrochloric acid strongly attack aluminiiun. The rate of attack increases with concentration. On the other hand, concentrated nitric acid does not react with aluminium. Due to its oxidative action, it even... 

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