Aluminium alloys composition effects

In addition to examining pre-exposure effects, the slow strain-rate testing technique has been used increasingly to examine and compare the stress-corrosion susceptibility of aluminium alloys of various compositions, heat treatments and forms. A recent extensive review draws attention to differences in response to the various groups of commonly employed alloys which are summarised in Fig. 8.57. The most effective test environment was found to be 3 Vo NaCl -F 0.3 Vo HjOj. The most useful strain rate depends upon the alloy classification. 

Thus, in summary, the composition can be divided into propellant, emitter and additives. The propellant is invariably gunpowder, whilst the emitter might be carbon, steel, iron, aluminium, magnesium/ aluminium alloy or even titanium. Additives are often used to promote the visual effects and to cheapen the composition. 

Aluminium alloys well with up to about 3-5 per cent, of tantalum, which has no effect, however, on the mechanical strength, ductility, and working properties of aluminium.3 Reduction of tantalum pentoxide by the thermite process yields hard, brittle alloys.1 A substance the composition of which corresponds with the formula TaAls has been obtained by reducing potassium tantalum fluoride, K2TaF7, with aluminium filings at a high temperature. It is described as an iron-grey crystalline powder, of density 7-02, which is scarcely attacked by acids.5... 

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 participants were instructed to expose these racks in representative spent nuclear fuel assembly storage basins. They were also encouraged either to add coupons made of site specific alloys to these CRP racks or to fabricate similar racks with site specific alloy coupons. None of the participants added site specific alloy coupons to the racks of Batch I. China, however, prepared and exposed a separate rack with Chinese alloy coupons. This rack was exposed prior to the second RCM. The Chinese rack included coupons of the aluminium alloys 305 and LT24L and of SUS 304-8K the composition of these alloys is given in Table 4.2. A photograph of this rack is shown in Fig. 4.3. Table 4.3 shows the position of these alloy coupons in their additional rack. Brazil manufactured a separate rack (the IPEN rack) of coupons. This included a large number of coupons in different configurations, to allow evaluation of the effect of fuel... 

The IPEN rack exposed aluminium alloys 1060,6061 and 6262, used in the fabrication of fuel assemblies for IPEN s lEA-Rl research reactor. The composition of these alloys is given in Table 4.4. Besides 80 mm diameter coupons of the three alloys, the rack included coupons of 1060 in the processed and scratched condition (to simulate the effect of scratches formed on fuel assemblies during handling in the reactor) and various combinations of galvanic and crevice couples. Table 4.5 lists the sequence of the coupons in the IPEN rack. 

In 1996 the IAEA initiated a CRP on the corrosion of aluminium clad spent research reactor fuels to help evaluate the state of the spent fuel assemblies and to inform pool/basin operators regarding maintenance and housekeeping procedures to extend the lives of the FAs. The main activities of this programme are related to exposing racks of aluminium alloy specimens (coupons) in different spent fuel basins around the world. Five racks were suspended in the 1EA-R1 reactor pool and were subsequently withdrawn after different time spans to evaluate the extent of corrosion of the coupons as a function of alloy composition, crevices, bimetallic effects and water chemistry. During this period the pool water was monitored for pH, conductivity, chloride ion content and radiometry (Table 6.3). The IAEA CRP racks are denoted as racks 1,2A, 2B, 3A and 3B. 

Iron-nickel alloys are known to dissolve in the aluminium melts non-selectively. " As seen from Table 5.3, during dissolution of a 50 mass % Fe-50 mass % Ni alloy the ratio, cFe cNi, of iron to nickel concentrations in the melt is 1.00 0.05, i.e. it is equal to that in the initial solid material. The same applies to other alloys over the whole range of compositions. Respective saturation concentrations are presented in Table 5.4. The data obtained display a strong mutual influence of the elements on their solubilities in liquid aluminium because in its absence the solubility diagram for a constant temperature would be like that shown by the dotted lines in Fig. 5.5, with the eutonic point, E, at 2.5 mass % Fe and 10.0 mass % Ni. The effect of iron on the nickel solubility is seen to be more pronounced than that of nickel on the iron solubility. 

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