Aluminium alloys dissolution

On the other hand, pit initiation which is the necessary precursor to propagation, is less well understood but is probably far more dependent on metallurgical structure. A detailed discussion of pit initiation is beyond the scope of this section. The two most widely accepted models are, however, as follows. Heine, etal. suggest that pit initiation on aluminium alloys occurs when chloride ions penetrate the passive oxide film by diffusion via lattice defects. McBee and Kruger indicate that this mechanism may also be applicable to pit initiation on iron. On the other hand, Evans has suggested that a pit initiates at a point on the surface where the rate of metal dissolution is momentarily high, with the result that more aggressive anions... 

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. 

Highly porous metal sponges can be made in aqueous solutions by dissolution of aluminium alloys by a strong base. The metal sponges are typically used for hydrogenation in laboratory syntheses. 

Table 2 shows results obtained by Campbell [3] for the determination of silicon in aluminium alloys. In this procedure the sample was dissolved in 20% sodium hydroxide/hydrogen peroxide. After dissolution the solution was treated with a mixture of nitric and hydrochloric acids and diluted to volume. 

D.X. Yuan, X.R. Wang, P.Y. Yang, B.L. Huang, On-line electrolytic dissolution of solid metal samples and determination of copper in aluminium alloys by flame atomic absorption spectrometry, Anal. Chim. Acta 243 (1991) 65. 

The lowest potential is measured in the center, where corrosion (i.e. anodic dissolution of iron) attacks most aggressively. At the edges, the potential increases somewhat in this zone oxygen reduction proceeds. The potential changes around the drop imply the presence of an ultrathin electrolyte film because the potential reaches values of the bare iron surface only at a considerable distance from the edges of the macroscopically observed drop [213]. Filiform corrosion of automotive aluminium alloy AA6016 has been studied with SKP [221]. 

The second molten salt battery to have received detailed attention is the lithium/ iron sulphide battery. During discharge, the negative electrode reaction is the dissolution of Hthium from a lithium/aluminium alloy (10—20% Li), while the positive electrode reaction is the reduction of iron disulphide which occurs in stages... 

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 corresponding corrosion potential (Ecorr), corrosion current density (icon), anodic Tafel slope (ba), cathodic Tafel slope (be) and CR for uninhibited and inhibited systems from PP measurement are listed in Table 3. The data demonstrates that the Ecorr values shift to more positive values as the concentration of added studied inhibitors are increased. On the other hand, the corrosion current densities are markedly declined upon addition of the studied corrosion inhibitors. The extent of its decline increases with increasing of the corrosion inhibitor concentration. Moreover, the numerical values of both anodic and cathodic Tafel slopes decreased as the concentration of inhibitors were increased. This means that the three natural products have significant effects on retarding the anodic dissolution of aluminium alloy and inhibiting the cathodic hydrogen evolution reaction. 

Conversion coating processes produce a thin film of predominantly chromium oxide on metal surfaces. The colour of this film depends on the substrate metal, and may vary in colour, from pale-yellow to gold to dark-brown or black. Today, the most commonly used CCC process for aluminium, zinc and cadmium (Biestek and Weber 1976) is an acid treatment (pH 1—2), based on a two-part solution containing a source of hexavalent chromium ion, e.g. chromate, dichromate or chromic acid. The solution for treating aluminium alloys, generally contains fluoride ion, which assists in the dissolution of the original oxide film, and an accelerator, e.g. ferricyanide, to facilitate the formation of the chromium oxide (Biestek and Weber 1976). 

Accelerated galvanic corrosion of the Mg alloy matrix due to general dissolution of the Mg alloy followed by the re-precipitation of metallic Fe, Ni and Cu on the alloy surface. The re-precipitation of metallic Fe, Ni and Cu on the alloy surface is the key part of this mechanism, which is similar to the acceleration of corrosion by alloyed copper for aluminium alloys. 

Dissolution potentials of the most common aluminium alloys are listed in T able B. 1.4 [ 16]. For a given alloy, variations by 50-100 mV from one author to another have been reported. 

Figure B.1.8. Influence of alloying elements on the dissolution potential of aluminium alloys [17].

Table B.1.4. Dissolution potentials of aluminium alloys (NaCl solution, H2O2, ASTM G69)...

The comparison of the dissolution potentials of aluminium alloys may reach absurdity, for example, leading to a preference for alloys of the 2000 series, which have a dissolution potential far less negative, about — 650 mV, over those of the 5000 series, which have a more electronegative potential, on the order of - 800 mV (Table B.1.3). And yet the latter show excellent corrosion resistance, while alloys of the 2000 series are highly susceptible to pitting corrosion in natural environments. 

Taking into account the dissolution potential of aluminium, anodes can be made either in zinc or in a special aluminium alloy called Hydral , which contains indium (0.015-0.025%) or tin (0.10-0.20%). Magnesium anodes must not be used, because they lower the potential too much and will thus lead to severe cathodic corrosion of aluminium. 

In general, the dissolution rate of aluminium alloys is even higher than that of aluminium grade of the 1000 series. 

This is a strong acid and one of the most aggressive products towards aluminium and even more so towards aluminium alloys [1]. For grades of the 1000 series, the dissolution rate slightly decreases with increasing aluminium content. 

Table E.5.2. Dissolution rate of aluminium alloys in hydrofluoric acid at — 10 °C...

Due to the rather low mechanical strength (/ = 90 MPa) of this material, alloys have been considered for this use. In general, the resistance of aluminium alloys at room temperature is slightly less good than that of unalloyed aluminium. Alloying elements such as silicon, copper, zinc or magnesium slightly increase the dissolution rate, especially at low acid concentrations. 

The dissolution rate in phosphorous acid is lower than in hydrochloric or sulphuric acid, but still too high to use aluminium alloys in contact with phosphoric acid solutions [21], except in a very narrow range of concentrations and temperatures [22] (Figure E.5.6). 

Phosphoric acid solutions are used for pickling of metals, especially aluminium alloys. The dissolution rate can be controlled by adding chromates, which slow down the attack, depending on the acid concentration [23] (Table E.5.12). 

In the absence of humidity, dry calcium chloride has no action on aluminium alloys. Solutions have a slight action the dissolution rate is low [8], in the order of a few tenths of a millimetre per year (Table E.6.1). Concentration and temperature have no clear influence. If pitting occurs, its depth normally does not exceed 0.1-0.2 mm. The addition of 2% sodium chromate to calcium chloride solution inhibits possible pitting corrosion. 

The resistance of aluminium alloys depends on the composition of the sludge the more alkaline, the quicker the metal s dissolution. In lime sludge (pH 11.9), the dissolution rate is 5 mm per year, while in betonitic sludge (pH 9.8) it amounts to 0.20 mm per year. 

As an example, in a solution containing 37% formaldehyde and 1% methanol, the dissolution rate is 200 JLm per year at room temperature. The presence of other organic products such as methanol or formic acid can slightly increase the dissolution rate [3]. Like formaldehyde, formol is stored and transported in equipment made in aluminium alloy. 

Pure pyridine does not attack aluminium its solutions are alkaline and lead to a very superficial pickling of aluminium. Equipment in aluminium is used for the extraction of pyridine. The azeotropic mixture water-pyridine (60 40) has only a very weak action on aluminium the dissolution rate is in the order of 25 p,m per year with alloys 3003, 5052 and 6061 [1]. 

CLSM and Near-Field Scanning Optical Microscopy has been used by R.W Smyrl to map the fluorescence during corrosion of 2024 and 6061 aluminium alloys. It has been shown that on 6061 alloy fluorescence is emitted from the Al-Mg-Si intermetal-lics. The fluorescence is in fact trapped by the corrosion products (Fig. 9a) due to the selective dissolution of Mg it is an indirect... 

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