Iron-aluminium alloys, diffusion

Physical metallurgy is a rather wide field of applications of Mossbauer spectroscopy and it is possible to enumerate only the main topics phase analysis, order-disorder alloys, surfaces, alloying, interstitial alloys, steel, ferromagnetic alloys, precipitation, diffusion, oxidation, lattice defects etc. Alloys are well represented by the iron-carbon system, the mechanism of martensite transformation, high-manganese and iron-aluminium alloys, iron-silicon and Fe-Ni-X alloys. 

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... 

Sprayed, vacuum-deposited and plated coatings can be applied to most metals and to many non-metals, e.g. vacuum deposition is applied to many substrates including plastics spray application can be used for coating fabric, plastic and paper. Hot dipping and other diffusion processes are dependent on the nature of the substrate for the properties of the coating. Most commercial applications of aluminium coatings are on iron and steel with smaller quantities applied to aluminium alloys and plastics. 

Dilute binary alloys of nickel with elements such as aluminium, beryllium and manganese which form more stable sulphides than does nickel, are more resistant to attack by sulphur than nickel itself. Pfeiffer measured the rate of attack in sulphur vapour (13 Pa) at 620°C. Values around 0- 15gm s were reported for Ni and Ni-0-5Fe, compared with about 0-07-0-1 gm s for dilute alloys with 0-05% Be, 0-5% Al or 1-5% Mn. In such alloys a parabolic rate law is obeyed the rate-determining factor is most probably the diffusion of nickel ions, which is impeded by the formation of very thin surface layers of the more stable sulphides of the solute elements. Iron additions have little effect on the resistance to attack of nickel as both metals have similar affinities for sulphur. Alloying with other elements, of which silver is an example, produced decreased resistance to sulphur attack. In the case of dilute chromium additions Mrowec reported that at low levels (<2%) rates of attack were increased, whereas at a level of 4% a reduction in the parabolic rate constant was observed. The increased rates were attributed to Wagner doping effects, while the reduction was believed to result from the... 

Hot Dipped Coatings Major attempts have been made to improve the quality of aluminised steel strip. Requirements on coating thickness and uniformity have been imposed. It is the speed of sheet movement, length of path in the molten bath, temperature and composition of the bath that control the thickness of the intermetallic layer which lies below the aluminium outer surface. The process of intermetallic alloy formation is diffusion controlled, and it is usual that some dissolution of iron into the molten aluminium does occur at a rate, Ac/At, which is given by... 

Little, M. V., Bonding Aluminium To Ferrous Alloys , Machinery, N. Y., 56, 173 (1950) Drewett, R., Diffusion Coatings for the Protection of Iron and Steel , Part I, Anti-Corrosion, 16 No. 4, 11-16, April (1969)... 

Internal oxidation was observed only at 900°C in samples with less than 10 at.% aluminium, whereas at 1000°C and 1100°C even 5 at.% aluminium was still enough to form a complete alumina outer layer. The internal oxides generate stresses, which cause formation of outer iron protrusions (Fig. 8). The pore formation beneath the oxide scales which is caused by Al-consumption and inward diffusion of Fe, is more marked for the ordered phases near 50 at.% and less for the disordered alloys with low A1 content. These, however, at 900 °C show a tendency to internal oxidation. Al203 is growing inward while Fe protrusions are pressed outward in the process [6,7],... 

The mole fractions of aluminium and chromium, as well as the molar volumes, can easily be determined. The diffusion coefficients have been chosen from the literature as given in Table 4.1. Only the solubility of oxygen in iron has not yet been measured without uncertainty [21], because even small levels of impurities lead to adulteration of the results by oxide formation. Thus in this work another method has been chosen. The critical value h has been set to 1/3. From [22] the lowest aluminium contents for the formation of a closed alumina layer on Fe-Al binary alloys have been chosen and the oxygen solubility is then calculated. The result is plotted in Fig. 4.1. The three data points are connected by the best fit straight line Xq = ll%e—kt— Compared... 

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