Origen y desarrollo de la marca país Ecuador

The oxidation behaviour of the three iron-manganese alloys investigated showed a very similar pattern to that of pure iron. The initial rate of oxidation was higher for the iron-manganese alloys and the level of oxygen picked up by the droplets increased with increasing manganese content.

The iron-manganese equilibrium diagram, Fig.171, shows complete solubility in both the solid and liquid phases, and manganese

(m.pt 12/,7,°C) forms an almost ideal solution in liquid iron

(m.pt 1535°C). Manganese oxide MnO is thermodynamically much more stable than wustite, Fe-j_xO, at all temperatures and at 1600°C there is a difference in standard £ree energy of approximately 195 kJ. However, it has been shown (Appendix IV) that when

dissolved in iron, at the composition levels encountered in these experiments, the manganese oxide is only marginally more stable than pure wustite when oxidised in air (P02 = 0.2). At 1600°C, for the 0.5$ Mn, 0.7$ Mn and 1.0$ Mn dissolved in iron the Gibbs free energy differences are approximately 30, 40 and 50 kJ.

The reaction product between manganese and oxygen dissolved in liquid iron can be either a solid or liquid oxide, depending on the composition of the liquid and the temperature. At 1600°C

the product is liquid with up to about 0.2 mass/6 Mn and is a solid at higher concentrations^-^) #

In the Fe-0 system there exists a composition range in which two immiscible liquids co-exist in equilibrium, Fig.172. Iron is the primary crystalline phase in this region and the wustite-iron eutectic is on the oxygen side of the two liquid region. The

Mr>-0 system probably has a phase diagram with a two liquid region^^) The system iron oxide-manganese oxide in air is shown in Fig. 173. A very flat minimum appears on the liquidus and solidus curves and

extensive solid solution formation takes place among the various oxides which are stable at temperatures below the solidus.

At the onset of oxidation of these iron-manganese alloys, i.e. with up to 1% manganese, the oxide will separate out as a liquid phase while a large proportion of the metal is liquid. The manganese content would have to be considerably higher before the oxide phase separated out as solid FeO-MnO crystals. Hence the situation is very similar to that of pure iron. Oxygen both reduces the surface tension and viscosity of iron and will assist in pro­ moting intefacial turbulence. Manganese has a negligible effect on surface tension^^*^. Compared to the iron droplets, the only major difference is the presence of manganese in the iron with a greater chemical potential for oxide formation.

EDX Analysis carried out on a small chord section of a

Fe-0.3^ Mn and on a section of a Fe-1.0$ Mn sample showed a change in the manganese profile across the specimens, Figs. 105 and 107. In the case of the 0.3 rnass^ Mn sample, the manganese level in the zone adjacent to the oxidised edge was increased. For the 1.0 mass$

Mn sample, the manganese level in the iron, again in the zone adjacent to the oxidised edge, had been reduced. It was evident that some migration of manganese in these regions had occurred. This migration could occur in two ways. Firstly, by diffusion in the iron-manganese alloy. Secondly, depletion of the sub­ surface layers of the alloy could occur by evaporation of manga­ nese or a combination of both processes. It has been reported that at low oxygen concentrations and high manganese contents,

(67)

evaporation occurs . This condition was possible in this work when the droplets were held molten in the levitation coil under hydrogen gas. Further, the manganese level in the oxide film formed on the surface was considerably higher than the initial alloy composition (Table 124). It must be noted that the actual manganese and iron concentrations were considerably higher than the figures quoted in Table 124• The figures, taken from Figs.140 and 142, were distorted by the presence of silicon (present due to interaction of the drcplet with the silicone oil quenchant). The important factor was the TFe/Mn ratio. For a 1% Mn alloy the original ratio was 99 8l, However, from SIMS analysis, the ratios in the oxide were between 2.1:1 and 0.4:l(Table 124). The highest ratio was 2.1:1 at the air-oxide interface. It is well known that between 700°C and 1200°C iron oxidises in air to give a sequence of three layers(^13)# ^he inner wustite (Fe-j_xO) occupies about 0,95 of the thickness, intermediate magnetite (Fe^O^) with the spinel structure about 0.045, and outer hematite (Fe20^) with the rhombohedral structure about 0,005. The ratio of 2 Fe(Fe20^) to 1 Mn(MnO) is 2.02:1; the ratio of 2 Mn to 1 Fe is 0.50:1. This

suggests a qjinel structure between (FejMn^O^ and (Fe,Mn)0 occurs immediately behind the air/oxide interface. Beyond this region the Fe/Mn ratio is approximately 1:1 and is indicative of (Fe,Mn)0 oxide. The oxygen profile, Fig.144, only extended to one third of the depth analysed for Mn and Fe. After the initial surface contamination the oxygen profile was constant.

The oxide film on the surface of the Fe-0.5$ Mn droplet, Plate 14, appeared two-phased. The dominant feature was the stria which appeared directional. However, amongst the stria were

pockets of an oxide with a wavy texture. Plate 15, at higher mag­ nification, shows this in more detail. A mounted ar*3 sectioned sample shows a region of porosity behind the surface, Plate IB.

The duplex nature of the oxide on the surface of a Fe-0.7$ Mn droplet is shown in Plate 20. The striated surface is not evident now and is replaced by the *sif! pattern more resembling the pure iron specimen.

The 1.0$ Mn sample shows some of the stria similar to that of