Duplex scale

The transition from non-protective internal oxidation to the formation of a protective external alumina layer on nickel aluminium alloys at 1 000-1 300°C was studied by Hindam and Smeltzer . Addition of 2% A1 led to an increase in the oxidation rate compared with pure nickel, and the development of a duplex scale of aluminium-doped nickel oxide and the nickel aluminate spinel with rod-like internal oxide of alumina. During the early stages of oxidation of a 6% A1 alloy somewhat irreproducible behaviour was observed while the a-alumina layer developed by the coalescence of the rodlike internal precipitates and lateral diffusion of aluminium. At a lower temperature (800°C) Stott and Wood observed that the rate of oxidation was reduced by the addition of 0-5-4% A1 which they attributed to the blocking action of internal precipitates accumulating at the scale/alloy interface. At higher temperatures up to 1 200°C, however, an increase in the oxidation rate was observed due to aluminium doping of the nickel oxide and the inability to establish a healing layer of alumina. 

The sulphide usually forms an interconnected network of particles within a matrix of oxide and thus provides paths for rapid diffusion of nickel to the interface with the gas. At high temperatures, when the liquid Ni-S phase is stable, a duplex scale forms with an inner region of sulphide and an outer porous NiO layer. The temperature dependence of the reaction is complex and is a function of gas pressure as indicated in Fig. 7.40 . A strong dependence on gas pressure is observed and, at the higher partial pressures, a maximum in the rate occurs at about 600°C corresponding to the point at which NiS04 becomes unstable. Further increases in temperature lead to the exclusive formation of NiO and a large decrease in the rate of the reaction, due to the fact that NijSj becomes unstable above about 806°C. 

However, the observation of existing duplex scales (and breakaway scales) during metal oxidation rather points to the second oxidation mode. The duplex morphology... 

Figure 7.10 Schematic diagram of the mechanism of formation of duplex scale (FeO + FeS) on iron from Ar-S02 atmospheres, showing how the FeS lamellae provide rapid cation transport and also supply the adjacent FeO lamellae.

Figure 7.11 The iron-sulphur-oxygen stability diagram at 900 °C showing reaction paths for duplex scale formation. The reaction path starting at X corresponds to the case for which Fe304 is stable at the bulk gas composition and the one starting at X corresponds to a bulk gas in which FeS is stable.

A similar argument can be made for the formation of duplex scales from atmospheres with high sulphur potentials using the CO-CO2-SO2 system (reaction path X -Z). 

Initially, from to the duplex scale grows at a constant rate until time h, when the activity of the metal at the scale surface has fallen to the value given by identity in Equation (7.33b). After this point, only oxide is stable and can form on the scale surface. After time h, the duplex scale is covered with an outer layer of oxide only, which slows down the reaction rate. Consequently, the metal activity at the duplex-layer-oxide-layer interface rises, reflecting the lower rate of transfer of iron ions... 

Figure 7.12 Corresponding rates of reaction, structure, and potential distribution for formation of a duplex scale in S02-containing atmospheres, eventually forming a surface layer of oxide.

Figure 7.13 Optical micrograph of a duplex scale formed on nickel heated to 500°CinAr-l%S02.

Although iron, nickel, and cobalt behave similarly in Ar-S02 atmospheres and they all grow duplex scales, the sulphide and oxide phases are arranged quite differently in the three cases, as can be seen from Figures 7.9, 7.13, and 7.15. The reason for this is not understood and, to the authors knowledge, has not been addressed. 

Figure 7.15 Optical micrograph showing the fine duplex scale formed on cobalt at 840 °C in Ar-2% SO2.

When the concentration of B is so low that a protective scale of B O cannot form, a zone of internal oxidation of B O particles in a matrix of A will form. The surface of the alloy, effectively pure A, can now react with the complex atmosphere to form a scale of either A 0 or duplex A 0 and A, S. Where a duplex scale is formed, the metal-scale interface will be at equilibrium with A + A 0 + A S sulphur will dissolve in the metal and diffuse inwards through the internal oxidation zone to form internal B S particles. This forms as a second, inner, sulphide-based internal zone of precipitation below the outer internal oxidation zone. Since BpO is assumed to be substantially more stable than B S, sulphide formation is not expected to be seen in the outer internal oxidation zone. As oxygen continues to diffuse inwards it wiU react with the internal sulphide particles, forming oxide and releasing sulphur to diffuse further into the metal. This is shown in Figure 7.17. Thus, once the internal sulphide zone is established, it can be driven into the alloy by this cascading mechanism, effectively removing the metal B from solution in the alloy. 

The observation of this pattern depends on the experimental conditions. (i) The layer thickness increases regularly from 5 to 40ym when the temperature is increased from 580 C to 740 C. At around 740 C the scale morphology changes brutally the stratification disappears whereas a duplex scale arises, (ii) The layer thickness also varies with the composition of the alloy If... 

Numerical simulations not only confirm these predictions but also help to obtain the essential characters observed by selection of realistic values of the parameters, these characters being oscillatory formation of the layers, the multilayered morphology and the bifurcation leading to the duplex scale (see Figure 8). 

During exposure in CO2, mild steels produce protective duplex scales (Fig. 3.15(a)), the interface between the two layers being the original metal surface. The zone where the inner oxide layer grows is observed in Fig. 3.15(b) through the presence of chromium, which does not diffuse during the oxidation process. 

After treatment at 950°C, on the other hand, Haynes 230 exhibits thicker surface layers. Figure 26.5 shows a cross-section of a 813 h coupon. The treatment has produced a duplex scale with a loose Cr-Mn spinel at the outside and a dense Cr-rich oxide inward. The alloy/oxide interface is rough and metallic islands remain included within the oxide scale. Beneath the surface layer, fine internal oxide precipitated, possibly rich in Cr. Oxidation of Al at the alloy grain boundaries is observed up to 40 pm deep. 

Such a duplex microstructure is commonly observed for NiO scales grown at a temperature lower than 1000°C (Peraldi et al., 2002 Haugsrud, 2003). For such duplex scales, inert marker location, 0 experiments and careful analysis of NiO scale microstructure show that the growth of the external columnar subscale is associated with the outward diffusion of Ni cations and occurs at the scale-gas interface, while the inward diffusion of oxygen is involved in the growth of the inner equiaxed subscale. Therefore, the internal interface between the equiaxed and columnar sublayers marks the initial location of the Ni surface before the formation and growth of NiO scales. 

There were several types of genuine porosities observed by various researchers. The first type was a void observed in the magnetite layers formed on iron and steel at temperatures below 570°C [91,93], where the gas used could be moist air, dry air or CO2. The voids were very fine in size and were observed at grain boundaries as well as inside magnetite grains. This type of porosity is also known as Kirkendall voids [100]. The formation of voids appeared to be associated with the formation of a duplex scale structure [101]. Recently, some theoretical treatments using conventional diffusion theories were made by Maruyama etal. [102] and Ueda etal. [92] to provide a semi-quantitative and quantitative explanation of their formation mechanism and their location in the scale. 

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