Scale adherence

Small additions of Ce have been shown to have a favourable influence on oxide growth of several Fe-Cr alloys by improving scale adherence and acting as nucleation sites for CrjO, . Levels of Ce as low as 0.024% reduce the carbon uptake of steels in carbonaceous atmospheres by several orders of magnitude. Trace concentrations of As and Sn have been found to improve the breakaway properties of mild and low alloy steels in CO/COj, whereas Cu has been found to be detrimental. ... 

The areas concerning monolithic intermetallics which have been studied in recent years are (i) the formation of mctastable aluminas, and their transformation to stable a-alumina, (ii) the formation of interfacial voids and scale adherence and how these are influenced by reactive elements and sulfur, and (iii) accelerated oxidation at intermediate temperatures. Additionally the applications oriented areas of (iv) coatings, (v) oxidation of composites, and (vi) life predictions have received attention. 

The Formation of Interfacial Voids and Scale Adherence and how these are Influenced by Reactive Elements and Sulfur... 

The mechanical behaviour of oxide scales has been investigated by a complex of appropriate techniques (cf. for example [12]), where the attention has been focused on the analysis of the stress development in the scale, as well as the measurement of the scale adherence. In the case of weak scale adherence, spontaneous scale failure is often observed during oxidation or cooling of specimens. For systematic investigations of the fracture-mechanical properties of oxide scales, scale failure is induced by a controlled loading of the scale which is produced by an appropriate deformation of the whole specimen. 

For weak scale adherence and low substrate ductility, through-scale cracking is accompanied by a delamination and spalling of the scale. This was observed to a large extent on Fe-doped NiAl specimens after bending at RT (Fig. 6).The weak adherence in this case was caused by a pronounced formation of interfacial voids. Void formation at the oxidc-metal interface has often been observed for NiAl. It was studied in more detail by Brumm and Grabke in [32. ... 

SEM observations on a series of NiAl specimens showed that the formation of large interfacial voids at 1223 K is more the exception than the rule. Typically, small faceted voids of only a few micrometre size were found. Larger voids are formed by a coalescence of small voids which leads to a complicated shape of the voids. Such a void structure with high void density was found on the specimen shown in Fig. 6. The high density of voids diminished the effective scale adherence and affected the large-scale delaminations observed. 

The compositions of the alloys investigated are listed in Table 3. The aluminium concentration was varied between 6 and 17 wt%, the chromium concentration between 2 and 10 wt%. All alloys contained 0.1 wt% hafnium to improve the scale adherence. Alloys 2, 5 and 6 additionally contained 0.13 to 0.18 wt% mischmetal (which is mainly consisting of cerium and lanthanum). The carbon concentration was normally between 300-500 ppm extra carbon was added to alloys 2,3,4,5 and 6 to increase the mechanical strength. The concentrations of sulphur and phosphorus were below 40 ppm in all alloys investigated. Alloy 800H (1.4958) was included in the test programme for comparison purposes its composition is listed in Table 3, too. 

Regarding the chemical processes, sediments are heterogeneous at various sample, aggregate, and particle scales. Adherent or entrapped nonaqueous-phase liquids and combustion residue particulate carbon (e.g., chars, soot, and ashes) can also function as sorbents. Complex assemblages of these constituents can cause complex mass transfer phenomena, and the term sequestration refers to some combination of diffusion limitation, adsorption, and partitioning (Luthy et al. 1997). Some geosorbents exhibit typical nonlinear sorption behavior (Farrell and Reinhard 1994 Huang and Weber 1998). 

As in many other cases, scale adherence and oxidation resistance are improved by microalloying with oxygen active elements, e.g. Y, Zr, Hf, Ce, or La (Mrowec... 

Jox> Cfm coefficients of thermal expansion for oxide and metal parameter indicative of alloy/scale adherence 7 dimensionless kinetic parameter for internal oxidation... 

Y ions into an aluminide ((3-NiAl) on a nickel-base alloy and confirm that while initially the implanted reactive element effectively imparts increased scale adhesion, both in air and oxygen at 1000 -1200 C, the beneficial influence is not long lasting. They attributed this loss to the influence of the substrate Ni-base superalloy, since lasting benefits of reduced rates of oxidation and improved scale adherence were maintained when Y was implanted into bulk 3-NiAl (Jedlinski and Mrowec 1987). 

The effect of Hf, which also belongs to the group IVa of the periodic system is to some extent different to that of Zr. Although both Zr and Hf improve the scale adherence, a Hf addition of 0.03 wt.% to the FeCrAlY base alloy does not lead to an increase of the oxide growth rate, which was... 

Their oxidation resistance relies on the formation of a protective chromia layer, on the good scale adherence and on the creep properties owing to the addition of stabilisers (Ti, Nb). Typical service temperatures are 700-950°C. As testing material in the COTEST programme the 18% Cr containing Ti and Nb stabilised AISI441 grade was used. 

Later studies by others [40-44] indicated that the presence of water vapour in air or oxygen had an even more profound effect on the oxidation of steel and Fe-C alloys. It was generally accepted that the presence of water vapour improved the scale adherence and hence increased the oxidation rate or prolonged the period when the parabolic oxidation kinetics were held. 

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