Environmental Engineering Reference
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close to the borderline between ''protective'' and ''nonprotective'' scaling, a
situation common in practice where alloys are being exploited as close as is
practicable to their useful limits. In industrial situations the pre-steady-state oxi-
dation data are very important in deciding the conditions for commissioning new
equipment. One has to recognize that the nature of the scales (protective or non-
protective) formed during the transition period, i.e., the spatial distribution,
amount, composition, and structure, will determine to a large extent the nature
of the steady-state scale. Even though the relative free energies of formation of
the oxides predict the formation of thermodynamically favored oxide, they do
not guarantee the composition of the initial oxide or of the steady state. The
composition of the bulk alloy plays a major role in determining whether the
thermodynamically favored oxide will appear as an external, often protective,
scale or as an internal oxide. The free energies of formation of the oxides also
determine the possibility of additional solid-state reactions between oxides of the
scale and between the oxide and the alloy. A large alloy interdiffusion coefficient
ensures rapid replenishment of the appropriate alloying element at the surface
and thus help in establishment of the healing layer (i.e., eventual development
of a complete layer of a protective oxide). Higher oxygen solubility and diffusiv-
ity in the alloy substrate tend to promote internal oxidation. The relative growth
rates of elemental oxides and complex oxides will eventually determine the rela-
tive development and overgrowth rates of the initial nuclei and the scale en-
croachment rate on the alloy. The main scale may tend to incorporate the internal
oxide particles, thus preventing them from producing a healing layer. The inter-
play of all of these factors under the prevailing oxidizing condition will determine
the oxidation behavior of the alloy. Chattopadhyay and Wood [20] made detailed
investigations on the pre-steady-state or transient oxidation behavior of a number
of alloys comprised of three different sets of binary alloy systems: (1) Fe-Cr, Ni-
Cr, and Co-Cr alloys (in which the reactive solute metal is the same but the
solvent noble metals are different and the oxides are partially miscible or they
react); (2) Ni-Al, Ni-Cr, Ni-Si, Ni-Mn, and Ni-Co alloys (in which the solvent
noble metal is the same but the less noble solutes are different having different
affinities for oxygen and the oxide phases produced include solid solutions,
largely immiscible simple oxides, and complex oxides); and (3) Cu-Ni, Cu-Zn,
and Cu-Al alloys (in which the solvent noble metal is the same but the less noble
solutes are different and their resultant oxides are immiscible). These studies have
advanced our understanding of the mode of development of transient oxides on
binary alloys which could be readily explained at least in a qualitative manner.
To illustrate, the coalescence of initially formed oxide nuclei to give rise to
the transient scale and its subsequent development into the steady-state scale is
presented schematically in Fig. 6.10 for two different alloys of the Ni-Cr system
[21]. At high temperatures (1273-1473 K), alloys in this system are of single
phase up to 40% Cr, but the respective oxides, such as NiO and Cr 2 O 3 , are only
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