Environmental Engineering Reference
In-Depth Information
scale consisted of a thin outer layer of CuO over Cu
2
O and an inner two-phase
layer of Cu
2
O
BeO. Internally oxidized BeO crystallites in the copper matrix
were also detected beneath the scale-metal interface (Fig. 6.13a). The mutual
solubilities of the two oxides (Cu
2
O and BeO) are known to be almost nil, and
the oxides do not react to form a ternary oxide as well. At a concentration of
12.6 at. % Be, the oxidation resulted in an external scale formation exclusively
of BeO, where the rate was approximately 10
4
times slower than that for Cu-Be
alloys with less than 7 at. % Be. The critical concentration of Be required for
the transition from internal oxidation to external scale formation is theoretically
calculated to be only 1.8 at. % against 12.6 at. % as experimentally observed.
The discrepancy between the experimental value and the predicted value was
attributed to additional factors such as (1) internal oxidation of Be in the alloy
phase and (2) development of porosity in the Cu
2
O layer—which were not con-
sidered in the ideal model. Pores are expected to develop when cations migrate
outwardly through the scale. Even though the pores act as solid-state diffusion
barriers to copper ion migration, the oxidation is sustained by dissociative trans-
port of oxygen across the pores, as schematically illustrated in the reaction mecha-
nism (Fig. 6.13b).
Formation of Oxide Solid Solution Having Complete
Solubility of Oxides
When both components of the binary alloy (A-B) are oxidized and the resulting
oxides are completely miscible in each other, both appear in the external scale
in a proportion determined by the oxidation potentials of A and B as well as by
their diffusion rates in the alloy and scale. Binary alloys of Co, Fe, Mn, and Ni
oxidized under most conditions of temperature and oxygen pressure fall into this
category. Their oxides, e.g., CoO, FeO, MnO, and NiO, all have a simple cubic,
NaCl-like structure and form solid solutions over their entire composition range.
Since the two cations involved in the reaction do not diffuse at the same rate
through the scale, different concentration gradients for the ions develop in the
scale. The situation is best illustrated by considering the oxidation of Ni-10.9%Co
alloy in 1 atm O
2
at 1273 K [30]. The alloy was found to oxidize at a relatively
uniform rate following parabolic growth kinetics, yielding a scale similar to that
on pure Ni or Co except for the occurrence of internal oxidation in a limited
way. Ni and Co produce a continuous range of solid solutions and their resultant
oxides, NiO and CoO are also mutually soluble, producing (Ni,Co)O solid solu-
tion of variable composition. The concentration profiles of Ni and Co as deter-
mined through microprobe analysis in the unoxidized alloy together with the
resultant scale are presented in Fig. 6.14. It is revealed that Co is enriched at the
oxide-oxygen interface. Concurrently, the alloy beneath the scale is depleted in
Co but enriched in Ni. This is possibly because Co has a higher affinity for
oxygen than Ni and so is enriched in the steady-state scale. Enrichment in Co
