Environmental Engineering Reference
In-Depth Information
such as Cr, Mn, Al, Si, Zr, Be, or In, when exposed to either single oxidant
or mixed environments such as a mixture of CO, CO 2 ,CH 4 ,H 2 O, H 2 S (i.e., in
simultaneous presence of oxygen, carbon, and sulfur potentials) exhibit consider-
able solubility and diffusivity of the oxidants in the alloys at high temperatures,
leading to internal oxidation. For simplicity and convenience, oxygen is consid-
ered here as the single oxidant. Then the process occurs by dissolution of the
oxidant in the base metal (either at the external surface or at the alloy-scale
interface in the presence of an external scale), which diffuses inward through the
base metal matrix containing previously precipitated internal oxide particles. At
an advancing reaction front (parallel to the alloy external surface), the critical
solubility product, a B a O (B
BO ν ) for the nucleation of precipitates is es-
tablished by the inward diffusion of oxygen and the outward diffusion of the
reactive solute atoms, B. Subsequently, nucleation and growth of the oxide pre-
cipitate occur until the reaction front moves forward and depletes the supply of
solute atoms arriving at the precipitate. Further growth of the precipitate takes
place only by capillary-driven coarsening (Ostwald ripening).
The necessary criteria for the occurrence of internal oxidation in a binary alloy,
A-B (B is a more reactive solute and A is a noble solvent) during its isothermal
oxidation at a constant oxygen pressure are as follows:
ν
O
1.
The standard free-energy change of formation (per mole O 2 ) for the solute
metal oxide, BO ν
is the number of oxygen ions per B ion in the oxide),
must be more negative than the standard free-energy change of formation
(per mole O 2 ) for the lowest oxide of the base metal.
(
ν
2.
The free-energy change for the reaction
B
ν
O
BO ν
must be negative. This implies that the base metal must have a significant
solubility and diffusivity for atomic oxygen at the oxidation temperature to
establish the required activity of dissolved oxygen (O) at the reaction front.
3.
The solute concentration of the bulk alloy must be lower than that required
for transition from internal to external oxidation.
4.
The surface layer formed during chemical or mechanical preparation of the
alloy surface must not prevent dissolution of oxygen into the alloy at the
start of oxidation.
In practice, internal oxidation of an alloy that fulfills conditions 1 and 2 may
be prevented by intentional elimination of conditions 3 and 4. From a practical
standpoint, internal oxidation was initially considered to be undesirable since by
such process unwanted inclusions can be introduced into an otherwise clean alloy.
However, internally oxidized copper alloys are now commercially available, and
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