Environmental Engineering Reference
In-Depth Information
are referred to as phase diagrams, provide valuable information in understanding
the mechanism of the sequential oxide layer formation. If the metal can possibly
form two or more oxides, the most oxygen-deficient one will be formed adjacent
to the metal substrate and the most oxygen-rich oxide will be in contact with the
gas phase. As illustrations, the Fe-O and the Ti-O phase diagrams are presented
in Fig. 5.3a and b, respectively. The Fe-O
2
system depicts formation of three
oxide phases, namely (1) w ¨ stite (Fe
1
δ
O), (2) magnetite (Fe
3
O
4
), and (3) Haem-
atite, (Fe
2
O
3
). W¨ stite phase is known to be highly metal-deficient but unstable
below 843 K where it disproportionates to Fe and Fe
3
O
4
. Therefore, during oxida-
tion of Fe above 843 K in air, the total scale will consist of three layers having
the most Fe-deficient oxide (w
¨
stite) adjacent to the metal and the most oxygen-
rich oxide (haematite) in contact with the gas phase with Fe
3
O
4
sandwiched be-
tween them [2]. Experience shows that the w
¨
stite layer will be the thickest and
the haematite will be the thinnest as shown in Fig. 5.4. Reasons for such observa-
tion will be discussed subsequently.
If one has to deal with binary or ternary alloys in a single-oxidant system,
the thermodynamic description of the condensed phase equilibria will be more
complicated due to the presence of a second or third element as a variable. In
such a situation, it will be more convenient to consider a stability diagram of the
alloy-oxidant system at a constant temperature [3]. Such an isothermal phase
diagram for the Fe-Cr-O system at 1573 K is presented in Fig. 5.5. Here, mole
fraction of Cr in binary Fe-Cr alloy has been plotted against logarithm of oxygen
partial pressure. This figure depicts the stability regions of different solid solu-
tions and mixed oxides with variation in Cr content and oxygen potential.
5.3 KINETIC ASPECTS AND RATE EQUATIONS
In general, whenever one examines the reaction behavior of any metal with a
single oxidant, one must consider a large number of phenomena and partial pro-
cesses of which one would be the slowest and hence the rate-determining step.
The various partial or sequential processes could be the following:
1.
Phase boundary reactions (chemisorption of the nonmetal molecule and si-
multaneous electron exchange with the substrate, thereafter splitting of the
molecules at the oxide-gas interface and transfer of the metal from the metal-
lic phase in the form of ions and electrons to the oxide film at the metal-
oxide interface with further reaction of the individual reactants and formation
of a distinct reaction product), nucleation, and growth of oxide crystals.
2.
Diffusional transport of cations, anions, electrons, or positive holes through
the scale, which gets complicated by a special migration mechanism because
of the development of electrochemical potential gradients set up across the
growing layer.
