Environmental Engineering Reference
In-Depth Information
From the above discussion, it can be concluded that high-temperature strength
or creep resistance is best offered by FCC (austenitic) alloys based on Fe, Ni,
or Co. Additions of Cr, Al, and perhaps Si to these alloys will impart oxidation
and hot corrosion resistance. Other alloying elements chosen for strengthening
purposes include C, Mo, W, V, Nb, Ta, Ti, and Zr, together with austenite-pro-
moting elements such as Ni and Mn to permit heat treatment. Under severe and
adverse environmental conditions, surface protection by means of coatings or
claddings is frequently used for improving resistance to environmental attack
with simultaneous retention of strength. The upper limit of temperature at which
alloys can be used in most engineering applications appears to be about 1473 K.
Above this temperature, they start losing their mechanical and chemical stabili-
ties. The presently available superalloys lose their strength above 1523 K and
melt at around 1723 K.
6.2 DOPING EFFECT
The phenomenon of doping is relevant to an understanding of the defect models
of the oxide film or scale material as well as the mechanism of film or scale
growth processes on metals. It is therefore important in the study of physical
chemistry and electrochemistry of ionic compounds and semiconductors.
It is known that electrically active aliovalent impurities dissolved in the corro-
sion product scale bring about changes in the concentrations of ionic and elec-
tronic defects. Since the transport of these defects through the film/scale decides
the rate of diffusion-controlled film/scale growth process, the presence of such
impurities in the film/scale is expected to bring about a change in the rate of
film/scale growth. This is widely known as Wagner-Hauffe rule or the valence
approach to alloy oxidation, also referred to as the doping effect [5]. According
to this rule, the effect of doping can be observed only when
1.
The impurity atom or intentionally added atom has a valence different from
that of the parent atom, i.e., host lattice atom,
2.
In practice, the doping effect is applicable to alloys with minor alloying addi-
tions only, and
3.
The impurity oxide (solute oxide) must be soluble to some extent in the host
oxide lattice.
The effects of a higher or lower valency foreign ion addition on the oxidation
rates of dilute alloys are discussed below where the host oxide lattice exhibits
predominance of p-type, n-type, or ionic conductivity. In such discussion, NiO
as a typical p-type oxide, ZnO as a typical n-type oxide, and AgBr as a typical
ionic conductor are considered.
