Environmental Engineering Reference
In-Depth Information
plastic property of the scale, (3) cohesive forces between the scale grains, and
(4) adhesive forces between the scale and the substrate.
Three methods have been used extensively to improve the scale adherence on
metals and alloys. They are:
1. The small addition of oxygen active elements like Y, Ce, Hf, and the rare
earths to the alloys
2. Incorporation of an oxide dispersoid like ThO
2
, into the alloy, and
3. Addition of noble metals, such as platinum.
The first two methods are widely used and they are somewhat related because
during exposure of the alloy to the environment the oxygen-active elements are
internally oxidized due to their high reactivity, forming highly stable oxides be-
low the scale-alloy interface. However, the third method finds limited applica-
tions due to high cost of the noble metal.
6.6.1 Active Element/Rare Earth Additions for Oxidation
Resistance
In 1937, Griffiths and Pfeil [37] observed that rare earths added as melt deoxidant
to Ni-20% Cr alloys had a beneficial effect on their lifetime when used as heating
elements under thermal cycling conditions. Such empirical observations subse-
quently prompted Pfeil [38] to demonstrate that surface coatings of the same
elements, their oxides, hydroxides, or other salts, which would get converted to
stable oxides at high temperatures, could also provide an alternative means to
alloy additions in improving oxidation resistance of the alloys without affecting
their creep resistance property.
However, this ''rare earth effect/reactive element effect/active element ef-
fect'' (REE/AEE) was not confined to rare earth element additions. In fact, Pfeil
[38] indicated that elements from groups II, III, IV, and V of the periodic table
could be used for such effects, although their effectiveness decreased on passing
from group II to group V but increased with increasing atomic mass within a
particular group. From the classification as suggested by Pfeil, subsequently only
some elements as depicted in Table 6.3 [39] emerged to be of practical impor-
tance. The active elements (AEs), having a higher affinity for oxygen than the
base metal constituents, as referred to in this table are those that when added in
small quantity improve the oxidation resistance of the alloys not only by curtail-
ing the degradation rates but by ensuring the integrity of the protective scales,
developing or developed on high-temperature alloys under isothermal or thermal
cycling exposure conditions. Accordingly, the choice of the active elements is
governed primarily by their properties [39] related to
1.
Free energy or enthalpy of formation of AE compounds (oxides, sulfides,
oxysulfides, etc.)
