Environmental Engineering Reference
In-Depth Information
2.
Pilling-Bedworth ratio (PBR) of the compounds
3.
Crystallographic structure of their compounds
4.
Solubility of active elements in the main scale constituents and in the metallic
substrate
5.
Diffusivity of the active elements in the scale and alloy
6.
Valency of the active element ion and its radius
7.
The thermal conductivity of oxides, which can play a significant role in the
temperature distribution and consequently on the stress generation in the
scale and at the scale-alloy interface.
On the consideration of the above-mentioned properties, it may be concluded
that the three most relevant active elements are Y, Ce, and La, which are charac-
terized by
1.
High stability of their oxides, sulfides, and oxysulfides
2.
Very low PBR values of their oxides
3.
Their fairly low melting points, which facilitate plasticity at high tempera-
tures.
The active elements can be incorporated into or onto the metallic substrates
by any of the following ways as depicted in Fig. 6.19:
1.
As addition of pure active elemental metal or an alloy (''mischmetal'') dur-
ing alloy preparation
2.
Added/formed as an active element oxide dispersion
3.
Active elements incorporated into the alloy surface by ion implantation
4.
Application of surface coatings of the active element or its oxide.
It is important to recognize that methods (1) and (2) may influence the mechan-
ical properties of the alloys and method (3) is yet to be commercialized, now
being restricted only to laboratory tests for limited surface area to receive protec-
tion. Method (4) is the most commonly used commercial technique, which con-
sists of applying an MCrAlY-type protective overlay coating, where M designates
the alloy substrate.
6.6.2 Beneficial Effects of Additions
Despite the numerous studies devoted to REE within the last six decades, their
beneficial or detrimental effects are not yet well understood and are sometimes
controversial [40-52]. However, available results indicate that the major benefi-
cial effect is a dramatic improvement in the scale adherence [39], as depicted in
Fig. 6.20 and 6.21. This is more marked when the exposure temperature is high
or when the conditions are severe involving thermal cyclings.
Detailed investigations on Cr
2
O
3
- and Al
2
O
3
-forming alloys, reported in an
exhaustive review by Whittle and Stringer [40], indicate that the addition of reac-
tive or REEs or a distribution of their oxides within the alloy or a surface coating
