Environmental Engineering Reference
In-Depth Information
Figure 6.18
Oxide map for alloys in Ni-Cr-Al system at 1273 K delineating the com-
position ranges for the formation of different types of oxide scales [32].
6.6 REACTIVE ELEMENT EFFECTS ON THE OXIDATION
BEHAVIOR OF ALLOYS
The alloys designed to withstand high-temperature degradation by oxidation dur-
ing their service must meet two essential requirements. Primarily they must form
a surface compound (oxide, sulfide, halide, etc.) that thickens only at a slow rate,
and secondly the respective compound layer must remain adherent to the alloy
substrate under service conditions. It is also well known that all of the technologi-
cally important high-temperature alloys used in an oxidizing environment (oxy-
gen as oxidant) receive protection from degradation by the formation and mainte-
nance of a thermodynamically stable, coherent, crack-free, well-adherent, oxide
scale of low diffusivity, such as chromia (Cr
2
O
3
) or alumina (Al
2
O
3
). Cr
2
O
3
-
forming alloys generally contain less than 2-3% Al and 15% Cr or more de-
pending on the base alloy. On the contrary, Al
2
O
3
-forming alloys usually contain
more than 5% Al with a substantial amount of chromium. On the basis of low
volatility, relative chemical inertness, and slow-growth characteristics, Al
2
O
3
is
often the scale of choice.
Although Cr
2
O
3
and Al
2
O
3
layers are effective barriers to the transport of
both oxygen and metal ions, they are susceptible to failure by loss of adhesion,
particularly spallation on thermal cyclings. However, continued resistance re-
quires the maintenance of the protective barrier separating the environment from
the alloy substrate. It has already been mentioned (Sec. 5.8) that the principal
sources of stress during high-temperature exposure of a metal/alloy are twofold:
oxide growth stresses, which develop during the isothermal formation of the
scale, and thermal stresses, which develop due to the differences in thermal
