Environmental Engineering Reference
In-Depth Information
Figure 6.42
Influence of chromium content on (a) oxidation resistance (
T
1123 K)
and (b) creep rupture strength (σ
200 MN m
2
,
t
10
4
h) of some superalloys [65].
strengthening mechanisms is such that higher strength can only be achieved at
the expense of oxidation resistance.
The situation is more adverse for refractory metals such as Mo, W, Nb, and
Ta. These metals are characterized by very high melting points, but even so their
alloys cannot be used in oxidizing environments without additional protection
due to their poor resistance to oxidation. During oxidation of Mo and W, volatile
oxides (MoO
3
,WO
3
) are formed; while Nb and Ta, apart from developing poor
protective scales have high affinity for interstitial elements such as O, N, C, and
H. These interstitial elements easily dissolve in the metals and form ordered and
