Hj. Grabke et Gh. Meier, ACCELERATED OXIDATION, INTERNAL OXIDATION, INTERGRANULAR OXIDATION, AND PESTING OF INTERMETALLIC COMPOUNDS, Oxidation of metals, 44(1-2), 1995, pp. 147-176
The compounds MoSi2, NiAl, and NbAl3 all form protective oxide films,
particularly at high temperatures where the diffusion of Si or Al is m
ore rapid and, for the case of MoSi2, the transient oxides evaporate.
However, at low temperatures, all three can undergo accelerated oxidat
ion. The mechanisms of degradation are unique to the particular compou
nd although there are some similarities. The accelerated oxidation of
MoSi2, occurs at temperatures below 600 degrees C by the rapid growth
of Mo oxides which prevent development of a continuous silica film. In
ternal or intergranular oxidation does not occur. rf the specimen cont
ains cracks or pores, the rapid oxidation in these defects leads to fr
acture of the specimen or ''pesting.'' The accelerated oxidation of Ni
Al occurs at temperatures below 1000 degrees C at reduced oxygen parti
al pressures as the result of internal oxidation and rapid intergranul
ar oxidation. The intergranular oxidation does not lead to pesting. Sp
ecial circumstances are required for the accelerated oxidation of NiAl
as it does not appear to occur in flowing gases unless sulfur is pres
ent. The accelerated oxidation of NbAl3 also occurs at temperatures le
ss than 1000 degrees C and at reduced oxygen partial pressures and tak
es the form of intergranular oxidation of Al. The intergranular oxidat
ion results in pesting of NbAl3. The phenomena of accelerated oxidatio
n, internal oxidation, intergranular oxidation, and pesting have not b
een investigated in detail for most other intermetallic compounds but
one or more of these phenomena seems to afflict mast aluminides and si
licides.