Environmental Engineering Reference
In-Depth Information
Tantalum and Niobium
Oxidation of Nb and Ta has been studied over wide temperature and oxygen
pressure ranges. Their poor oxidation resistance presents a major problem re-
stricting their application in oxidizing environments at elevated temperatures.
These metals exhibit a complex oxidation behavior, and various stages of the
process, such as oxygen adsorption, oxygen dissolution, oxide nucleation and
growth, diffusion through compact and cracked scales, and oxide evaporation,
have been identified to be the rate-limiting factors depending on temperature,
oxygen pressure, exposure time, etc. Many problems have been encountered in
the interpretation of oxidation behavior of both of these metals [57,63] for which
more contemplated and careful experiments are essential to unravel the true
mechanisms of film/scale growth on Nb and Ta.
5.10 EFFECTS OF METAL SURFACE PREPARATION
AND PRETREATMENT
It is now accepted that the oxidation rate of metals is strongly influenced by the
oxide grain size because grain boundary diffusion often governs the cation or
anion transport rate, particularly at low and moderate temperatures. Studies car-
ried out by different groups have further established that fine-grained polycrystal-
line oxide thickens most rapidly due to the presence of many grain boundaries
that act as preferential paths for rapid diffusion. In contrast, monocrystalline ox-
ide, which forms on some preferred orientations of metals, thickens very slowly
showing the highest activation energy. Oxide grain size and morphology are
found be highly dependent on factors like metal orientation, chemical and other
surface pretreatments, cold work and oxidation procedures. The following section
is an attempt to review the correlation of oxidation rates with different surface
pretreatments on the oxidation behavior of pure metals like Ni, Cr, and Fe, which
are usually used as base metals in the design of high-temperature alloys. The
common observations of Graham et al. [64], who did extensive studies in these
directions, strongly suggest that the oxidation rates vary considerably with sur-
face pretreatments and employed oxidation procedures, which govern the initial
oxide grain size and structure of the oxide films.
5.10.1 Oxidation Procedures and Different Methods
of Specimen Surface Preparation
The specimens normally used for oxidation experiments could be polycrystalline
sheets or single crystals. A proper surface for oxidation studies should have a
low roughness factor and be representative of bulk composition. The methods
