Environmental Engineering Reference
In-Depth Information
Figure 5.7
Oxidation-time relationships [Ref. 8].
aqueous corrosion, which is frequently referred to as ''wet corrosion,'' emphasis
is mainly placed on the electrochemical aspects, i.e., electrode kinetic studies are
more important. In contrast, in the study of elevated temperature gaseous reaction
of metals or alloys, referred to as ''dry corrosion,'' emphasis is placed on thermo-
dynamics or phase stabilities of the various reaction products, the defect struc-
tures and transport properties of the scale, and the morphology of the product
scale, besides the usual electrochemical aspects.
5.4 DEFECT CHEMISTRY OF OXIDES AND OTHER
INORGANIC COMPOUNDS
Before one can understand the mechanisms of oxide film or scale growth pro-
cesses in the thin- and thick-film ranges for protective oxidation of metals, one
is required to have knowledge of the mechanisms of solid state diffusion. It is
well established that diffusion in solids takes place because of the occurrence of
imperfections or point defects within them. The presence of such point defects
in the compounds, their concentrations, and the nature of the defect structure
would play the most important role in controlling the progress of oxidation reac-
tions. Continued degradation of the metal becomes possible only when the re-
acting species, i.e., either cations or anions, migrate through the oxide layer so
