Environmental Engineering Reference
In-Depth Information
However, the films formed on active-passive metals when exposed to highly oxi-
dizing solutions are often very thin and invisible. Though investigators [4] have
become successful in identifying
-Fe
2
O
3
in thin films isolated from the surface
of passivated iron, controversy exists regarding the formation of a stoichiometric
oxide film during passivation.
The adsorption theory [5] holds that the passivity arises out of the chemisorp-
tion of oxygen on the metal surface. The adsorption layer may only be monoa-
tomic, but it is effective in providing a kinetic limitation reducing the exchange
current density for the dissolution reaction. Chemisorption of oxygen is favored
by the presence of uncoupled
γ
electrons in transition metals; since the active-
passive metals like Fe, Ni, Cr, and Ti are transition metals, the adsorption theory
tends to get support. However, it has also been argued that both theories are
supplementary in that the adsorbed film in the process of thickening gradually
develops into an oxide film.
Studies on the amount of charge required to produce passivation have given
information on the thickness of passivating films produced on initially film-free
surfaces [6]. Platinum, gold, cobalt, nickel, and iron in alkaline solutions require
only a monolayer of oxide for passivation, which has been indicated by a rise
of potential at constant current density. Copper and silver require approximately
four molecular layers of oxide in alkaline solutions. In acid solutions, silver and
lead require films of visible thickness. All of these indicate that the nucleation
and growth characteristics of the film are important in deciding how thick it must
become before being compact enough to produce passivation.
α
2.5 CORROSION RATE MEASUREMENT
Measurement of corrosion rate is essential for the purpose of material selection.
The compatibility of a metal to its environment is a prime requirement for its
reliable performance. Corrosion rate measurement may become necessary for the
evaluation and selection of materials for a specific environment or a given definite
application or for the evaluation of new or old metals or alloys to determine the
environments in which they are suitable. Often the corrosive environment is
treated to make it less aggressive, and corrosion rate measurement of a specific
material in the untreated and treated environments will reflect the efficacy of the
treatment. These apart, corrosion rate measurement is also essential in the study
of the mechanisms of corrosion.
2.5.1 Corrosion Rate Expressions
Corrosion involves dissolution of metal as a result of which the metallic part
loses its mass (or weight) and becomes thinner. Corrosion rate expressions are,
