Environmental Engineering Reference
In-Depth Information
Figure 2.21
Corrosion behavior of an active-passive metal under different reduction
reactions.
crease in the rate of cathodic reaction induces a higher rate of dissolution, taking
the value to i cr . This is achieved either through the availability of more of oxygen
at the metal surface or through the use of oxidizers or by increasing the exchange
current density for cathodic reaction. A low value of i cr ensures passivity under
relatively low oxidizing conditions and this is achieved by suitable alloying addi-
tions. Passivation can also be effected by maintaining the potential in the passive
range by using a potentiostat, as described above; this forms the basis of anodic
protection of metals (Section 4.5). These factors have been discussed in subse-
quent sections.
2.4.2 Effect of Environmental Factors
Various environmental factors, such as temperature, pH, and velocity of the cor-
rosive medium, addition of oxidizers or halides to the corrosion medium, and
galvanic coupling, influence the passivity vis-`-vis the corrosion behavior of an
active-passive metal. The effects are spectacularly different from those on a metal
that does not show active-passive transition.
1.
Temperature and pH . An increase in temperature and increasing acidity of
the medium (higher H concentration or lower pH) increases the value of i cr
and decreases the passive potential range as shown in Fig. 2.22. The primary
passive potential, E pp , is affected only slightly.
2.
Velocity . An increasing velocity of the corrosive medium increases the lim-
iting diffusion current density because of the increasing availability of oxy-
gen. The effect of increasing velocity on the corrosion rate of a normal metal
has been illustrated in Fig. 2.18. The corrosion rate increases up to the
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