Environmental Engineering Reference
In-Depth Information
Figure 2.23
(a) Electrochemical behavior of an active-passive metal corroding with
a diffusion-controlled cathodic process showing the effect of velocity. (b) Corrosion rate
as a function of velocity.
4.
Addition of halides
. The presence of halides, especially chloride, in the elec-
trolyte has the similar effect of increasing temperature or increasing acidity
in that
i
cr
increases and the passive region is shortened. Halides have a ten-
dency to destroy the passive film and hence in its presence the passivity is
achieved with difficulty, i.e., requiring higher
i
cr
. The passivity shows a local
breakdown at some potential below the normal transpassive region. This po-
tential is often termed breakdown potential and is associated with the onset
of pitting (Section 3.4).
5.
Galvanic coupling
. Galvanic coupling with a more noble metal increases the
corrosion rate of a normal corroding metal (Section 3.3). In the case of metals
exhibiting passivity, the situation turns out to be different. Coupling of Pt
