Page 119 - Corrosion Engineering Principles and Practice

94 C h a p t e r 5 C o r r o s i o n K i n e t i c s a n d A p p l i c a t i o n s o f E l e c t r o c h e m i s t r y 95

This is represented by the horizontal line where C = C . There is also a
O
region where the concentration drops, falling to zero at the electrode
surface. The Nernst diffusion layer associated with this drop has a
specific thickness (d ) that depends upon the nature of the solution into
which it extends. For stirred aqueous solutions the thickness of the
diffusion layer varies between 0.01 and 0.001 mm.
For a chemical species O that is consumed by the cathodic reaction
at the corroding surface, the concentration gradient (d C /d x) is
O
greatest when the concentration of that species is completely depleted
at the surface, that is, C = 0. It follows that the cathodic current is
O
limited in that condition, as expressed by Eq. (5.14).

i = i = − nFD O C bulk (5.14)
O
c
d
L
For intermediate cases, that is, when the cathodic current is
smaller than i , h conc can be evaluated using an expression [Eq. (5.15)]
L
derived from Nernst equation:
2 303 × RT   i
.
h conc = nF log 10   1 − i L   (5.15)
where 2.303 × R × T/F = 0.059 V when T = 298.16 K.

5.4 Ohmic Drop
The ohmic overpotential appears in Eq. (5.2) as the simple product of
a resistance and a current between the anodic and cathodic sites of a
corrosion process. For many corrosion situations these sites are
adjacent to each other and the ohmic drop is negligible, particularly
so when the environment itself is a good electrolytic conductor, that
is, seawater. However, there are special conditions where the
separation of the anodic and cathodic sites can be an important factor
in the corrosion progress, for example, galvanic corrosion, or even an
integral part of a particular protection scheme, for example, anodic
and cathodic protection.

5.4.1 Water Resistivity Measurements
The conductivity of an environment can itself be a complex function.
When a salt dissociates, the resulting ions interact with surrounding
water molecules to form charged clusters known as solvated ions.
These ions can move through the solution under the influence of an
externally applied electric field. Such motion of charge is known as
ionic conduction and the resulting conductance is the reciprocal
function of the resistance of an environment.
The dependence upon the size and shape of the conductor can be
corrected by using the resistivity r rather than the resistance R, as