Page 117 - Corrosion Engineering Principles and Practice

92 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 93

Fe 2+

Fe 2+

2e –

H +
e –
H + Diffusion
Mass Transport Migration
Convection
H +
e – H +

Exchange Current Density (i )
0
Charge Transfer Tafel Slope (b)
Activation Barrier (α)
FIGURE 5.5 Graphical representation of the processes occurring at an
electrochemical interface.

mass transport, the flux of a species O to a surface from the bulk is
described with Fick’s first law in Eq. (5.12):

d  C 
O
J = − D O   d x   (5.12)
O
−2
where J is the flux of species O (mol s cm )
−1
O
D is the diffusion coefficient of species O (cm s )
2
−1
O
d C is the concentration gradient of species O across the inter-
O
face between the metallic surface and the bulk environ-
ment (mol cm )
−3
d x is the Nernst diffusion layer or diffuse layer (cm)
Table 5.3 contains values for D of some common ions. For more
O
practical situations the diffusion coefficient can be approximated
with the help of Eq. (5.13), that relates D to the viscosity of the
O
solution (m) and absolute temperature:
⋅
D = T A (5.13)
m
O
where A is a constant for the system.