METALS TUTORIAL 2.pdf - MS3013 Tutorials – Corrosion...

MS3013 Tutorials – Corrosion Failure
Tutorial 2

2-1. A univalent metal, M, has b
a
= +0.2 V, b
c
= 0.2V, and i
o
= 20 mA/m
2
.
Calculate values for i
a
and i
c
when the metal is anodically polarized to +
0.20 V. Determine the experimentally measured net current density for the
same polarization.
2
Figure 2-1 plot of the anodic and cathodic current densities for M.
i
a
i
c

Method II by calculation: Tafel equation is used
η = ± b log
࠵?
࠵?
࠵?
So,
࠵?
±࠵?
= log
࠵?
࠵?
࠵?
࠵?
࠵?
࠵?
= ࠵?࠵?
࠵?
±࠵?
. ࠵?࠵?࠵?࠵?࠵? η = E – E
o
= 0.2 – 0 = 0.2 V
i
a
= ࠵?࠵?
࠵?
.࠵?
⋅ i
o
= ࠵?࠵?
࠵?.࠵?
࠵?.࠵?
⋅ 20 =10 × 20= 200 mA/m
2
i
c
= ࠵?࠵?
࠵?
1࠵?
⋅ i
o
= ࠵?࠵?
࠵?.࠵?
1࠵?.࠵?
⋅ 20 =10
-1
× 20= 2 mA/m
2
i
net
= i
c
- i
a
= 2-200 = -198 mA/m
2
Note: E o is obtained by reading from the plot

2-2.