Environmental Engineering Reference
In-Depth Information
ally very low. For example, type 316 stainless steel requires 0.6 mA/cm
2
for
passivation in 67% H
2
SO
4
at 24
A/cm
2
for the maintenance of passiv-
ity. The requirement of current in cathodic protection of the same system will
be several thousand times higher.
The decrease in the rate of corrosion is significant. For example, the corrosion
rate of type 304 stainless steel in 10NH
2
SO
4
drops from 1930 mpy to 0.016 mpy
when protected anodically. Corrosion rates of type 316 stainless steel heat ex-
changers used for cooling of sulfuric acid have been reported to reduce from 200
mpy to 1 mpy.
Anodic protection has also found application in the protection of stainless
steels in phosphoric acid and caustic soda solutions, in carbon steels in oxalic acid
and in NH
4
NO
3
fertilizer mixtures. However, anodic protection is not possible in
hydrochloric acid or in solutions of large chloride content as the passivity is
destroyed by chloride ions and the current requirement becomes high (high
i
critical
and
i
passivity
).
When compared with cathodic protection, it can be concluded that both sys-
tems are often complementary to each other. Cathodic protection is effective in
low to moderately high corrosive environments, whereas anodic protection is
effective in low to highly aggressive media. The cost of installation for anodic
protection is higher than that of cathodic protection, but this is compensated by
low maintenance cost.
°
C and 0.1
µ
4.6 DESIGN IMPROVEMENT
Many corrosion problems can be eliminated or reduced if proper considerations
are made at the design stage. It is obviously wiser to reduce the possibilities of
corrosion in the beginning than to take elaborate arrangements for the prevention
of corrosion that might be avoidable. Broadly, the design considerations should
include appropriate site selection for the plant, plant layout, selection of material
for various components, design of components, and selection of corrosion preven-
tion measure. A design engineer is not basically a corrosion engineer, but it is
necessary to convey the basic knowledge of corrosion to the designer so that the
obvious factors aggravating corrosion are not overlooked.
For the location of the plant and its layout due considerations are to be given
to the direction of winds. Airborne particulates and saline droplets from the
nearby sea can lead to adverse deterioration of structures. The stacks should be
suitably located so that the effluent emitted by one does not directly hit the other
structures in the plant or another stack located nearby.
The basic requirements of a component from a corrosion point of view need
to be established before the selection of material for the component and its design
proper. The considerations include the environment to which the component is
likely to be exposed, the expected life of the component, accessibility for mainte-
