Page 274 - Corrosion Engineering Principles and Practice

248 C h a p t e r 7 C o r r o s i o n F a i l u r e s , F a c t o r s , a n d C e l l s 249

3. Provide complete drainage
4. Inspect regularly; clean thoroughly
5. Be careful when using packing materials or thermal insulation
which can hold moisture in contact with metals

7.5.3 Galvanic Corrosion Mitigation
There are a number of ways that galvanic corrosion may be prevented.
These can be used singly or in combination. All of these preventive
measures follow directly from the basic mechanism of galvanic
corrosion.

1. Avoid the use of dissimilar metals wherever possible. If this
is not practical, try to use metals which are close together in
the galvanic series (Fig. 6.31 in Chap. 6).
2. Avoid an unfavorable area ratio whenever possible, particularly
in the presence of an electrolytically conductive environment.
3. If dissimilar metals are used, insulate these electrically from
one another.
4. If it is necessary to use dissimilar metals, and these cannot be
insulated, then the more anodic part should be designed for
easy replacement or should be constructed of thick materials
to longer absorb the effects of corrosion.
5. Coat the cathode (or both anode and cathode) near the
junction to reduce the effective cathodic area. Never coat
the anode alone since any pinhole or holiday would be the
site of rapid anodic attack due to the large S /S surface
a
c
area ratio.
7.5.4 Fretting Corrosion Mitigation
Fretting corrosion can be prevented by eliminating any slipping
movement between two surfaces in contact. Thus, it is sometimes
possible to overcome fretting by increasing the load on the surfaces,
if it prevents relative motion. Alternatively, decreasing the load can
minimize the effect of vibratory motion. In other cases, roughening
the surfaces can increase the friction between the surfaces and stop
the movement. Fretting corrosion can also be greatly retarded by
lubricating the contacting surfaces with an oil or grease with sufficient
load-bearing characteristics that it will separate the surface from the
environment.
7.5.5 Mitigation of Stress Corrosion Cracking
In reference to Fig. 7.39, one could be tempted to say that it is easy to
prevent mitigation of SCC. Simply eliminate the stress, isolate the