Page 66 - Corrosion Engineering Principles and Practice

46 C h a p t e r 3 C o r r o s i o n E l e c t r o c h e m i s t r y 47

a much larger area. This factor is expressed in Eq. (3.25) which states
that the ratio of cathodic to anodic surfaces is an important amplifying
factor of the anodic current when S /S is >> 1 and a stifling factor
c
a
when it is << 1.
This area effect in terms of current density is illustrated by
combinations of steel and copper as either plates or the fasteners used
to join them and immersed in a corrosive solution. If steel rivets are
used to join copper plates, the current density on the relatively large
cathodic copper plates will be low, cathodic polarization of the copper
will be slight, and the voltage of the galvanic couple will maintain a
value close to the open circuit potential. At the same time, the current
density on the small anodic steel rivets will be high and the consequent
corrosion quite severe, giving rise to a particularly vicious form of
corrosion called galvanic corrosion [Fig. 3.8(a)].
With the opposite arrangement of copper rivets joining steel
plates, the current density on the copper cathodes will be high, with
consequently considerable cathodic polarization of the copper reduc-
ing the open circuit potential below its initial value. The diminished
anodic current will be spread over the relatively large steel plates and
the undesirable galvanic effect will hardly be noticeable [Fig. 3.8(b)].
Open circuit potential measurements are grossly inadequate for
predicting the magnitude of galvanic effects since they do not take
into account area and polarization effects. They are reliable only for
predicting the direction of such effects.

Reference
1. Roberge PR. Corrosion Basics—An Introduction. 2nd ed. Houston, Tex: NACE
International, 2006.