Properties of Metals
DOE-HDBK-1017/1-93
CORROSION
Galvanic corrosion occurs when two dissimilar metals with different potentials are placed in
electrical contact in an electrolyte. It may also take place with one metal with heterogeneities
(dissimilarities) (for example, impurity inclusions, grains of different sizes, difference in
composition of grains, or differences in mechanical stress). A difference in electrical potential
exists between the different metals and serves as the driving force for electrical current flow
through the corrodant or electrolyte. This current results in corrosion of one of the metals. The
larger the potential difference, the greater the probability of galvanic corrosion. Galvanic
corrosion only causes deterioration of one of the metals. The less resistant, more active one
becomes the anodic (negative) corrosion site. The stronger, more noble one is cathodic (positive)
and protected. If there were no electrical contact, the two metals would be uniformly attacked
by the corrosive medium. This would then be called general corrosion.
For any particular medium, a list can be made arranging metals sequentially from most active,
or least noble, to passive, or most noble. The galvanic series for sea water is discussed in the
Chemistry Fundamentals Handbook.
Galvanic corrosion is of particular concern in design and material selection. Material selection
is important because different metals come into contact with each other and may form galvanic
cells. Design is important to minimize differing flow conditions and resultant areas of corrosion
buildup. Loose corrosion products are important because they can be transported to the reactor
core and irradiated.
In some instances, galvanic corrosion can be helpful in the plant. For example, if pieces of zinc
are attached to the bottom of a steel water tank, the zinc will become the anode, and it will
corrode. The steel in the tank becomes the cathode, and it will not be effected by the corrosion.
This technique is known as cathodic protection. The metal to be protected is forced to become
a cathode, and it will corrode at a much slower rate than the other metal, which is used as a
sacrificial anode.
Localized corrosion is defined as the selective removal of metal by corrosion at small areas or
zones on a metal surface in contact with a corrosive environment, usually a liquid. It usually
takes place when small local sites are attacked at a much higher rate than the rest of the original
surface. Localized corrosion takes place when corrosion works with other destructive processes
such as stress, fatigue, erosion, and other forms of chemical attack. Localized corrosion
mechanisms can cause more damage than any one of those destructive processes individually.
There are many different types of localized corrosion. Pitting, stress corrosion cracking, chloride
stress corrosion, caustic stress corrosion, primary side stress corrosion, heat exchanger tube
denting, wastage, and intergranular attack corrosion are discussed in detail in the Chemistry
Fundamentals Handbook.
Rev. 0
Page 33
MS-02
