Figure 2-32.Simplified corrosion cell.
Electrochemical attack is evidenced in several
forms depending on the metal involved, its size and
shape, its specific function, the atmospheric conditions,
and the type of corrosion-producing agent (electrolyte)
present. A great deal is known about the many forms of
metal deterioration that result from electrochemical
a t t a c k .
B u t d e s p i t e e x t e n s i v e r e s e a r c h a n d
experimentation, there is still much to be learned about
other more complex and subtle forms of metal
deterioration. Descriptions are provided later in this
chapter for the more common forms of corrosion.
Since there are so many contributing factors to the
process of corrosion, selection of materials by the
manufacturer must be made with weight versus strength
as a primary consideration and corrosion properties as a
However, close attention
during design and production is given to heat treating
and annealing procedures, protective coatings, choice
and application of moisture barrier materials, dissimilar
metal contacts and accesses. Every logical precaution
is taken by the manufacturers to inhibit the onset and
spread of corrosive attacks.
There are many factors that affect the type, speed,
cause, and seriousness of metal corrosion. Some of
these factors can be controlled; others cannot.
Preventive maintenance such as inspection, cleaning,
painting, and preservation are within the control of
offers the most positive means of corrosion
The electrochemical reaction that causes metal to
corrode is a much more serious factor under wet, humid
conditions. The salt in seawater and in the air is the
greatest single cause of corrosion. Hot environments
speed the corrosion process because the electrochemical
reaction develops faster in a warm solution. Warm
moist air is usually sufficient to start corrosion if the
metal surfaces are unprotected.
Another corrosion factor is the relationship between
dissimilar metals. When two dissimilar metals come in
contact, if the more active metal is smaller than the less
active one, corrosive attack will be severe and extensive.
Insulation between such contact will inhibit this process.
If the area of the less active metal is small compared to
the other metal, corrosive attack will be relatively slight
Figure 2-33.Effects of area relationships in dissimilar metal