Page 28 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

6 INTRODUCTION AND FORMS OF CORROSION

the current of the galvanic cell depend on the properties of the electrolyte and the
polarization characteristics of the anodic and the cathodic reactions. Galvanic corro-
sion is caused by the contact of two metals or conductors with different potentials.
The galvanic corrosion is also called as dissimilar metallic corrosion or bimetallic
corrosion where the metal is the conductor material.
Galvanic corrosion can lead to general corrosion, localized corrosion, and some-
times both. Although the dissolution of active metals in acids is because of the numer-
ous galvanic cells on the same metallic surface, it is generally referred to as general
corrosion. In less aggressive media such as natural media consisting of dissimilar
electrode cells, galvanic corrosion can start as general corrosion that can lead to local-
ized corrosion because of different microstructures or impurities in several cases.
Localized galvanic attack depends on the distribution and morphology of metallic
phases, solution properties, agitation, and temperature. Localized galvanic corrosion
can result in the perforation or failure of the structure.
Galvanic corrosion occurs when two metals with different electrochemical poten-
tials are in contact in the same solution (Figs. 1.5 and 1.6). In both cases (5,7) the
corrosion of iron/steel is exothermic, and the cathodic reaction controls the rate of
corrosion. The more noble metal, copper, increases the corrosion rate through the
cathodic reaction of hydrogen ion reduction and hydrogen evolution. A passive oxide
film on stainless steel can accelerate hydrogen reduction reaction.
In engineering design, a junction of two different metals is seldom recommended.
It is possible to use alloys with close values of potential in a certain medium. Some of
the factors that can be deleterious are mechanical shaping, bending, or lamination of
part of the metal, thermal treatment of metallic structure, welding, and cooking coils
in vessels and heat exchangers can create galvanic cells of the same metal. These
cells are known as macrogalvanic cells, which are different from microgalvanic cells
present even in pure metals in a corrosive medium (5, 7).
In general, the galvanic cell is influenced by (i) the difference in potential between
two metals/materials, (ii) the nature of the medium or environment, (iii) polarization
of the metals, and (iv) the geometry of the cathodic and anodic sites such as shape,
relative surface areas, distance.

H O
2
Fe 2+
Fe
H + H +

e – e –
Fe Cu
Fe Cu
Figure 1.5 Galvanic corrosion of mild steel elbow fixed to a copper pipe. (Reproduced by
permission, National Association of Corrosion Engineers International (5).)