Page 29 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

GALVANIC CORROSION 7

Figure 1.6 Galvanic corrosion of painted auto panel in contact with stainless steel wheel
opening molding. (Reproduced by permission, ASM International (6).)

For example, it is not desirable to have a small anode connected to a large cathode
as this favors accelerated localized anodic dissolution. Rivets of copper on a steel
plate and steel rivets on a copper plate on immersion in seawater for a period of
15 months resulted in the steel plate covered with corrosion products while the steel
rivets were corroded completely and disappeared. As copper is more noble than iron,
it accelerated the hydrogen reduction reaction for the oxidation of the steel plate.
In the case of the copper plate with steel rivets, the steel rivets corroded because of
the relatively important cathodic surface of copper. The same reasoning applies to
the corrosion of noncoated auto parts in contact with a large stainless steel surface
(Fig. 1.6).
Galvanic corrosion occurs because of : (i) nonmetallic conductors and corrosion
products, (ii) metallic coatings and sacrificial anodes, (iii) polarity inversion,
(iv) deposition corrosion, (v) hydrogen cracking or damage, (vi) high temperature.

(i) Nonmetallic Conductors and Corrosion Products. Carbon brick in vessels,
graphite in heat exchangers, carbon-filled polymers, oxides such as mill scale
(magnetite Fe O ), iron sulfides on steel, lead sulfate on lead can act as
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effective cathodes with an important area to that of anodes. Very often the
pores of the conductive film are the preferred anodic sites that lead to pitting
corrosion.
(ii) Metallic Coatings and Sacrificial Anodes. Some sacrificial metal coatings pro-
vide cathodic protection for base metals, such as galvanized steel or Alclad
aluminum. If the metal coating is more noble than the base metal such as nickel
on steel pitting of the base metal at pores, damage sites and edges can occur.
It is important to note that the coatings should be kept free of pores, scratches,
or any penetrating chemical attack or deterioration of the coat such as paint.
Sacrificial anodes, such as magnesium, zinc, and aluminum are used
extensively for cathodic protection in some locations where impressed current
systems are forbidden because of stray currents.