Corrosion handbook
12 06/21
1.2.6 Galvanic (contact) corrosion
Galvanic corrosion refers to damage caused by two dissimilar metals having
an electrically conducting connection while being in contact with a common
corrosive electrolyte (e.g.: humidity in the air). In the electrochemical model of
corrosion, one of the two partial reactions (anodic metal dissolution and cathodic
oxygen reduction) takes place almost exclusively on one metal.
Generally, the less noble metal will be dissolved (anodic metal dissolution),
whereas the more noble part is not attacked by corrosion (it serves only as the
cathode for oxygen reduction). Where galvanic corrosion takes place, the rate
of corrosion of the less noble metal is higher than it would be in a free corroding
environment without contact to another metal.
Using thermodynamic data and taking into account common experience gained
in typical applications, it is possible to predict which material combinations will
be affected by galvanic corrosion (see 3.3). A positive application of the galvanic
corrosion phenomenon is the way in which zinc or zinc alloys such as Zinc
Magnesium (here on referred to as ZM) protect carbon steels and low-alloyed
steels. Zinc or zinc alloys such as ZM are the less noble metals which actively
protect steel by being corroded themselves (see Fig.12).
0RUHQREOH /HVVQREOH
Fig. 12: This is a typical case of contact corrosion. A zinc-plated carbon steel
(washer) and stainless steel (screw and part) were used together. The surface area
of the more noble metal – the stainless steel – is larger, causing strong corrosion
of the washer.