4.3.1 DEFINITION

Galvanic corrosion occurs when two metals with different electrochemical potentials or with different tendencies to corrode are in metal-to-metal contact in a corrosive electrolyte.

4.3.2 DESCRIPTION

When two metals with different potentials are joined, such as copper (+0.334 V) and iron (-0.440 V), a galvanic cell is formed. A cell in which the chemical change is the source of energy, is called a galvanic cell. The corrosion which is caused due to the formation of the galvanic cell is, therefore, called galvanic corrosion. The driving force for corrosion is a potential difference between different materials. This force was described by Luigi Galvani, late in the eighteenth century. Between the two different materials connected through an electrolyte, the less noble will become the anode and tend to corrode.

The tendency of a metal to corrode in a galvanic cell is determined by its position in the galvanic series of metals and alloys. A galvanic series is a list of metals and alloys arranged according to their relative potentials in a given environment (Table 4.1). Galvanic series have already been discussed in Chapter 3. In the galvanic series, a metal tends to corrode when connected to a metal which is more cathodic to it. The further apart the metals or alloys are in the series, the more rapid will be the corrosion of the more anodic (baser) metal. For instance, chromium steel (12 14% chromium) in the active condition cannot be joined to chromium steel...