Corrosion couples can be categorized into three broad categories: (i) composition cells, (ii) concentration cells, and (iii) stress cells.

5.3.2.1 Composition Cells

A composition cell can be established between two dissimilar metals. One example of it is the galvanized steel in which zinc coating is applied to carbon steel to protect it from rust and other environmental attacks. Here, zinc coating acts as a sacrificial anode, which means that the steel underneath is protected while zinc coating gets corroded. Thus, a metal higher on the electrochemical series can be used to reduce corrosion to a less active one. Note that from the relative positions of iron and zinc on the electrochemical series in Table 5.4, zinc is more anodic than iron. The unique aspect of sacrificial anode is that even if there is an accidental scratching on the zinc coating exposing the steel surface, the steel will still be protected until all zinc coating gets corroded. However, tin coating on iron or steel sheets is also used for protection in which case protection remains only up to the point when the tin coating acts as a barrier and is not compromised. However, if tin coating gets scratched or punctured, the corrosion effect will be concentrated in that punctured region (anode) with respect to remaining tin coating (cathode).

In the galvanized steel example, we found out benefits of the composition cell in corrosion protection. However, there are many other engineering examples of com­position cells wherever dissimilar metals/alloys are made to work in contact and that situation is not desirable at all from an engineering point of view. A couple of examples include steel bolts in brass naval components and steel piping connected to a copper valve. Whenever a materials selection decision is made to use dissimilar metals/alloys in engineering applications, due care must be exercised. If not, com­position cells will be set up and would cause undue galvanic corrosion problems. Galvanic cells can also be created at microlevel. Usually the single-phase metallic alloys are more corrosion resistant. As more second-phase particles are formed through precipitation, galvanic microcorrosion cells are set up between the particle and the matrix metal and lead to a poor corrosion resistance.