When dissimilar metals are in electrical or physical contact (the former through an electrolyte), galvanic corrosion can take place. The process is akin to a simple DC cell in which the more active metal becomes the anode and corrodes, where as the less active metal becomes the cathode and is protected.

The galvanic series shown in Table 1 can be used to predict the metal which will corrode in contact with another metal, based on whether it is cathodic or anodic with respect to another.

Cathodic protection was first developed by Sir Humphrey Davy in 1824 as a means of controlling corrosion on British naval ships.

Virtually all modern pipelines are coated with an organic protective coating that is supplemented by cathodic protection systems sized to prevent corrosion at holidays in the protective coating.

This combination of protective coating and cathodic protection is used on virtually all immersed or buried carbon steel structures, with the exception of offshore petroleum production platforms and reinforced concrete structures.

Table 1  shows the theoretical electrochemical potentials obtained by pure metals in 1 N solutions of their own ions.

Figure 1  shows two of these metals--iron and zinc--separately immersed in a weak mineral acid. The chemical reactions that occur in Figure 1  are :

Both metals corrode, and both corrosion (oxidation) reactions are balanced by an equal reduction reaction, which in both cases involves the liberation of hydrogen gas from the acid environments. 

Almost all of the oxidation reaction (corrosion of zinc) has been concentrated at the zinc electrode (anode) in   Figure 2  and almost all of the reduction reaction (hydrogen liberation) has been concentrated at the iron electrode (cathode).

The oxidation of the zinc anode in Figure 2   is much faster than that in Figure 1. At the same time, most of the corrosion of iron in Figure 1  has stopped in Figure 2   As shown schematically, the zinc anode in  Figure 1  has been used to cathodically protect the iron cathode in Figure 2  .

Of course, some corrosion of the iron may still occur; whether or not this happens depends on the relative sizes of the zinc and iron electrodes.

Some reduction of hydrogen may still occur on the zinc anode. The anode is the electrode at which a net oxidation reaction occurs, whereas cathodes are electrodes at which net reduction reactions occur. 

All cathodic protection systems require an anode, a cathode, an electric circuit between the anode and cathode, and an electrolyte. 

Thus, cathodic protection will not work on structures exposed to air environments. The air is a poor electrolyte, and it prevents current from flowing from the anode to the cathode.