6.5.4.2 Formation of galvanic elements

Corrosion processes can be explained using a galvanic element, or galvanic cell. When a metal rod is immersed in an electrolyte, positively charged ions pass into the electrolyte and, conversely, positive ions from the electrolyte are absorbed by the metal compound. This phenomenon is referred to as the 'solution pressure' of the metal and the 'osmotic pressure' of the solution. Depending on the magnitude of these two pressures, either the metal ions from the rod pass into the solution (the rod becomes negative compared to the solution) or the ions of the electrolyte deposit on the rod (the rod becomes positive compared to the electrolyte). This creates a voltage between two metal rods in the same electrolyte. In practice, the potential of metals in the ground is measured using a copper sulphate electrode (Figure 6.5.4.2.1). It consists of a copper rod immersed in a saturated copper sulphate solution (the reference potential of this reference electrode remains constant).

In the following, it will be described how two rods made of different metals are immersed into the same electrolyte. A voltage of a certain magnitude is now created on each rod in the electrolyte. A voltmeter can be used to measure the voltage between the rods (electrodes). It is the difference between the potentials of the individual electrodes relative to the electrolyte.

How does it come that current flows in the electrolyte and that material is transported, that is corrosion occurs?

If the copper and iron electrodes are connected via an ammeter outside the electrolyte, the following can be observed (Figure 6.5.4.2.2): In the outer circuit, the current i flows from + to –, namely from the “more precious” copper electrode according to Table 6.5.4.2.1 to the iron electrode. In the electrolyte, in contrast, the current i must therefore flow from the “more negative” iron electrode to the copper electrode to close the circuit. This means that the more negative pole releases positive ions into the electrolyte and thus becomes the anode of the galvanic element; in other words, it dissolves. The metal is dissolved at those points where the current enters the electrolyte.