By coupling a given structure (say Fe) with a more active metal such as zinc or magnesium. This produces a galvanic cell in which the active metal works as an anode and provides a flux of electrons to the structure, which then becomes the cathode. The cathode is protected and the anode progressively gets destroyed, and is hence, called a sacrificial anode.

The second method involves impressing a direct current between an inert anode and the structure to be protected. Since electrons flow to the structure, it is protected from becoming the source of electrons (anode). In impressed current systems, the anode is buried and a low voltage DC current is impressed between the anode and the cathode.

Sacrificial anode systems are simpler. They require only a material anodic to the protected steel in the environment of interest. Figure 3  shows an impressed-current system used to protect a pipeline.

The buried anodes and the pipeline are both connected to an electrical rectifier, which supplies direct current to the buried electrodes (anodes and protected cathode) of the system. 

Unlike sacrificial anodes, impressed-current anodes need not be naturally anodic to steel, and in fact, they seldom are most impressed-current anodes are made from non-consumable electrode materials that are naturally cathodic to steel.

If these electrodes were wired directly to a structure, they would act as cathodes and would cause accelerated corrosion of the structure they are intended to protect.

The direct current source reverses the natural polarity and allows the materials to act like anodes. Instead of corrosion of the anodes, some other oxidation reaction, that is, oxygen or chlorine evolution, occurs at the anodes, and the anodes are not consumed.

Impressed-current systems are more complex than sacrificial anode systems. The capital expenses necessary to supply direct current to the system are higher than for a simple connection between an anode and a cathode.

The voltage differences between anode and cathode are limited in sacrificial anode systems to approximately 1 V or even less, depending on the anode material and the specific environment. Impressed-current systems can use larger voltage differences.

The larger voltages available with impressed-currents allow remote anode locations, which produce more efficient current distribution patterns along the protected cathode.

These larger voltages are also useful in low-conductivity environments, such as freshwater and concrete, in which sacrificial anodes would have insufficient throwing power.