SACRIFICIAL ANODE CATHODIC PROTECTION SYSTEMS

SACRIFICIAL ANODE CATHODIC PROTECTION SYSTEMS

In sacrificial anode systems the high energy electrons required for cathodic protection are supplied by the corrosion of an active metal.

This is illustrated in Figure cathodic protection sacrificial anodes

Sacrificial cathodic anodes systems depend on the differences in corrosion potential that are established by the corrosion reactions that occur on different metals or alloys. (see Galvanic Corrosion & Galvanic Corrosion Chart)

For example, the natural corrosion potential of iron is about -0.550 volts in seawater. The natural corrosion potential of zinc in seawater is about -1.2 volts. Thus if the two metals are electrically connected, the corrosion of the zinc becomes a source of negative charge which prevents corrosion of the iron.

The materials used for sacrificial anodes are either relatively pure active metals, such as zinc or magnesium, or are magnesium or aluminum alloys that have been specifically developed for use as sacrificial anodes.

In applications where the cathodic protection anodes are buried, a special backfill material surrounds the anode in order to insure that the anode will produce the desired output.

Sacrificial cathodic protection anodes are normally supplied with either lead wires or cast-m straps to facilitate their connection to the structure being protected.

The lead wires may be attached to the structure by welding or mechanical connections. These should have a low resistance and should be insulated to prevent increased resistance or damage due to corrosion.

When cathodic protection anodes with cast-in straps are used, the straps can either be welded directly to the structure or the straps can be used as locations for attachment

A low resistance mechanically adequate attachment is required for good protection and resistance to mechanical damage.

In the process of providing electrons for the cathodic protection of a less active metal the more active metal corrodes.

The more active metal (anode) is sacrificed to protect the less active metal (cathode). The amount of corrosion depends on the metal being used as an anode but is directly proportional to the amount of current supplied.

The anodes in sacrificial anode cathodic protection systems must be periodically inspected and replaced when consumed.

Source : "Corrosion Control" NAVFAC MO-307 September 1992