Cathodic protection is a technique used to reduce the rate of corrosion of a metallic surface by making the metal the cathode. This can be achieved by attaching galvanic (sacrificial) anodes or impressed current anodes.

Due to its extensive use in marine and other corrosive environments, steel is often the focus of Cathodic protection. Other metals widely used for mechanical and structural applications are aluminium alloys, stainless steels, brasses and bronzes, all of which require some level of protection against corrosion.

GALVANIC ANODES

Galvanic anodes are one of the most effective methods of protecting metallic structures or vessels against corrosion. Galvanic anodes, as distinct from impressed current anodes, do not require an external power source. They are a cheap and simple method of protecting against corrosion.

Galvanic anodes are cast from high purity primary magnesium, aluminium and zinc ingot, with additional “activating” elements added to the melt to ensure that the anode offers the most effective protection. The addition of the activating elements is strictly controlled according to national and international standards and verified by spectrographic and electrochemical analysis.

RATE OF CORROSION

When a metal is immersed in an electrolyte (water, soil, concrete) it generates an electrical current. The current is dependent on the type of electrolyte. Aluminium and zinc anode alloys are used in sea water which is a very conductive (low resistance) electrolyte. Magnesium alloy anodes are used in fresh water which is much less conductive (high resistivity).

Any metal will corrode much faster in sea water than in fresh water, typically ten times faster. The resistivity of salt water is 0.25 ohm metres, while fresh water (in most locations) is typically 50 ohm metres, a difference of 200 times.

It is important to remember that the aim of cathodic protection is to shift the natural voltage of a metal in a negative direction to a point at which corrosion is significantly reduced. For example steel has a natural potential of -500mV in sea water (with respect to a standard silver/silver chloride reference electrode, such as a Rust Seeker). The whole point of coating the steel with paint and fitting anodes is to shift the potential to around -800mV. At this point cathodic protection is achieved and corrosion of the steel is significantly reduced.

THE GALVANIC SERIES

All metals have an energy level which can be measured. This level is measured as the metal’s natural voltage within a particular electrolyte, e.g. sea water. Thus metals can be tabulated as a function of their natural voltage for a particular electrolyte. This table is referred to as the Galvanic Series.

THE GALVANIC SERIES IN SEA WATER
Metal or Alloy
ANODIC actively corrodeMagnesium anode alloy - high potential
Magnesium anode alloy - low potentia
Aluminium anode alloys
Zinc anode alloys
Aluminium alloys
Cast iron
Carbon steel (mild steel)
Copper alloys (brass/bronze)
Cupronickels
Copper
Nickel
306 stainless steel (active)
Silver
Titanium
304 stainless steel (passive)
CATHODIC noble passive316 stainless steel (passive)
Platinum
Gold
Graphite

The Galvanic Series ranges from the most active to the least active. Metals shown low in the table are more “noble” (more resistant to corrosion), whilst metals at the top of the series are more active or anodic (actively corrode).

bargeAlloys higher on the table cathodically protect all alloys below them when electrically connected. The further apart they are on the table, the more active the higher alloy.

Magnesium, aluminium and zinc anode alloys are at the top of the Galvanic Series. They have a large voltage difference when connected to metals such as copper, platinum, gold, and titanium and therefore the rate of corrosion and loss of magnesium, aluminium and zinc would, on comparative surface areas, be quite rapid.

SIZE OF ANODES & CATHODES

A small anode electrically connected to a large cathode will result in the anode being rapidly consumed. A large anode electrically connected to a small cathode will result in the anode being slowly consumed.

It is the surface area of the anode and the cathode and the level of resistivity of the electrolyte (e.g. water) that determines the amount of DC current that flows from an anode. It is the flow of electrons (mA) driven by the galvanic energy (voltage) of the metal/s that results in the amount of corrosion, oxidation or degradation of any metal.

Using Ohms Law (V=IR, where V=volts, I=amps and R=resistance), and the surface area of an anode, corrosion engineers calculate the amount of protective current (milliamps) able to be delivered when a galvanic anode is connected to a metallic structure or vessel.