R14 Combating corrosion: sacrificial protection and electrolytic protection

Sacrificial protection

Examples of the use of sacrificial blocks of metal to combat corrosion are to be found in illustration R14. The table of standard reduction potentials in the text accompanying illustration R13 are useful in clarifying the processes involved.
Sacrificial protection of metals involves the use of small blocks of metal which due to their higher -values will oxidise before the metal which they are there to protect.
They are replaced at regular intervals as they are oxidized away.

Magnesium, aluminium or zinc are used in sacrificial blocks, all having higher -values than iron. Examples of applications are :

• zinc blocks welded to the steel hulls of ships, as shown in illustration R14;
• zinc blocks welded to the steel supports of oil rigs; and
• magnesium, aluminium zinc or even graphite blocks
  electrically connected to pipe lines at regular intervals, as shown at the top right of illustration R14.

As these sacrificial electrodes are oxidised away ions are released, which in the cases of zinc and aluminium ions have a polluting effect on the sea or ground water.

Electrolytic protection

An alternative to the use of sacrificial blocks in the protection of the steel hulls of ships is to apply a negative charge to the iron surface. An electrolytic cell is thereby set-up with the same half-cells which in the absence of this electron source make up the electrochemical cell responsible for the corrosion process. Whereas in the electrochemical cell the iron surface acts as the anode at which the iron is oxidised and oxygen in the water acts as the cathode at which oxygen is reduced to hydroxide ions, the application of an electric current results in iron acting as the cathode making oxidation more difficult.

A decrease in the potential at the iron surface of
100 mV reduces the rate of its oxidation by a factor of between 5 and 100.

In practice a cathodic current is applied to the metal being protected and the metal is surrounded by a good but inert conductor such as graphite which acts as the anode, such is shown in the top left of illustration R14.
This method has the advantage over sacrificial blocks of being easier to control and quantify, of requiring less sacrificial material and of reducing pollution due to corrosion products. It is used mainly in the petrochemical industry to protect pipelines and storage tanks, in concrete structures to protect the iron reinforcement and in the shipping world to protect ships, oil platforms and harbour installations. It is however an expensive form of corrosion protection.