Cathodic protection can, in principle, be applied to any metallic structure in contact with a bulk electrolyte. In practice its main use is to protect steel structures buried in soil or immersed in water. It cannot be used to prevent atmospheric corrosion.

Cathodic protection - An application of a galvanic cell

In a corrosion cell, steel will corrode when coupled to a more noble material. If however the steel is coupled to a material being more active than steel (less noble), the direction of the current will change. The less noble material will corrode and the steel will be protected from corrosion. The less noble material becomes a sacrificial anode.

Principle of Cathodic Protection

The principle is based on “supply of electrons to the base material”. This is done either by:

• connecting the structure to more active material (Sacrificial anode)
• connecting the structure to an external source of electrons (Impressed current)

Both systems supply electrons to the structure. The structure will become more noble, and metal dissolution (corrosion) of the structure will be prevented.

Sacrificial anodes are made of a metal that is more active (less noble) than the metal it is intended to protect. The anode “sacrifices” itself by corroding when connected to a more noble metal. This provides the noble metal with cathodic protection.

Sacrificial anodes will only function when they and part of the structure are submerged. A calcareous deposit is formed on the steel surface when submerged in seawater.

Anodes

Many different sizes and shapes are available. Anodes can be welded, clamped or bolted on to the structure. They can be made of:

• Zinc alloy anodes
• Aluminium alloy anodes
• Magnesium

Aluminium is often recommended instead of zinc because aluminium anode weight is approx. 1/3 of zinc. This makes a difference in total price for equal protection with lower installation costs due to the weight difference (and less weight added to the structure).

Sacrificial anodes on a ship

Sacrificial anodes will protect areas where the protective coating has suffered mechanical damages. Both zinc alloy and aluminium alloy anodes can be used on ships. A sacrificial anode cathodic protection system (SACP) will only work in submerged conditions, such as the submerged part of the ship’s hull or in its ballast tanks (but will have no effect when the tank is empty). Sacrificial anodes on the hull of a ship will increase its friction in the water.

Bracelet sacrificial anodes on rig or pipeline

The anodes are clamped on to the construction like a bracelet. Additional welding to the construction ensures good electrical contact.

Impressed Current Cathodic Protection (ICCP)

For larger structures, it is often more cost effective to use impressed current cathodic protection (ICCP) systems instead of sacrificial anodes to provide corrosion protection. ICCP systems use permanent anodes connected to a power source (e.g. AC rectifier or DC Solar Permanent anodes for ICCP systems are available in a variety of shapes and sizes. Common anodes are tubular and solid rod shapes or continuous ribbons of various noble materials. These are made from high silicon cast iron, graphite, mixed metal oxide (MMO) and platinum.

ICCP system

The ICCP system on ships comprise of these components:

Rectifier / Control Panel

This unit supplies DC electrical current (Direct Current) to the permanent anodes. A rectifier can convert an AC (Alternate Current) power supply to a DC output.

These units are available in many sizes (e.g. 50 amps to 1,200 amps) with instruments showing the output to each anode. They can incorporate over/under protection alarms and be wired to a central monitoring computer, if required.

Reference electrodes

These are flush mounted on the hull and measure the electrical potential at the hull/seawater interface. A signal is fed to the control panel which increases or decreases the anode output, so both under- and over-protection is avoided.

Permanent anodes

These anodes provide the electrical current (electrons) which protect the steel against corrosion. They can be linear anodes which are mounted on the surface of the hull or circular anodes which are flush mounted to minimise drag. These current emitting faces are made from noble materials, mixed metal oxide (MMO), graphite, platinum, etc. A typical installation may be two circular anodes in the bow area (one on each side) and two linear anodes in the stern area (one on each side).

Anode Shield

The current released by the permanent anodes can damage the protective coatings in the surrounding area. A thick, putty like coating is applied around the permanent anodes to reduce the effect of overprotection.

Shaft grounding equipment

This installation will extend cathodic protection to propeller and propeller shaft surfaces. It will reduce spark erosion damage to shaft bearings. It consists of high efficiency silver coated slip rings and silver graphite brushes attached to the propeller shaft. The effectiveness may be monitored by an optional Shaft Condition Monitor. Rudder and stabilizer bonding cables are provided in the same system.

ICCP for Buried Pipelines

Impressed Current Cathodic Protection can also be used for buried pipelines. Hazardous product pipelines are routinely protected by a protective coating supplemented with cathodic protection. An impressed current cathodic protection system (ICCP) for a pipeline consists of a DC power source, often an AC powered transformer rectifier and an anode, or a range of anodes buried in the ground (the anode groundbed).

The strength of the impressed current will depend on several factors, such as the size of the pipeline and the coating quality. The positive DC output terminal would be connected via cables to the anode range, while another cable would connect the negative terminal of the rectifier to the pipeline, preferably through junction boxes to allow measurements to be taken.

Anodes can be installed in a groundbed consisting of a vertical hole backfilled with conductive coke (a material that improves the performance and life of the anodes) or laid in a prepared trench, surrounded by conductive coke and backfilled. The choice of groundbed type and size depends on the application, location and soil resistivity. The DC cathodic protection current is then adjusted to the optimum level after conducting various tests including measurements of pipe-to-soil potentials or electrode potential.

For a pipeline, the environment will consist of the soil and anything in the soil such as other pipelines, buried cable, minerals, and water. Attention must be paid to other current sources. A stray current source can affect the amount of protection a corrosion control system can provide. In an unprotected system, stray currents can increase the rate of corrosion.

The system consists of Copper and Aluminium (or soft iron) anodes strategically located in sea chests or sometimes in-board, but as close to the sea water intake point as possible. One such set of anodes is recommended for each sea water service. The anodes are connected to a control panel that feeds a current to the anodes. This causes the Copper anodes to release cupric ions and the Aluminium anodes to release aluminium ions that form an aluminium hydroxide. The resultant ions produced by the anodes are carried by the sea water, spreads through the pipe work and creates an environment that is distinctly unfriendly to the marine life. Thus any marine life larvae that enter the pipeline will not settle but will pass right through to discharge. An added benefit is that the Aluminium hydroxide  creates a protective film on the pipe lines thereby significantly reducing pipeline corrosion. If the seawater service pipelines, condensers, etc., are of aluminium brass, cupro-nickel, etc., the aluminium anodes would generally be replaced or supplemented by soft iron anodes. The design and control panel functioning ensures that just the right concentrations of Cupric ions are introduced to keep the pipelines free but not affect the marine life outside the ship after discharge.

The result is cleaner pipelines with much longer and trouble free life resulting in lower maintenance and running costs achieved in an environmentally acceptable manner.

CP can be used to back up the paint system

No paint system is 100 % perfect, weak spots and holidays will exist. For submerged structures a CP system will protect such areas. A calcareous deposit precipitates on the substrate and reduces the corrosion rate.

This is a process of adhesion loss of protective coatings from the protected structure due to the formation of hydroxyl (or in extreme cases hydrogen) between the coating and the protected material (cathode)

If a CP system is not managed correctly, excess current flow to the anode (called over-protection) can result in blistering and loss of adhesion of the paint. This effect is known as Cathodic Disbondment. Over protection will result in the formation of Hydrogen ions and Hydroxyl ions.

How can overprotection occur?

• Active corrosion of steel are under 0.80 to - 0.85 Volts
• Critical potential (-0.85 Volts) is set for CP system to deliver electrons to the cathode
• However, there is a risk of increasing polarization as potential rises
• Result of that will generate hydrogen gas around -1.10 Volts.

Hydrogen ions induced problems caused by overprotection:

• Entrapment of hydrogen gas results in blistering between coating and the steel
• Hydrogen embrittlement of steel
• Risk of explosive atmospheres

Hydroxyl ions induce problems:

• Saponification of the coating binder

Controlling hydrogen gas formation

Apart from controlling the current flow between the noble and more active (anode) metal, the choice of anode metal is also important to avoid over protection. Use of a very active metal such as Magnesium is not recommended due to the rapid degradation of the metal and the increased production of hydrogen gas which increases the cathodic disbondment of the coating

• Ships
• Offshore platforms and rigs
• Sub-sea installations
• Sub-sea pipelines
• Harbour facilities
• Storage tanks
• Buried tanks and pipelines (onshore)