Cathodic protection explained

Cathodic protection prevents corrosion on submerged and underground metallic structures.

Cathodic protection is commonly used to protect numerous structures against corrosion, such as ships, offshore floaters, subsea equipment, harbours, pipelines, tanks; basically all submerged or buried metal structures.

Basic principles of cathodic protection

The technique is based on convertering active areas on a metal surface to passive, in other words making them the cathode of an electrochemical cell. Read more about electrochemical cells and corrosion.

By supply of current, the potential of the metal is reduced, the corrosion attack will cease and cathodic protection is achieved. Cathodic protection can be achieved by either:

• Sacrificial anode cathodic protection
• Impressed current cathodic protection, often referred to as ICCP

Sacrifical anode cathodic protecion

The simplest method to apply cathodic protection is by connecting the metal to be protected with another more easily corroded metal to act as the anode. Zinc, aluminium and magnesium are the metals commonly used as anodes.

Read more about the galvanic series and nobility of metals.

The most active metal (whis also is the less noble) becomes the anode to the others, and sacrifices itself by corroding (giving up metal) to protect the cathode. Hence, the term sacrificial anode.

As the driving voltage of sacrificial anodes is low compared with impressed current anodes, the sacrificial anodes must be well distributed and located closer to the area being protected.

Which to use; aluminium or zinc anodes in salt water environments?

Due to the potential difference between the anodic (less noble) and the cathodic area (steel), positively charged metal ions leave the anode surface, while electrons leave the surface at the cathode. For aluminium alloy anodes, the reaction at the anode surface is: 4Al → 4Al^{+ + +} + 12e^–.

Impressed current cathodic protection (ICCP)

ICCP systems uses an external source of electrical power provided by a regulated DC power supply, often referred to as control panel. The control panel provides the current necessary to polarise the surface to be protected.

The protective current is distributed by specially designed inert anodes, generally a conductive material of a type that is not easily dissolved into metallic ions, but rather sustain alternative anodic reactions.

An ICCP system continuously monitors the level of protection and adapts to the current required to stop corrosion.

In good sea water environmental conditions oxidation of the dissolved chloride ions will be the predominant anodic reaction resulting chlorine gas developed at the anode surface: 2Cl^– → Cl₂ + 2e^–. In low salinity waters the predominant anodic reaction will be decomposition of water: 2H₂O → O₂ + 4H⁺ + 4e^–.

One of the most common ICCP anode types for seawater application is the “MMO/Ti”, which consists of titanium substrate (Ti) coated with a noble metal or metal oxide catalyst (MMO).

When is cathodic protection obtained?

Cathodic protection current flow onto any metal will shift its normal potential in the negative direction. Full cathodic protection of steel is obtained by a potential at a certain level. Read more about level of cathodic protection.