Cathodic protection

Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell.

It is a method used to protect metal structures from corrosion. Cathodic protection systems are most commonly used to protect steel water and fuel pipelines and tanks, steel pier piles, ships, and offshore oil platforms.

The first use of CP was in 1824, when Sir Humphry Davy, of the British Navy, attached chunks of iron to the external, below water line, hull of a copper clad ship. Iron has a stronger tendency to corrode than copper and when connected to the hull, the corrosion rate of the copper was dramatically reduced.

Today, galvanic or sacrificial anodes are made in various shapes using alloys of zinc, magnesium and aluminum. The electrochemical potential, current capacity, and consumption rate of these alloys are superior for CP than iron.

Galvanic anodes are designed and selected to have a more "active" voltage (technically a more negative electrochemical potential) than the metal of the structure (typically steel). For effective CP, the potential of the steel surface is polarized (pushed) more negative until the surface has a uniform potential. At that stage, the driving force for the corrosion reaction is halted. The galvanic anode continues to corrode, consuming the anode material until eventually it must be replaced. The polarization is caused by the current flow from the anode to the cathode. The driving force for the CP current flow is the difference in electrochemical potential between the anode and the cathode.

For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection. Impressed Current Cathodic Protection (ICCP) systems use anodes connected to a direct current power source (AC powered rectifier). Anodes for ICCP systems are tubular and solid rod shapes of various specialized materials. These include high silicon cast iron, graphite, mixed metal oxide, platinum and niobium coated wire and others.

A typical ICCP system for a pipeline would include an AC powered DC rectifier with a maximum rated DC output of between 10 and 50 amperes and 50 volts. The positive DC output terminal is connected via cables to the array of anodes buried in the ground (the anode groundbed). For many applications the anodes are installed in a 60 m (200 foot) deep 25 cm (10-inch) diameter vertical hole and backfilled with conductive coke (a material that improves the performance and life of the anodes). A cable rated for the expected current output connects the negative terminal of the rectifier to the pipeline. The operating output of the rectifier is adjusted to the optimum level by a CP expert after conducting various potential measurements.

Electrochemical potential is measured with reference electrodes. A copper copper sulphate electrode is used for structures in contact with soil or fresh water. Silver chloride electrodes are used for seawater applications.

Most modern cars have galvanized (zinc-coated) steel frames and panels. Unprotected steel forms a layer of iron oxide, which is permeable to air and water and allows corrosion to continue underneath. However, zinc oxide (produced on the surface of zinc-protected objects) is impermeable. As long as the zinc and zinc oxide layers are undisturbed (i.e. not scraped or sanded off), the steel underneath will not rust. Galvanised steel has some self repairing properties; small scratches, where the steel is exposed will be covered by zinc, as zinc from the surrounding area will dissolve and be deposited on the steel.

External links

• NACE International (http://www.nace.org) (formerly the National Association of Corrosion Engineers) - largest professional association of CP experts