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, offshore oil platforms, and onshore oil well casings.

A side effect of improperly performed cathodic protection may be production of molecular hydrogen, leading to its absorption in the protected metal and subsequent hydrogen embrittlement.

Cathodic protection is an effective method of preventing stress corrosion cracking.

Origins

The first use of CP was in 1852, 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 (rust) than copper and when connected to the hull, the corrosion rate of the copper was dramatically reduced.

Galvanic CP

Today, galvanic or sacrificial anodes are made in various shapes using alloys of zinc, magnesium and aluminium. 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.

Impressed Current CP

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 DC power source (a cathodic protection rectifier). Anodes for ICCP systems are tubular and solid rod shapes or continuous ribbons 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 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 tests including measurements of electrochemical potential.

Testing

Electrochemical potential is measured with reference electrodes. Copper-copper(II) sulfate electrodes are used for structures in contact with soil or fresh water. Silver chloride electrodes are used for seawater applications.

Galvanized Steel

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 (see passivation). 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 becomes exposed will be recovered by the zinc. This happens because the zinc from the surrounding area will dissolve and be deposited on the steel, replacing what was lost to the scratch.

External links

NACE International (formerly the National Association of Corrosion Engineers) - largest professional association of CP experts
US Army Corps of Engineers, "Engineering and Design - Cathodic Protection Systems for Civil Works Structures", Engineering manual 1110-2-2704, 12 July 2004