Buried tank evaluation
Introduction
Cathodic protection (CP) is a technique to control the galvanic corrosion of a metal surface by converting it into the cathode of an electrochemical cell.[1] The simplest method of applying CP is by connecting the metal to be protected with another more easily corroded metal by acting as the anode of an electrochemical cell. Cathodic protection systems are most commonly used to protect steel, water transport pipelines, fuel storage tanks, ship hulls, or oil platform structures both offshore and onshore.
Cathodic protection (CP) can, in many cases, prevent galvanic corrosion.
History
Cathodic protection was first described by Humphry Davy in a series of papers presented to the Royal Society[2] in London in 1824. After a series of tests, the first application was on the ship HMS Samarang")[3] in 1824. An iron sacrificial anode was attached to the copper hull plate below the waterline and this dramatically reduced the rate of corrosion of the copper. However, a side effect of PC was that it increased algae growth. Copper, when corroded, releases copper ions that have an anti-algae effect. Since excessive algae growth affects the ship's performance, the Royal Navy decided that it was better to allow the copper to corrode and have the benefit of reduced algae growth (anti-fouling effect), so the PC was discontinued.
Types of cathodic protection
sacrifice PC
Currently, the galvanic anode or sacrificial anode is made in various forms with zinc, magnesium and aluminum alloy. The electrochemical potential, current capacity, and consumption rate of these alloys are higher for aluminum than for iron. ASTM International publishes standards for the composition and manufacture of galvanic anodes.[4][5].
Galvanic anodes are designed and selected to have a more "active" voltage (more negative electrochemical potential) than the metal of the structure (generally steel). For effective PC, the potential of the steel surface must be biased more negatively until the surface has a uniform potential. At this time, the driving force for the corrosion reaction is removed. The galvanic anode continues to corrode, the anode material is consumed until finally it must be replaced. Polarization is caused by the flow of electrons from the anode to the cathode. The driving force for current flow is the electrochemical potential difference between the anode and cathode.