Cathodic protection explained
Cathodic protection
explained
Cathodic protection prevents corrosion on submerged and
underground metallic structures.
WHAT IS CATHODIC PROTECTION?
Cathodic protection is one of the most effective methods for preventing corrosion on a metal surface.
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
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Monitoring the level of cathodic protection
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.
Principles of sacrificial cathodic protection
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.
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Corrosion explained
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–.
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Different sacrificial anode models and designs
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.
Principles of ICCP – impressed current cathodic protection
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).
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Different ICCP anode models
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.
