CUPROBAN Sacrificial Anode Cathodic Protection System
The Principle of Cathodic Protection
All metals and alloys undergo a natural process of corrosion depending on the metal and the surrounding environment. Metal atoms have loosely bonded electrons which they tend to lose. When a metal is immersed in an electrolyte such as sea-water this tendency results in the setting up of an electric potential. The loss of electrons from the metal leads to its dissolution or corrosion.
Further if two dissimilar metals are electrically in contact and immersed in the same electrolyte, the more reactive (or baser) metal will part with its electrons more readily and will corrode in preference to the less reactive (or nobler) metal. The surface or material where loss of electrons and hence corrosion takes place is called the anode and the surface where electrons are absorbed and where there is no corrosion is called the cathode.
Even in a seemingly uniform single metal structure (for instance a ship’s hull or the legs of an off shore platform) there will be enough metallic dissimilarities to create internal anode anodic and Cathodic spots on the surface leading to corrosion. In a painted surface if there is a small area where the paint has deteriorated that area will become anodic with reference to its neighbouring areas leading to corrosion. If there are bi-metallic connections – for example the hull of a ship is steel while the propellers are bronze (a metal that is Cathodic to steel) – anodes and cathodes are automatically created leading to corrosion.
In summary, corrosion can thus be seen to be an electro-chemical phenomenon. Its prevention by cathodic protection is also an electro-chemical process. The principle of cathodic protection is to create a potential gradient opposing the flow of electrons from the surface to be protected.
One of the methods of doing this is by galvanic reaction of a Sacrificial Anode system.
Sacrificial Anode Cathodic Protection System
If a baser metal is connected electrically with the surface to be protected inside the electrolyte, the baser metal will become the anode and will ‘corrode’ in preference to the surface to be protected which becomes the cathode. The baser metal is thus called a ‘sacrificial anode’. For protecting steel from corrosion Zinc, Aluminium and Magnesium are all more reactive (baser) and can be used as sacrificial anodes. For sea water applications Zinc and Aluminium are preferred. Such sacrificial anodes have a definite life after which they should be replaced with fresh anodes.
Choosing Between Aluminium and Zinc
Zinc has been in use as a sacrificial anode for longer than Aluminium and is considered the traditional anode material. However, aluminium has several outstanding advantages as a sacrificial anode material and is fast becoming the material of choice.
The efficiency of an anode material depends on its electrochemical properties. There are two electrochemical properties that are of relevance: the first is the material’s open circuit potential and the second its electrochemical capacity and consumption rate.
The open circuit potential is the potential the anode material develops when immersed in an electrolyte. For Aluminium it is 1.05V to 1.07V while for Zinc it will be 0.95V to 1.0V. Thus Aluminium can develop 50V to 70V more than Zinc and this makes a significant difference in their respective current delivery capabilities with Aluminium capable of delivering more current.
Secondly Aluminium anodes can develop 2500 amp-hr of current for every Kg of material that it sacrifices while the comparative figure for Zinc is 780 amp-hrs. Higher the current capacity lower is the consumption rate. Thus 3.4 Kgs of Alumnium gets consumed for generating 1-amp of current for 1 year while the corresponding figure for Zinc is 11.2 to 11.5 Kgs! Thus a lesser mass of Aluminium anodes will protect a given surface for a given period of time as compared to Zinc anodes. This leads to greater economies and improved performance in using Aluminium as opposed to Zinc.
Comparison of Common Sacrificial Anode Materials
For protecting steel from corrosion in marine environments the two commonly used sacrificial anode materials are suitably alloyed – Aluminium and Zinc. They compare as follows:
| Aluminium Anodes |
Zinc Anodes |
| Composition |
|
|
|
| Zinc |
2 to 5% |
Aluminium |
0.1 to 0.5% |
| Indium |
0.01 to 0.05% |
Cadmium |
0.025 to 0.15% |
| Copper |
0.05% max. |
Lead |
0.006% max |
| Silicon |
0.13% max. |
Iron |
0.005% max |
| Iron |
0.13% max |
Silicon |
0.125% max |
| Aluminium |
Balance |
Zinc |
Balance |
| Electrochemical Properties |
|
|
| Open circuit potential |
-1.05V to 1.07V (Ag/AgCl) |
Open Circuit Potential |
-0.95 to -1.00V (Ag/AgCl) |
| Electrochemical capacity |
2550 Amp-Hr / Kg |
Electrochemical capacity |
780 Amp-Hr/Kg |
| Consumption Rate |
3.4 Kg/ Amp-Yr |
Consumption Rate |
11.25 Kg/Amp-Yr |
Cuproban Range of Sacrificial Anodes
Cuproban offers high quality Aluminium and Zinc sacrificial anodes made to exacting international standards. This is possible through an exclusive cooperation and brand manufacturing arrangement with Sargam Metals, South Asia’s oldest Lloyd’s Register of Shipping Type Approved, ISO 9001 certified Anode manufacturer and a contract supplier to Jotun (Singapore) Pte Ltd for more than a decade with experience of having supplied several thousand tons of anodes to various Singapore yards, vessels and projects.
Cuproban Aluminium anodes will typically have composition and properties as above.
Cuproban Zinc anodes meet the widely used US MIL 18001 specifications that are in line with the above.
New Range of Environmentally Friendly Zinc anodes
In addition Cuproban also offers a unique eco-friendly range of Cadmium free Zinc anodes that also perform better than the conventional Zinc anodes at higher operating temperatures.
All the above anodes are offered in a wide variety of sizes and weights with weld-on, clamp-on and bolt-on types of inserts for fixing to the structure to be protected.
Off Shore Applications
Cuproban’s range of off-shore Aluminium anodes are made from low impurity Aluminium and are offered as large weight ‘jacket’ anodes with pipe cores for protecting off-shore platforms and structures or as bracelet anodes for sub-sea pipeline protection. Cuproban can also provide segmented bracelet anodes for retrofitting on existing pipelines and structures.
Cuproban provides complete chemical composition certificates (analyzed using advanced spectrometers) as well as electro-chemical test certificates (as per DNV testing standards) for its supplies. Third party inspection can also be organized and requirements complied with if required.
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