MMO Coated DSA Anodes

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Dimensionally Stable Anodes (DSA) Normally, Electrodes (Anodes) consist of a basic component, which is responsible for mechanical stability and uniform current distribution. As Titanium has all these....

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Dimensionally Stable Anodes (DSA) Normally, Electrodes (Anodes) consist of a basic component, which is responsible for mechanical stability and uniform current distribution. As Titanium has all these characteristics, insoluble electrodes are manufactured with titanium as basic component. The excellent stability of mixed metal oxide (MMO) coated titanium anodes have been used for almost 30 years. MMO coated titanium anodes are used in various environments such as seawater, brackish water, fresh water, carbon backfill, and concrete. However, many structure owners and CP engineers use MMO coated titanium anodes only specify the sizes without understanding the characteristics, limitations and evaluation method of the anodes. There are many different size and shapes are available, wire, ribbon, mesh, expanded mesh, rod, tubular, disc, probe etc.

MMO coatings for cathodic protection are mainly divided into three types, iridium oxide (IrO₂-x), ruthenium oxide (RuO₂-x) and its mixture. In addition, titanium oxide (TiO₂-x) and tantalum oxide (Ta₂O₅) are also used as the bulk materials. When MMO coated anodes discharge CP current, the coating is slowly consumed. The consumption rates of MMO coatings are varies based on the chemical compositions and the environments.

Generally, iridium oxide based MMO coating is mainly used for oxygen evolution and ruthenium oxide coating is used for chlorine gas evolution. Recently, NACE developed and modified the acceleration anode life test method using acid solutions (H₂SO₄, NaSO₄, etc.). Recently, NACE published the life acceleration tests using acid solution. Ruthenium coating is consumed at higher rates under working condition in strong acid solution, it needs to considered in actual anode performance. Iridium MMO coated anode can provide much longer life than ruthenium MMO coated anode with the same thickness when they discharge current in the same environment. The cost of iridium is much more expensive than that of ruthenium; however, iridium MMO coating is typically used when chlorine evolution is concerned.

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Cathodic protection was first described by Humphry Davy in a series of papers presented to the Royal Society in London in 1824. The first application was to the HMS Samarang in 1824. Sacrificial anodes made from iron attached to the copper sheath of the hull below the waterline dramatically reduced the corrosion rate of the copper. However, a side effect of the cathodic protection was to increase marine growth. Copper, when corroding, releases copper ions which have an anti-fouling effect. Since excess marine growth affected the performance of the ship, the Royal Navy decided that it was better to allow the copper to corrode and have the benefit of reduced marine growth, so cathodic protection was not used further.
Davy was assisted in his experiments by his pupil Michael Faraday, who continued his research after Davy’s death. In 1834, Faraday discovered the quantitative connection between corrosion weight loss and electric current and thus laid the foundation for the future application of cathodic protection.
Thomas Edison experimented with impressed current cathodic protection on ships in 1890, but was unsuccessful due to the lack of a suitable current source and anode materials. It would be 100 years after Davy’s experiment before cathodic protection was used widely on oil pipelines in the United States – cathodic protection was applied to steel gas pipelines beginning in 1928 and more widely in the 1930s.
Cathodic protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. A simple method of protection connects the metal to be protected to a more easily corroded “sacrificial metal” to act as the anode. The sacrificial metal then corrodes instead of the protected metal. For structures such as long pipelines, where passive galvanic cathodic protection is not adequate, an external DC electrical power source is used to provide sufficient current.
Cathodic protection systems protect a wide range of metallic structures in various environments. Common applications are: steel water or fuel pipelines and steel storage tanks such as home water heaters; steel pier piles; ship and boat hulls; offshore oil platforms and onshore oil well casings; offshore wind farm foundations and metal reinforcement bars in concrete buildings and structures. Another common application is in galvanized steel, in which a sacrificial coating of zinc on steel parts protects them from rust.
Cathodic protection can, in some cases, prevent stress corrosion cracking. (Source: Wikipedia )