Mixed metal oxide (MMO) electrodes, also called Dimensionally Stable Anodes (DSA), are devices with high conductivity and corrosion resistance for use as anodes in electrolysis. They are made by coating a substrate, such as pure titanium plate or expanded mesh, with several kinds of metal oxides. One oxide is usually RuO2, IrO2, or PtO2, which conducts electricity and catalyzes the desired reaction such as the production of chlorine gas. The other metal oxide is typically titanium dioxide which does not conduct or catalyze the reaction, but is cheaper and prevents corrosion of the interior.
Read more The loading or amount of precious metal on the substrate (that is, other than the titanium) can be in the order of around 10 to 12 grams per square metre.[1]
Applications [ edit ] Applications include use as anodes in electrolytic cells for producing free chlorine from saltwater in swimming pools, in electrowinning of metals, in printed circuit board manufacture, electrotinning and zinc electro-galvanising of steel, as anodes for cathodic protection of buried or submerged structures.
History [ edit ] Henri Bernard Beer registered his patent on mixed metal oxide electrodes in 1965.[2] The patent named "Beer 65", also known as "Beer I", which Beer claimed the deposition of Ruthenium oxide, and admixing a soluble titanium compound to the paint, to approximately 50% (with molar percentage RuO2:TiO2 50:50).
His second patent, Beer II,[3] reduced the Ruthenium oxide content below 50%.[4]
For more information, please visit GIANT ANODE. See also [ edit ] References [ edit ] MMO & PLATINUM COATED TITANIUM ANODES by Titan
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 specfy the sizes without understanding the characteristics limitations and evaluation method of the anodes.
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.
This paper will discuss the characteristics limitations and evaluation method of various MMO coatings. In addition the distributions of iridium and ruthenium in various commercially available MMO coatings are also discussed.
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