Product Introduction
Mixed Metal Oxide Anodes (MMO Anodes), also called dimensionally stable anodes (DSA), are a type of Impressed Current Protection Anode, consisting of rare earth oxides coated onto a titanium substrate.
MMO anodes have imperceptibly low wear rates and are often referred to as DSA as they do not change geometry over time. MMO anodes are quickly becoming the most popular impressed current anode gaining market share over conventional anodes such as graphite and high silicon cast iron.
What is mixed metal oxide?
There is no precise industry definition for mixed metal oxide. Each supplier has a unique recipe for their mixed metal oxide coating. The coating recipe consists of an electrocatalytic conductive component that catalyzes the reaction to generate current flow, and bulk oxides (cheaper fill materials) that prevent corrosion of the substrate material. For MMO Anode for Cathodic Protection applications two primary electrocatalysts are used:
Ruthenium Oxide (RuO2)
Ruthenium oxide, which is the cheaper of the two electrocatalysts, should only be used in chlorinated environments (seawater). The catalytic life is significantly reduced when the primary reaction is the evolution of oxygen (freshwater, soil, coke backfill, and concrete environments).
Iridium Oxide (IrO2)
Most CP manufacturers use Iridium Oxide mixed metal oxide for their cathodic protection anodes because it is suitable for both chlorinating and oxygenating environments. The bulk oxides are typically a mixture of Titanium Dioxide (TiO2) and Tantalum Oxide (TaO5). Their primary purpose is to provide an oxide film over the substrate material to prevent corrosion of the substrate.
Commercially pure titanium is available in a range of grades. For MMO anodes, Grades 1 and 2 are used. Gr1 titanium is slightly more malleable than Gr2 titanium but is also less commonly used in industry. The specifications for Gr1 and Gr2 are very similar and overlap so often the same material meets the requirements of both Gr1 and Gr2 and can be dual-certified. There is no "better" between Gr1 and Gr2 – they can be used interchangeably with no impact on performance.
When first used as an anode substrate, there were significant, although unfounded, concerns over the potential for the titanium to exceed its "break down" voltage. Bare titanium when polarized above 8 to 9 volts in seawater (or above 10 volts in soil or coke backfill) is subject to intense oxygen evolution of the titanium surface resulting in passive film break-down and dissolution (pitting corrosion) of titanium. The concerns led some to believe that any anode operating with more than 8-10 volt power supplies could be at risk of damaging the anode. This does not take into consideration the role of the highly conductive electrocatalytic oxide and the protective oxide films that make up the MMO coating. Unless the catalytic coating is significantly damaged or consumed, the titanium substrate never sees sufficient polarization to reach the breakdown voltage for dissolution. The titanium merely acts as a conductor.
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