We are manufacturer of MMO (Mixed Metal Oxide) Anodes / Dimensionally Stable Anodes (DSA). Normally, Electrodes (Anodes & Cathodes) 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. Occasionally electrodes are also made from Nb, Ta and or Zr. In general, electrodes are available in the form of plate, mesh, rod, wire, ribbon, and customs built. Electro catalytically selective Electrodes from Ti are superior to conventional Nickel, Lead, Lead alloy, Steel, Graphite & Magnetite Electrodes.
• Electro-dialysis in waste water treatment, water conditioning and the synthesis of organic compounds.
• Cathodic protection of various structures like offshore, ships, reinforcement bars in concrete.
• Wet electro deposition of precious metals like Gold, Rhodium, Platinum, Palladium & Non-ferrous metals like Chromium, Nickel, Copper, Tin, Cobalt & Zinc.
• Production of Chlorates, Perchlorate, Chlorine / Caustic, Hypochlorite.
• Electrolytic production of chromic acid and Hydrogen from water.
• Electrochemical Cleaning of electronic components.
• Eelectrochemical Electro-galvanizing of Steel Strip.
• Electrolytic metal recovery.
MMO Anodes Overview
Originally developed to produce chlorine from seawater electrolysis, MMO anodes have been used in cathodic protection (CP) applications since the 1980s. They have continued to gain acceptance in a wide range of applications due to significant advantages.
MMO Anode Advantages
Exceptionally long life
Available in numerous anode configurations
MMO anodes have imperceptibly low wear rates and are often referred to as Dimensionally Stable Anodes 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?
The coating recipe consists of an electro catalytic conductive component that catalyzes the reaction to generate current flow, and bulk oxides (cheaper fill materials) that prevent corrosion of the substrate material. For cathodic protection applications two primary electrocatalysts are used:
1-Ruthenium Oxide (RuO2)
Ruthenium oxide, which is the cheaper of the two electrocatalysts, should only be used in chlorinated environments (sea water). The catalytic life is significantly reduced when the primary reaction is the evolution of oxygen (fresh water, soil, coke backfill and concrete environments).
2-Iridium Oxide (IrO2)
Most CP Anodes use Iridium Oxide based mixed metal oxide for their cathodic protection anodes. 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.
MMO Coated Titanium Anodes
Mixed metal oxide anodes are applied over a commercially pure titanium substrate. Titanium offers several key advantages as a substrate:
Titanium MMO Anode Advantages
Titanium is readily available in commercially pure form
Excellent adherence properties with proper surface preparation
Outstanding mechanical strength
Outstanding chemical resistance to highly acidic conditions – typical for oxygen and chlorine evolving environments common in many cathodic protection applications
Commercially pure titanium is available in a range of grades. For MMO anodes, Grades 1 and 2 are used for MMO anodes. Grade 1 titanium is slightly more malleable than Grade 2 titanium but is also less commonly used in industry. The specifications for Grade 1 and Grade 2 are very similar and overlap so that often the same material meets the requirements of both Grade 1 and Grade 2 and can be dual certified. There is no “better” between Grade 1 and Grade 2 – 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 break down voltage for dissolution. The titanium merely acts as a conductor.
MMO Anode Reactions
Conventional anodes (magnesium, high silicon cast iron, aluminum, graphite, etc.) generate current flow through reactions that include the anode itself as a primary reactant. These anodes are physically consumed from their initial moment of operation up until the anode mass has been used up and can no longer continue to fuel the reaction. Mixed metal oxide anodes, however, are not themselves reactants in the process of generating current in a cathodic protection system. They operate as electrocatalysts that cause the reaction to occur while not actually participating in the reaction. With electrocatalysts such as iridium and ruthenium oxide, over time the catalyst is subject to both physical wear rate and passivation of the active element. These catalysts have a finite but predictable operating life.
The predominate reactions in seawater, soil and coke backfills are as follows:
Typically, in an electrolyte containing chlorides, first oxygen and then chlorine would be generated. The generation of oxygen versus chlorine depends on a number of parameters including thermodynamics (oxygen discharge is preferred), kinetics, ionic concentration, the ratio of chlorides to other anions, mass transport (migration, diffusion and convection due to liquid flow) and applied current density.
One important note is that from a thermodynamic state, reactions (3) and (4) are strongly favored where coke backfills are involved. This can be important in confined areas such as under tank bottoms, where the evolution of oxygen creates a strong depolarizing effect on the structure when bare MMO anodes in sand/soil are used. When the MMO anode is packaged in coke backfill the generation of oxygen is minimized.