MMO Anodes (Mixed Metal Oxide) for Cathodic Protection

MMO Anodes (Mixed Metal Oxide) for Cathodic Protection

Originally introduced to generate chlorine through seawater electrolysis, MMO anodes have been widely utilized in cathodic protection (CP) applications since the 1980s, consistently gaining acceptance due to their notable advantages, such as their extensive lifespan and reliability.

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MMO Anode Advantages

• Demonstrated reliability
• Exceptionally long life
• Available in numerous anode configurations
• Light weight
• Low cost

MMO anodes, known for their minimal wear rates, are often referred to as Dimensionally Stable Anodes due to their consistent geometry over time. These anodes are rapidly becoming favored over traditional anodes like graphite and high silicon cast iron in impressed current applications.

What is Mixed Metal Oxide?

No universally exact definition of “mixed metal oxide” (MMO) exists. Each supplier's unique recipe for MMO coating consists of an electro catalytic conductive component to drive current flow and bulk oxides that protect the substrate material. For cathodic protection, two primary electrocatalysts are commonly used:

1. Ruthenium Oxide (RuO2)

Ruthenium oxide, being cost-effective, is ideal for chlorinated environments like seawater but less effective in fresh water or soil due to reduced catalytic life.

2. Iridium Oxide (IrO2)

Widely used by CP manufacturers, Iridium Oxide-based MMOs excel in both chlorinating and oxygenating environments. Bulk oxides like Titanium Dioxide (TiO2) and Tantalum Oxide (TaO5) help prevent substrate corrosion.

MMO Coated Titanium Anodes

An MMO coating is applied over commercially pure titanium substrates, offering several advantages:

Titanium MMO Anode Advantages

• Readily available in pure form
• Excellent adherence with proper surface preparation
• Lightweight
• High mechanical strength
• Resistance to highly acidic conditions (common in many CP environments)

Commercially pure titanium in Grades 1 and 2 is used for MMO anodes. Both grades have similar properties and can interchangeably be used without impacting performance. Concerns over titanium exceeding its breakdown voltage have been proven to be unfounded due to the protective MMO coating which prevents exposure to elevated voltages.

Some suppliers use a bimetallic substrate for MMO wire anodes, incorporating a thin titanium layer over a copper core to reduce cost and improve conductivity. However, using copper complicates resistance to corrosion in acidic environments.

MMO Anode Manufacturing Process

The manufacturing process initiates with cleaning the titanium surface, followed by chemical etching and sometimes mechanical abrasion. The MMO coating mixture is then applied and thermally decomposed at 400 to 600 °C to form a stable MMO layer. Multiple layers and heat treatments ensure the coating meets current capacity requirements for CP applications.

MMO Anode Life Calculations

In 2008, NACE standardized the test method TM 0108 for evaluating anode life based on their current discharge performance in controlled environments. This rigorous testing helps predict the operating life of an MMO coating mixture, enabling accurate lifespan estimations for different anode applications. For example, a 5 amp anode rated for 20 years will last 40 years if operated at 2.5 amps.

MMO Anode Reactions

Unlike conventional anodes consumed during current generation, MMO anodes serve as electrocatalysts, promoting reactions without being part of them. Ruthenium and iridium oxides, though subject to physical wear and passivation, offer predictable lifespans. Reactions in seawater, soil, and coke backfills predominantly generate oxygen and then chlorine, depending on factors like ionic concentration and applied current density. Using coke backfill with MMO anodes helps minimize oxygen generation in confined areas.

Summary

MMO anodes represent a significant advancement over traditional anodes. With their dimensional stability and long service life, they are replacing conventional anodes in many CP applications. The benefits of MMO technology, especially iridium oxide, are integrated into all core cathodic protection anode products by MATCOR, a leader in this technology for over 40 years. For additional details on MMO anode technology, visit our website.

Development and Advantages of Titanium Anodes

In 1965, Dr. H. Beer from the Netherlands invented the ruthenium-coated titanium electrode, marking the beginning of the titanium electrode era. Titanium electrodes have evolved significantly, offering several benefits, and overcoming limitations associated with graphite electrodes, such as deformation and dissolution.

1. Relatively stable size and consistent electrode distance during electrolysis
2. High catalytic activity with low working voltage
3. Reduced power consumption by 10-20%
4. Long working life, with resistance to chlorine and alkali corrosion in chlor-alkali production
5. Prevents contamination of electrolyte and cathode products, improving metal product purity
6. Increases current density and enhances electrolysis efficiency
7. Strong corrosion resistance due to iridium and ruthenium oxides
8. Avoids short-circuiting associated with deformed lead anodes
9. Lightweight, reducing labor intensity
10. Easy shaping and fabrication with high precision
11. Reusability of the matrix without damage

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