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How Do MMO Titanium Anodes Work To Protect Cathodic

Jul. 31, 2021

Before 1893, the electrolytic industry had been using graphite anodes for more than 70 years, until 1968, when the research of titanium nail coating (RuO2-TiO2) anodes was successful, creating the era of MixedMetal Oxide coated anodes.  

MMO  Coated titanium anode in the applied current cathodic protection is a new type of auxiliary anode material, which has been used for nearly 30 years since 1980, and is developing rapidly. This paper describes the selection of coating composition, working principle and precautions for use of MMO titanium anodes.  

MMO Titanium Wire Anode

Working principle of MMO

Many reactions can occur on the anode surface during electrochemical reactions, including oxidation (dissolution) of the anode metal, precipitation of oxygen or chlorine, oxidation of electrolyte components, and so on. In the case of titanium anode, some noble metal oxides are involved in the electrochemical reaction, and oxidation of anode metal is not possible. Under cathodic protection conditions, the service life of MMO anodes can exceed 20 years in seawater, soil and concrete components due to the low anode current density.   

In soil, fresh water and concrete, only oxygen precipitation reaction occurs on the anode: reaction

The hydrolysis reaction products create an acidic environment in the anode area, and because of the high acid resistance of MMO titanium anodes, they can withstand higher current densities than other auxiliary anodes. Moreover, even if the MMO coating is partially broken, the majority of the coated area can continue to provide cathodic protection current.  

In seawater, there is a possibility of chlorine precipitation on the MMO coated titanium anode. The amount of chlorine precipitated depends on the CI concentration and the current density of the anode, and the chlorine precipitated will react with water to form hypochlorous acid. Hypochlorous acid is a weak acid and is not easily ionized, so the acidity of the anode area is relatively low.  

According to the above discussion, suitable MMO coating formulations should be used in different environmental media. This is the Mmo coating on titanium:

how MMO Coating looks like on Titanium

Matters needing attention

MMO coated titanium anodes can be used as auxiliary anodes for applied current cathodic protection in seawater, freshwater, soil and concrete environmental media. Domestic and foreign cathodic protection engineering practice shows that this new anode material has many outstanding advantages. However, in order to achieve satisfactory protection effect, the following points should be noted when using MMO coated titanium anodes: 

(1) According to the environmental medium in which the object is protected, the formulation of MMO coating should be reasonably selected. 

(2) In the process of preparing MMO coated titanium anode, strict quality control should be carried out, such as the content of precious metals in the coating, coating thickness, coating uniformity, coating and titanium substrate bonding, etc. should meet certain requirements in order to ensure the best anode performance. 

(3) MMO coated titanium anode and wire connection resistance should be low, and the watertightness should be good to prevent moisture intrusion. 

(4) The wire should have good insulation performance and be corrosion resistant. 

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