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51313-02107-What You Need to Know About MMO Coated Metal Anodes

Picture for What You Need to Know About MMO Coated Metal Anodes
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Product Number: 51313-02107-SG
ISBN: 02107 2013 CP
Author: Miki Funahashi
Publication Date: 2013
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$20.00
$20.00

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.
 

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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Optimization of Accelerated Life Testing of Anodes

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The performance of titanium mixed metal oxide (MMO-Ti) anodes — provided by five global vendors — targeted for coke breeze backfilled soil impressed current cathodic protection (ICCP) applications, was investigated in this study. The time to failure of the MMO anodes was measured in accordance with NACE TM0-108. Accelerated lifetime testing was performed on MMO anodes to measure sample durability and to adequately meet the current density design requirement (0.06A/cm2). The anodes were immersed in 1M sulfuric acid under varying current densities (1A/cm2, 1.4A/cm2 and 2A/cm2) under controlled temperature, until the samples lost their electro-catalytic properties. The results measured at 1A/cm2 illustrated that time to failure of the tested anodes ranged from 10 days to more than 90 days. While conducting the same test at 1.4A/cm2, time to failure of MMO anodes was reduced to a range of 13 days to a little over 30 days yielding results of anode ranking consistent with those measured at 1A/cm2. Therefore, for the sake of time, the optimum applied accelerated current density was recommended to be 1.4 A/cm2 for Ru/Ta MMO anodes, to push them to their limits at a faster rate in a shorter time.
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