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MMO Coated Titanium Anodes for Cathodic Protection

David H. Kroon, P.E. Corrpro Companies Inc.

7000B Hollister Houston, Texas 77040

Lynne M. Ernes

Industrie De Nora S.p.A. Via Bistolfi, 35

Milan, Italy 20134

ABSTRACT Not all mixed metal oxide (MMO) coated titanium anodes are the same. MMO coatings have different compositions, some of which perform better for specific applications. Certain environments can cause increased wear rates or anode fouling. The performance of iridium oxide and ruthenium oxide coatings are compared. In addition to the coating, preparation of the titanium substrate, application of the coating, lead wire insulation, electrical connection and quality control procedures have substantial impact on anode performance and life. Case histories for underground conventional and concrete cathodic protection applications are included. Key Words: Cathodic protection, impressed current, anode, mixed metal oxide coated titanium, MMO, well casings, tank bottoms, AST, corrosion of reinforcing steel.

INTRODUCTION

The application of MMO titanium electrodes for both anode bed and concrete impressed current cathodic protection (CP) installations dates back to the 19�0’s. MMO anodes have been increasingly used in both applications for the last 20-25 years due to the demonstrated reliability of these anode materials. Understanding the electrochemical reactions on the surface of an anode is important for selecting the anode material, the lead wire connection and the wire insulation during design of cathodic protection for underground applications. In the case of MMO coated titanium anodes, it is also necessary to examine the composition of the coating, the method of application and the quality control program.

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ELECTROCHEMISTRY AT AN MMO ANODE

There are many oxidation reactions that can occur on the surface of an anode. These include metal oxidation, oxygen evolution and chlorine evolution. The reaction that dominates is largely determined by the anode material and the electrolyte in which it operates.1

For MMO coated titanium anode, metal oxidation is nearly insignificant, since the anode surface is fully oxidized prior to being energized. MMO anodes are a composite structure consisting of a valve metal (typically titanium) substrate covered by an electrocatalytic film of noble metal oxide. These anodes are characterized by very little dissolution of the metal oxide and uniform wear rates. The low consumption rate of the electrocatalytic layer of these MMO anodes operated at the recommended low current densities of CP applications has been demonstrated by lifetimes of over 20 years for underground, seawater and concrete installations. For soil, concrete and freshwater applications where chloride ions are either not present or are present in low concentrations, the evolution of oxygen will be the dominant reaction:

2H2O = O2 + 4H+ + 4e-

This water decomposition reaction produces an acid environment around the anode. When MMO coated titanium anodes are used for cathodic protection, they are operated at significantly higher current densities than most other impressed current anodes making their resistance to acidic conditions an extremely important characteristic. The MMO anode is designed to tolerate an acidic environment. The titanium valve metal substrate will form a protective oxide film if there is any defect in the electrocatalytic layer. Assuming the damage to the electrocatalytic coating is not significant, the remaining electrocatalytic coating will continue to support the current required for cathodic protection. The presence of chloride ions (such as in seawater applications) can lead to the evolution of chlorine. The relative amounts of chlorine and oxygen will depend on the chloride concentration and current density applied to the anode:

2Cl- = Cl2 + 2e-

The chlorine generated at the anode then reacts with water to form hypochlorous acid. Hypochlorous acid is a weak acid that does not dissociate to the extent of other acids, thereby lowering the pH at the surface of the anode to a lesser degree than when oxygen evolution is the dominant anodic reaction. Kinetics are also an important consideration in the relative amount of acid near the anode surface. In seawater, chloride ions can easily move to the surface of the anode and chlorine gas and the resultant acids can easily move away. Underground, ionic movement is much more restricted. Reactants and reaction products move less freely resulting in more highly acidic conditions on the anode surface. Where chloride ions are present in the soil, they may become depleted in the immediate vicinity of the anode, favoring oxygen evolution and a lower pH. The application of the appropriate type of MMO anode electrocatalyst is critical to meet the lifetime expectation in these environments.

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MMO COMPOSITION AND CONTROL OF MANUFACTURE

Not all mixed metal oxide (MMO) electrocatalytic coatings are the same. Figure 1 illustrates anode life as a function of current density for iridium oxide and ruthenium oxide coatings in sulfuric acid2. At a current density of 300 A/m2 (30 A/ft2), the Elgard iridium oxide coating achieved a life of approximately 3,000 days whereas the ruthenium oxide coating lifetime was only 200 days. Therefore the lifetime of this iridium oxide electrocatalytic coating is 15 times greater than the ruthenium oxide coating under the same, highly acidic operating condition. The composition of the MMO electrocatalytic coating is not the only variable to affect the performance. The application of the coating is also extremely important. The anode should start with a Grade 1 or 2 titanium substrate as per ASTM B-33� for titanium tube material. The titanium must be cleaned to remove all organic materials, such as cutting oils, which could interfere with adhesion of the electrocatalytic coating. The surface is then roughened by chemical etching, as a minimum, to further enhance coating adhesion. Abrasive blasting is sometimes used in addition to chemical etching, but should never be used as a substitute. The substrate is now ready to receive the MMO coating. The loading of the coating must be controlled to achieve optimum performance. Anode coating manufacturers with long histories of adequate anode lifetime in commercial CP applications and also relevant accelerated laboratory test data have developed correlations to ensure the optimum coating loading for CP applications. These manufacturers have also developed appropriate manufacturing and QC tools to ensure controlled application of the coatings to the substrate. Another important variable for optimum performance of an MMO anode is the adhesion of the electrocatalytic coating to the titanium substrate. Again, appropriate substrate pretreatment and application rate of the electrocatalytic coating will ensure this performance. The adhesion quality is further assured by testing the adhesion of the coating in accordance with ASTM D 3359-973. The ultimate stability of the electrocatalyst, which is a noble metal oxide produced from a thermal decomposition of noble metal salt paints, must be measured during the manufacturing process. Appropriate process controls must be applied to insure that the noble metal salts are completely converted to the stable, acid resistant, conductive noble metal oxide coating. Coating activity can be measured by a SEP (single electrode potential) test to demonstrate that the coating has been correctly applied. Measurements are made in sulfuric acid at several current densities to demonstrate an acceptable activity and complete conversion to the stable oxide form.

OPERATING CONDITION EFFECTS ON MMO ANODES When MMO coated titanium anodes are used for cathodic protection, they are operated at significantly higher current densities than most other impressed current anodes making their resistance to acidic conditions an extremely important characteristic. This is particularly true for underground and fresh water applications where oxygen evolution is the primary anodic reaction which lowers the pH at the surface of the anode to a greater degree than when chlorine evolution is the dominant reaction.. Further, when chlorine evolution occurs, the anode lead wire insulation must be resistant to attack by chlorine gas.

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The lifetime of MMO anodes can be compromised by deposits formed on the anode coating or by accelerated wear of the electrocatalytic coating when certain chemicals are present in the electrolyte at sufficient concentration. After thirty years and several millions of anodes installed in a wide spectrum of electrolyte compositions and operating conditions, the information in Table 1 is a guide for anticipated reduction in MMO coating performance when certain chemical constituents are present.

TABLE 1

Factors to Consider to Affect Anode Performance CHEMICAL CONCENTRATION POSSIBLE EFFECT

Fluoride >2 ppm free fluoride Premature loss of coating due to attack of

titanium substrate.

Bromide 50 ppm Reduced titanium breakdown potential if electrode is deactivated.

Cyanide 1 ppm Premature coating failure due to complexing

of the precious metal in the coating.

Manganese 50 ppb Higher anode potential due to deposition of MnO2 resulting in reduction of coating life.

Lead 2 ppm

(no chlorides present) Lead dioxide plating on anode surface causing localized high current densities and possible shorter coating life.

Barium 1 ppm

(sulfates present) Deposition of barium sulfate on anode surface will cause high localized current densities and reduced coating life.

Strontium 30 ppm

(sulfates present) Deposition of strontium sulfate will result in high current densities, high anode voltage, and possible shorter anode coating life.

Organics 1 ppm EDTA causes coating loss and premature

coating failure. Some organics form a deposit on the anode coating that can result in shorter anode lifetime.

These conditions are rarely a factor in underground or fresh water applications, but for wastewater and process vessels, the possible impact should be considered.

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LEAD WIRES AND CONNECTION QUALITY

The lead wire insulation and connection to the anode is critical to performance and achieving the desired service life. Where chlorine evolution is anticipated, the external jacket of the lead wires must be insulated with chlorine resistant polymers such as Halar® or Kynar®. High molecular weight polyethylene (HMWPE) insulation which is often used for conventional, surface anode beds in underground cathodic protection applications will be attacked by chlorine gas. For underground applications of MMO anodes, tubular titanium shapes are preferred which allow for anode lead wires to be attached internally at the longitudinal center of the anode. This can be accomplished with the Wedgelock® connection which uses a brass, wedge connector to which the anode lead wire is soldered or the LIDA® connection where the anode tube is compressed upon the lead wire. For the Wedgelock® connection, the wedge is secured to the sides of the tube to achieve a finished connection resistance of < 0.001 ohms. Heat shrink tubing is used to seal the end where the lead wire exits the tube and the entire tube is filled with SPL sealant. A very low resistance connection with a long-life, positive moisture seal is created. The Quality Program for the assembled anode structure should address the lead wire connection to the MMO coated titanium anode to ensure a low resistance and a complete moisture seal.

♦ Resistance: The electrical resistance of each anode should be tested and must be < 0.001 ohms.

♦ Moisture Seal: The moisture seal should be assured by testing one anode from each batch with helium at 2 atmospheres of pressure. Integrity is demonstrated using a helium detector with a sensitivity of 4 ppb.

♦ Cable Insulation: Where the evolution of chlorine gas may occur, the lead wire insulation must be immune from attack.

The Quality Program should also address documentation, shipping and handling, and field inspections.

♦ Documentation: The anodes should be traceable to the manufacturing batch. All laboratory and shop testing should also be documented.

♦ Shipping and Handling: Shipping, handling and storage procedures should ensure that the MMO anode and lead wire are not damaged.

♦ Field Inspections: Immediately prior to installation, the anode and lead wire should be physically inspected to make certain that there is no damage.

® Halar is a Registered Trademark of Allied Chemical. ® Kynar is a Registered Trademark of Elf Atochem. ® Wedgelock is a Registered Trademark of Corrpro Companies, Inc. ® LIDA is a registered Trademark of Oronzio De Nora SA licensed exclusively to the De Nora Eletroddi Network

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APPLICATION OF MMO ANODES IN CATHODIC PROTECTION

MMO coated titanium anodes exhibit excellent performance characteristics in most cathodic protection applications due to their ability to be operated at high current densities for initial structure polarization, resistance to low pH and ability to operate in fresh water, soil, sea water and concrete electrolytes with or without backfill. Three typical applications are impressed current cathodic protection using tubular MMO electrodes in deep anode configurations for pipelines and well casings; MMO coated titanium ribbon for cathodic protection of above ground storage tank bottoms; and expanded titanium ribbon mesh for protecting reinforcing steel in concrete. MMO Tubulars for Deep Anodes MMO tubulars are the cathodic protection anode of choice for many underground applications as demonstrated by the fact that the author’s company has fabricated 42,000 anode tubes in the last five years. There was a 1300 well casing cathodic protection project in the Middle East which began in 2001, when Front-End Engineering Design (FEED) work was completed. Well casings associated with oil producers, gas producers, water injection, water supply and observation wells were considered. The FEED scope included life-cycle cost sensitivity analyses of alternative designs and materials. The specific anode material that was shown to be the most economical for this application was the mixed metal oxide (MMO) coated titanium tubular with a center connection. These type electrodes were used in both surface and deep anode configurations. Since the anodic reactions are anticipated to be both oxygen evolution and chlorine evolution, it is important that the coating be a combination of iridium and tantalum oxides, and not the more common (and less expensive) ruthenium oxide that is designed to be used under conditions of chlorine evolution. The tubular shape is superior to rods or mesh since a low resistant, well sealed wedge connector can be placed at the center of the anode for connecting the anode lead wires. As demonstrated in a laboratory research project that simulated gas evolution of a deep anode, Halar® lead wire insulation is preferable to either HMWPE or Kynar® when the anodic reaction is chlorine evolution. Lead wires from individual anodes were extended, without splices, from the anode to a junction box for both surface and deep anodes.

There were four anode configurations selected for cathodic protection of the well casings. Two were deep anodes for either one or two-well clusters (see Figures 2 and 3); and two were surface anodes for either one or a two-well clusters. In the case of deep anodes, the depth to the top of the active anode column varied by location to account for variations in subsurface geology and depth of water. Surface anode configurations were used only in stable soils with low resistivities. This limited their use to sabkha areas. The anodes in both the deep and surface anode beds were backfilled with calcined petroleum coke.

Both 20-25 ampere and 40-50 ampere deep and surface anode configurations were developed. Using a very conservative anode rating of 50% of the manufacturers' published data, the capacity of each anode is 170 amp-years. For ten electrode anode beds rated at 20 amperes, and 20 anode beds rated at 40 amperes, the theoretical design life of the anode systems is �5 years. For 25 and 50 amp capacity systems with transformer-rectifiers, the design life of the anode system is 6� years.4

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The advantages of these cathodic protection groundbed designs were evident during construction and commissioning:

♦ The anodes are light weight, thereby reducing costs for packaging and shipping.

♦ The anode connections are low resistance and well sealed against moisture penetration.

♦ The anodes are easy to handle due to the size, shape and weight.

♦ The hole diameter can be minimized and still allow for sufficient calcined coke to surround the anode.

♦ The MMO coated titanium anodes can be operated at high current densities during initial polarization without compromising long term anode performance.

♦ The extended life of the anode columns was achieved at minimal incremental cost.

MMO Ribbon for Tank Bottoms A low profile, cathodic protection system for tank bottom protection above a secondary containment liner or in the space between an old bottom and a new bottom retrofit was developed. It is commonly referred to as the Grid® system. This has become a very popular design as evidenced by the author’s company providing over 3.5 million feet of anode ribbon and conductor bar in the last five years. The first system with this configuration was designed and installed in 19�7 for a 105 m (345 feet) diameter tank in a U.S. Gulf Coast Refinery5. It consisted of coated ribbon anodes placed on 1.5 m (5 feet) centers with conductor bars every 7.6 m (25 feet). At every point of intersection, the anode ribbon was resistance welded to the conductor bar (see Figure 4). The ribbon was 6 mm (0.025 inch) thick by 6.4 mm (0.25 inch) wide; and the conductor bar was 1 mm (0.04 inch) thick by 13 mm (0.50 inch) wide. Eight power feeds and six reference cells were installed as part of the system (see Figure 5). The tank is used for storage of surface water run-off from heavy rains and is therefore operated at ambient temperature. For this application, the anode grid was designed for a 50 year life at a current capacity of 1.1µA/cm2 (1 mA/ft2). The protection criterion selected was 100 mV polarization decay due to the changing conditions under an above ground tank from condensation, water accumulation, temperature variations and varying oxygen levels created as the tank is filled and emptied, causing the bottom to flex. These factors can impact the free corrosion potential. It took 60 days for polarization at a current density of 0.4µA/cm2 (0.36mA/ft2). Cathodic protection with the MMO coated titanium ribbon in the grid configuration provides many advantages:

♦ Low profile allowing for use between the tank bottom and any secondary containment liner or existing tank bottom in the case of a double bottom tank.

♦ Easy to handle and install. ♦ Even current distribution over the entire tank bottom.

® Grid is a Registered Trademark of Corrpro Companies, Inc. ® ELGARD is a Registered Trademark of ELTECH Systems Corp. U.S.A.

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♦ Redundant resistance welds for electrical continuity across the grid. ♦ No requirement for calcined petroleum coke backfill, saving time during installation and

eliminating the possibility of the coke creating a system short. ♦ No field splices. ♦ Low DC circuit resistance resulting in low power requirements. ♦ Redundant, shop assembled power feeds. ♦ No electrical isolation required. ♦ Extended anode life of over 50 years. ♦ No interference with adjacent structures. ♦ Environmentally friendly avoiding the need for drilling into subsurface strata. ♦ Lowest life-cycle costs compared to other sacrificial and impressed current systems.

MMO Ribbon Mesh for Reinforcing Steel in Concrete Reinforced concrete jetties, piers, decks and piles should be protected from corrosion (see Figure 6). Normally, the pH of Portland cement is 12.4. In this alkaline environment, steel reinforcements corrode very slowly. When exposed to a marine environment, however, chloride ions penetrate the concrete and reduce the pH at the steel surface, resulting in high rates of corrosion.6 Corrosion product occupies approximately twice the volume of the steel which places the concrete in tension and results in spalling of the concrete. The use of epoxy coated reinforcing steel or concrete sealers has not been effective. Cathodic protection provides positive corrosion control. Mixed metal oxide coated, expanded mesh and ribbon mesh, titanium anodes are often used for impressed current cathodic protection (ICCP) of steel reinforcements as evidenced by the author’s company providing 2.74 million meters (9 million feet) of mesh and ribbon mesh over the last five years. Many of coastal structures along the Gulf of Mexico and along the Florida coast suffer from premature deterioration of substructure elements. Reinforcing steel or prestressing strands corrode, sometimes at accelerated rates due to overdriving concrete piles during construction, causing hairline cracks in the concrete that serve as a direct path for moisture, oxygen, and chlorides to reach the reinforcing steel. For many years, the Florida Department of Transportation (FDOT) has combated the deterioration from corrosion on the piles with conventional patching. They have also placed fiberglass jackets around the piles and backfilled with grout in an attempt to keep moisture and oxygen out of the pile. These repair procedures were found to have a short life with failures after as little as three years. Further, the pile jackets were only hiding an increasingly dangerous condition as many prestressing strands were found to be completely severed by corrosion. Through FDOT’s Corrosion Research Lab in Gainesville, Florida, cathodic protection was introduced and developed into a standard repair procedure for existing substructures suffering corrosion damage. Cathodic protection has been used to control corrosion in piles and substructures since 1974. The FDOT standard requires an impressed cathodic protection system utilizing MMO anode mesh attached to the inside a fiberglass jacket (see Figure 7). The jacket is placed around all piles (or other members) and the annular space filled with grout. This method, using the MMO anode mesh has a life in excess of 50 years. Further, the system can be safely designed to protect

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prestressed concrete members. Since 1991, nearly 1000 piles have been protected with this system (see Figure �).

Expanded mesh and ribbon mesh MMO anodes demonstrate superior performance for cathodic protection of reinforcing steel in concrete structures:

♦ Proven performance. ♦ Easy to handle and install. ♦ Low profile for use with concrete overlays. ♦ Easily shaped for concrete embedments. ♦ Even current distribution. ♦ Low DC circuit resistance resulting in low power requirements. ♦ Long life. ♦ Lowest life-cycle cost for protection.

CONCLUSIONS

Mixed metal oxide coated titanium electrodes have become the anode of choice for many impressed current cathodic protection applications. Case histories clearly demonstrate that MMO anodes will meet all performance objectives. To ensure MMO anode performance which satisfies system design criteria, it is important that:

♦ The anode is supplied with the optimum MMO coating for the application. ♦ The MMO coating is properly applied, with the necessary process control and quality

control procedures to assure optimum performance ♦ The electrical connection to the anode is very low resistance and positively sealed from

moisture. ♦ The lead wire insulation is resistant to attack from gases that evolve as a result of the

oxidation reactions at the anode surface.

REFERENCES

1. Kroon, D.H. and Schrieber, C.F., “Performance of Impressed Current Anodes for Cathodic Protection Underground.” NACE Corrosion/�4, April 2-6, 19�4, New Orleans, Louisiana, paper 44.

2. Kus, R.A. World Pipelines, 3:22, pp.33-37, Palladian Publications, 2003. 3. ASTM International, “Standard Test Method for Measuring Adhesion by Tape Test.”

Standard D3359-97, ASTM, West Conshohocken, Pennsylvania, 1997. 4. Kroon, D.H., Williams, G., and Moosavi, A.N., “Cathodic Protection of Well Casings in

Abu Dhabi.” NACE Middle East Conference, Bahrain, March 7-10, 2004. 5. Kroon, D.H., “Tank Bottom Cathodic Protection with Secondary Containment.” NACE

Corrosion/91, March 11-15, 1991, Cincinnati, Ohio, paper 579. 6. Kroon, D.H., Williams, G., and Narayan, R., “Corrosion Protection at LNG Facilities.”

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0

500

1000

1500

2000

2500

3000

Life

(Day

s)

Ruthenium OxideCoating

Iridium OxideCoating

FIGURE I MMO Coating Life vs. Coating Composition

oOperation in Aqueous Sulfuric Acid

Figure 1: MMO Coating Performance

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(-)(+)

Solar Panels

Well Casing

Batteries & Controller

Junction Box

Individual No. 8 AWGHalar Leads

Top of Coke

MMO Tubular AnodesCentralizersCalcined Petroleum Coke

Figure 2: Deep Anode Configuration

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Figure 3: MMO Tubes with Centralizers Attached

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AnodeRibbon

ReferenceCells

ConductorBar

JunctionBox

(-)

(+)

Rectifier

TankShell

Power Feeds

Figure 4: Tank Bottom System Configuration

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Figure 5: Typical Tank Bottom System

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Figure 6: Marine Pile Corrosion

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Figure 7: MMO Anode in Pile Jacket

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Figure �: Completed Pile Protection System

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