Dimensionally Stable Anodes

Platinum, mixed metal oxide, and polymer anodes represent some common DSA options. Platinum-coated anodes were first introduced to the cathodic protection field in the early 1960s for offshore applications. Mixed metal oxide anodes, on the other hand, arrived on the cathodic protection scene in Europe in the early 1980s. Both of these have active anode surfaces covering an inactive substrate metal such as titanium or niobium. These substrate metals, known as “valve metals,” form thin, adherent, protective, self-healing, high-resistance oxide films when anodically polarized. These high-resistance surface films will not permit the passage of anodic current until a voltage sufficiently high to destroy the film is applied directly across the oxide interface. The magnitude of the breakdown voltage depends on the environment. In fresh waters, where chloride concentrations are low, the breakdown voltage of titanium is greater than 60 volts. However, in high-chloride environments the breakdown voltage is in the range of 8 to 10 volts. It is reduced even further in the presence of bromides or iodides, at higher temperatures, and with impurities present in the titanium, especially iron. The breakdown voltage for niobium, however, exceeds 100 volts even in the presence of high chloride concentrations. When the protective film is destroyed, the substrate metal corrodes and the anode surface is undercut.117,118,119

2.4.2.2(a) Platinum Anodes

Platinum anodes are available with a very thin layer of platinum electroplated or clad onto the substrate metal. Common platinum thicknesses range from about 1.2 to 7.5 microns (50 to 300 micro-inches). Platinum anodes are available in many different sizes and shapes, but wires, rods, and strips are the most common. The consumption rate for platinum in seawater, which is relatively constant at current densities from 540 to 5400 A/m2 (50 to 500 A/ft2), is in the range of 2.4 to 12 mg/A-y with 8 mg/A-y most often quoted. In fresh water, the consumption rate is approximately two to five times higher, and in brackish water is even higher. At chloride concentrations of approximately 25% of seawater, the consumption rate

116_{Thomas H. Lewis, Jr., “End Effect Phenomena,” CORROSION/78, paper no. 163, (Houston, TX: NACE,}

1978).

117_{NACE Publication 10A196, “Impressed Current Anodes for Underground Cathodic Protection Systems,”}

(Houston, TX: NACE International, May 1996).

118_{David H. Kroon and Charles F. Schrieber, “Performance of Impressed Current Anodes for Cathodic}

Protection Underground,” CORROSION/84, paper no. 44, (Houston, TX: NACE, 1984).

119_{M. A. Warne, “Precious Metal Anodes – The Options for Cathodic Protection,” CORROSION/78, paper}

 NACE International, 2005 CP 3–Cathodic Protection Technologist in undiluted seawater.

Some considerations and concerns when using platinum anodes are silting and deposits, abrasion damage, and attenuation problems. Platinum anodes do not perform well in environments where silting or deposits can cover the anode surface. Under these conditions, the anode life can be shortened to only 10% of the anticipated life in open seawater. The significantly increased platinum consumption rate and corrosion of the substrate occur because of the low pH environment created by the restricted mass flow. Abrasion can damage the active platinum surface; therefore, the installer must exercise care during installation and operation of these anodes. Attenuation of current along the length of the long, small diameter platinum wire anodes can prevent effective use of the entire anode surface. In these cases, copper-cored platinum anodes are often used to reduce attenuation effects.123,124,125

Use of platinum anodes in soil environments has had mixed results. Even at relatively low current densities, platinum anodes have failed prematurely in both shallow and deep groundbed applications. The failures have primarily manifested as loss of the electrical connection due to corrosion of the titanium substrate, perhaps due to low pH. However, there are examples of good long-term performance of platinum when installed in a clean, conductive carbon backfill within a uniform, homogeneous soil stratum and operated at low current densities of 55 to 75 A/m2 (5 to 7 A/ft2).120,122,126

120_{Cathodic Protection – Theory and Data Interpretation Course Manual, (Houston, TX: NACE, 1998), p.}

6:25-6:28.

121_{M. A. Warne and P. C. S. Hayfield, “Platinized Titanium Anodes for Use in Cathodic Protection,” MP,}

vol. 15, no. 3 (1976): p. 39-42.

122_{A. C. Toncre and P. C. S. Hayfield, “Consumption Rates and Operating Limits for Platinized Anodes in}

Brackish Waters,” CORROSION/83, paper no. 148, (Houston, TX: NACE, 1983).

123_{Ibid. 115}

124_{M. A. Warne and P. C. S. Hayfield, “Platinized Titanium Anodes for Use in Cathodic Protection,” MP,}

15, 3 (1976): p. 39-42.

125

P. C. S. Hayfield and M. A. Warne, “Variables Affecting Platinized Anodes in Cathodic Protection Systems,” CORROSION/82, paper no. 38, (Houston, TX: NACE, 1982).

126_{NACE Publication 10A196, “Impressed Current Anodes for Underground Cathodic Protection Systems,”}

 NACE International, 2005 CP 3–Cathodic Protection Technologist

2.4.2.2(b) Mixed Metal Oxide Anodes

The active surface of mixed metal oxide anodes consists of a solid state solution of rare metal oxides (Group IV and Group VIII metals in the periodic table) with other nonprecious metal oxides. Some of the oxides often found in this application include iridium, ruthenium, tantalum, and titanium oxides. Since the active surface is preoxidized, the consumption rate of the surface coating is very low. In fact, anode failure is not attributed to consumption of the mixed metal oxide, but rather to the formation of a high-resistance, passive oxide film between the active surface coating and the titanium substrate blocking current between them. Formation of this nonconductive oxide film and, therefore, anode life are a function of the anode current density.123,127

Because major characteristics of the oxide film, such as specific oxide formulation, application technique, and film thickness, may vary for a specific anode or manufacturer, it is important to use design information (recommended environment, current density, and anticipated life) supplied for a specific anode by the manufacturer. However, Table 2-14 provides general guidelines for current density ratings in various environments along with the specific design life based on information from several manufacturers. Mixed metal oxide anodes designed specifically for installation in carbon backfill generally have reduced film thickness to make the anode more economical. Consequently, the recommended current density for a specific design life is also reduced for this particular anode.128,129,130

Table 2-14: Mixed Metal Oxide Anodes

Carbon Backfill Fresh Water Brackish Water Seawater Mud Saline High Current Special Current Density, A/m2 (A/ft2) (7.7-13) 83-140 (3.3-3.8) 35-40 (7.7-16) 83-170 (7.7-24) 83-260 480-610 (45-57) (7.7-22) 83-240 Life, yrs 20 20 20 15 15 15

Comments: Above ratings do not apply to expanded mesh anodes.

Current densities must be dearated at temperatures below 5-10° C. Electrolyte impurities can affect ratings.

Mixed metal oxide surface is susceptible to abrasion damage. Attenuation should be considered in long, thin wires & rods.

127_{Richard A. Kus, “Advances in Coating Technology,” World Pipelines (November/December, 2002).} 128

Ibid. 123

129_{CerAnode Technologies International, Product Catalog, August, 1999.} 130_{Eltech Systems Corporation, Lida Products Catalogue,}

 NACE International, 2005 CP 3–Cathodic Protection Technologist including tubes, wires, rods, meshes, and strips. Wires and rods may also include copper cores to improve conductivity, thus reducing attenuation. However, as for any long, small diameter anode, the designer should evaluate attenuation for each specific case and carefully evaluate the effects of differing soil resistivities in soil applications. Significant variation in environmental resistivities can cause excessive discharge within the low resistivity areas resulting in premature anode failure.125

2.4.2.2(c) Polymer Anodes

Polymer anodes, introduced in the early 1980s, are manufactured by extruding a semiconductive polymer coating over a copper wire. The active anode material consists of a polymer matrix loaded with conductive carbon. This anode is designed for use in carbon backfill where the backfill surface provides the primary oxidation reaction site. The polymer anode primarily serves as an electrical contact to the carbon backfill.131

Currently only one size polymer anode is commercially available. This anode has an outside diameter of 13 mm (0.5 in.) and contains a #6 AWG stranded, copper conductor. When used with a high quality carbon backfill, the anode is rated for a 20-year life at a current output of 52 mA/m (16 mA/ft) of length.128,132

The primary design concerns with long, polymer anodes are attenuation and variations in current discharge along the anode due to soil resistivity variations. In addition, abrasion or penetration by sharp surfaces can damage the anode. Polymer anodes are available either bare or prepackaged with carbon backfill.128 When packaged in carbon backfill, the anode must be centered in the backfill to prevent premature failure.

2.4.3 Polarization Diagram

In constructing a polarization curve for an impressed current system, note that the corrosion potential (open CP circuit) of the anode is more noble than the corrosion potential for the cathode (structure). Of course, the anode potential must become even more electropositive as the anode polarizes (positive polarization slope); therefore, the entire polarization curve for the anode must be more electropositive

131_{NACE Publication 10A196, “Impressed Current Anodes for Underground Cathodic Protection Systems,”}

(Houston, TX: NACE International, May 1996).

132_{Tyco Adhesives, Reychem AnodeFlex 500, http://tycoadhesives.com/site/pdf/DCANODEFLEX_500.pdf}

 NACE International, 2005 CP 3–Cathodic Protection Technologist than the cathode curve. Ignoring the corrosion cells on the cathodic protection anode and structure, Figure 2-23 is the polarization graph for an impressed current system.

It is apparent from the polarization plot that the rectifier output voltage must be equal to the sum of the galvanic potential difference between the CP anode and the structure (Egal), the polarization of the anode (cpa), the IR drop at the anode (IRcpa), the polarization of the structure (s), and the IR drop at the structure (IRs) as indicated in Equation 2-37. The polarization curves for both the anode and cathode are straight lines on the E log I plot due the logarithmic relationship between the current and polarization potential (Tafel equation). Since the IR drop associated with each electrode has a linear relationship with the current, this plot must be curvilinear on a logarithmic scale as seen in Figure 2-23.

log I E Es,corr I_cp E_cpa,p Erect Ecpa,corr IR_cpa _cpa Ecpa _s IR_s E_gal E_s,p E_s

Figure 2-23: Polarization Diagram for Impressed Current CP System

ERECT = Egal + S + IRS + cpa + IRcpa [2-37]