Massive (Large) Anodes

The two primary types of massive (large) anodes are soluble and semisoluble. One of the first materials used as an impressed current anode, iron, is a soluble anode. This material consumes at a rate of approximately 9.1 kg/A-y (20 lb/A-y). Therefore, a relatively large quantity of material is necessary for a reasonable output capacity. Aluminum is another soluble anode material sometimes used as an impressed current anode in potable water tanks.

Although soluble type anodes are not as commonly used today as in the past due to the need for large masses of material, one significant advantage to this type of anode can make it useful. Soluble anodes produce current through metal consumption reactions, not gas evolution reactions as with semisoluble anodes. This major difference can be useful in applications involving confined or restricted spacing. If the anode is very near the cathode, products of the anodic reaction,

 NACE International, 2005 CP 3–Cathodic Protection Technologist (cathodic depolarizers). Using a soluble anode would minimize the production of these depolarizers.

Although several other massive type impressed current anodes are used, by far the two most common semi-soluble anodes in this classification are graphite and high- silicon cast iron (HISI). Both anodes have a long history of use in the cathodic protection industry with graphite anodes first used in the 1940s and HISI anodes first introduced in 1954. Both anodes are generally available in long cylindrical shapes; HISI anodes are also commonly available in hollow, tubular forms. Diameters from 5 - 10 cm (2 - 4 in.) and lengths from 152 – 213 cm (60 - 84 in.) are the most common sizes for soil applications. Other sizes are available for special applications.104,105

Graphite anodes perform well in environments where either oxygen or chlorine evolution occurs. However, the graphite consumption rate is higher when oxygen is evolved (fresh water and soil) due to the chemical reaction with oxygen producing carbon dioxide. In the case of chlorine evolution (sea or brackish water), hypochlorous acid is produced, which chemically reacts with graphite to produce carbon dioxide and hydrochloric acid. However, in this case the quantity of hypochlorous acid is generally small and easily moved away from the graphite surface without reacting.106,107

When high-silicon iron anodes are alloyed with a minimum of 14.5% silicon, the anode forms a protective oxide film on its surface when anodically polarized. This oxide film lowers the consumption rate of the anode. The protective film consists of hydrated, silicon dioxide. Although silicon dioxide is normally very high in resistivity, when anodically formed under wet conditions the film becomes conductive. However, if the film is formed with insufficient moisture present, the film becomes high in resistivity and contact resistance of the anode-to-earth is high.108,109,110

104

R. L. Bianchetti, ed., Control of Pipeline Corrosion, Second ed. (Houston, TX: NACE, 2001), p. 90, 166- 173, 308-310, and 315-317.

105_{NACE Publication 10A196, “Impressed Current Anodes for Underground Cathodic Protection Systems,”}

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

106

W. von Baechmann and W. Schwenk, Handbook of Cathodic Protection, (Surrey, England: Portcullis Press Ltd., 1975), p. 37-38, 58-59, 153-173, 185-198, and 208-217.

107_{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).

108

CP 4–Cathodic Protection Specialist Course Manual, (Houston, TX: NACE, 2002) p. 1:25-1:27, 3:4- 3:14, 3:18-3:33, and 8:34-8:35.

109_{NACE Publication 10A196, “Impressed Current Anodes for Underground Cathodic Protection Systems,”}

 NACE International, 2005 CP 3–Cathodic Protection Technologist In 1959, chromium was added to the high-silicon iron alloy. The addition of 3 to 5% chromium reduced pitting attack of high-silicon iron anodes in chloride environments, thus reducing the overall corrosion rate.106

Table 2-13 provides a summary of helpful application information for massive type impressed current anodes. Current density and consumption rate information for these anodes varies considerably from source to source, due in part to lack of consideration of variations in different environments. In the case of HISI anodes, some of the variation is due to differences in alloy composition. The information provided in Table 2-13 represents an integration of the recommendations from a number of sources with extreme values eliminated. The current densities provided represent nominal recommended values, not maximum values.105,106,107,111,112,113

110_{Ibid. 104.}

111_{W. von Baechmann and W. Schwenk, Handbook of Cathodic Protection, (Surrey, England: Portcullis}

Press Ltd., 1975), p. 37-38, 58-59, 153-173, 185-198, and 208-217.

112_{John Morgan, Cathodic Protection, 2}nd _{Ed. (Houston, TX: NACE, 1987}

), p. 37, 152-175, 205, and 254-258.

113_{R. L. Bianchetti, ed., Control of Pipeline Corrosion, Second ed. (Houston, TX: NACE, 2001), p. 90, 166-}

 NACE International, 2005 CP 3–Cathodic Protection Technologist

Graphite HISI

Nominal Current Density:

Soil/fresh water, A/m2 2 – 10 2 – 5

(A/ft2) (0.2 – 1) (0.2 – 0.5)

Carbon backfill, A/m2 5 – 10 5 – 10

(A/ft2) (0.5 – 1) (0.5 – 1)

Seawater A/m2 5 – 10 10 – 50

(A/ft2) (0.5 – 1) (1 – 5)

Consumption Rate:

Soil/fresh water, kg/A-y 0.5 – 0.9 0.1 – 0.5

(lb/A-y) (1 – 2) (0.2 – 1.2)

Carbon backfill, kg/A-y 0.1 – 0.2 0.05 – 0.3

(lb/A-y) (0.2 – 0.5) (0.1 – 0.7)

Seawater, kg/A-y 0.1 – 0.3 0.3 – 0.5

(lb/A-y) (0.2 – 0.7) (0.7 – 1)

Comments / Limitations: Avoid:

Low pH High sulfate Temp. > 50° C Consider: End effect Treatment Brittle Avoid: Dry soils High pH High sulfate Consider: End effect Brittle

Chrome alloy - halides

One major consideration in the application of impressed current anodes, especially massive type anodes, is end effect. End effect manifests itself as accelerated corrosion at the ends of long, cylindrical anodes often described as “penciling” of the anode. This accelerated corrosion is due to the increased current density discharged from the ends of long, cylindrically shaped anodes. The most significant result of end effect is the premature loss of electrical connection to the anode since the electrical connection is most often within about 15 cm (6 in.) of the end of the anode. One cost-effective solution is to place the electrical connection at the center of the anode. This step is recommended to increase the operational life of deep and horizontally buried anodes. Center connection is unnecessary if the top of the anode is located near the surface of the earth since the current density from the top of the anode is reduced due to the nonconducting plane (air) near the anode end.114,115,116

114

BP 4–Cathodic Protection Specialist Course Manual, (Houston, TX: NACE, 2002) p. 1:25-1:27, 3:4- 3:14, 3:18-3:33, and 8:34-8:35.

115_{T. H. Lewis, Jr., Deep Anode Systems: Design, Installation, and Operation, (Houston, TX: NACE}

 NACE International, 2005 CP 3–Cathodic Protection Technologist