Anode Types - PEABODY’S CONTROL OF PIPELINE CORROSION SECOND EDITION

Materials currently popular for use as anode material include graphite, high silicon cast iron, mixed metal oxide, platinum, and steel.

Graphite anodesmay be obtained in various sizes (Figure 8.7), although 3-in. diam-eters by 60-in. rods are most commonly used for pipeline ground beds. These anodes are typically supplied by the manufacturers with insulated copper leads usually high molecular weight polyethylene (HMW/PE). Standard lead length and size usually is

Figure 8.7 Graphite anodes typical sizes and chemi-cal composition. (Corrpro Companies Inc.)

20 ft of No. 8 HMW/PE insulated wire. Leads of any length may be special ordered.

Larger wire sizes are available within limits established by the manufacturer. Cata-logs for CP material give full details on available sizes and ordering information. The

“specially treated” graphite anodes (NA treated) are impregnated with linseed oil to pre-vent interparticle attack and sloughing off of active material under adverse conditions.

The additional cost for treated anodes is justified.

Graphite is brittle so anodes of this material should be handled accordingly. The con-nection between insulated anode lead and the anode proper is mechanical, and insulating material protects it from moisture penetration. Anode caps are designed to safeguard against early deterioration of the connection end. Caps may be cast in place or may be in the form of a prefabricated heat shrink cap.

When anodes are backfilled with carbonaceous backfill material, they are usually rated at a current output up to one ampere per square foot (1 A/ft²). A 3-in. by 60-in.

anode would have a maximum output of approximately four amperes. This must be taken into account when designing ground beds, as outlined in Chapter 7.

Graphite anodes are consumed at no more than two pounds per ampere per year when discharging current into an electrolyte. When used with carbonaceous backfill by direct electrical contact, most of the material consumed is backfill material rather than the anode itself.

High silicon cast iron anodesnormally contain between 14 and 15% silicon plus lesser quantities of other alloying elements. Chemical composition and other information is shown inTable 8.1.

Silicon alloy behaves differently from ordinary cast iron when discharging current.

Ordinary cast iron loses approximately 20 lbs of its iron content per ampere per year.

High silicon cast iron, on the other hand, loses material at a much lower rate. Typical reported rates are as shown inTable 8.1for ground bed applications in soil.

High silicon cast iron anodes also have insulated leads as does graphite. The material is somewhat brittle and must be handled with care. They are available in various sizes and available from suppliers of CP material. A common size used in impressed current ground beds is 2-in. diameter by 60-in. length. They are used commonly with carbona-ceous backfill, which absorbs most of the consumption resulting from current discharge.

Tables 8.2and8.3show sizes and output of rated operating current of high silicon iron anodes.

Mixed-metal oxide anodesare available in a variety of sizes and shapes. Mixed metal oxide coated titanium anodes are based on electrode technology developed in the early 1960s for production of chlorine and caustic soda. Usually the mixed metal oxide films are thermally applied to precious metal such as titanium or niobium cores. These oxide coatings have excellent conductivity, are resistant to acidic environments, are chemically stable, and have relatively low consumption rates. Table 8.4, shows sizes and rated operating current outputs of solid rod anodes in mixed metal for ground bed installations.

Table 8.5, shows sizes and rated operating current outputs of tubular mixed metal anodes for ground bed installations. Ground bed installation in soils usually specifies that the anode be prepackaged in a canister with carbonaceous backfill material. Standard lead wire is 10-ft of No. 8 HMW/PE; however, any length and type of cable including Kynar^TM,

Table 8.1 High Silicon Cast Iron Typical Types and Chemical Composition. (Corrpro Companies Inc.)

Halar^TM, and PVC can be specified. Mixed metal oxide anodes can also be configured for use in deep well anode beds.

Platinum and platinized-niobium anodes are available in a variety of sizes and shapes. Platinum anodes are available in wire and rod configurations and in different

TYPE SIZE In (mm)

AREA

ft²(m²) GENERAL APPLICATION SPECIAL FEATURES

B 1 × 60 (25 × 1524) 12 (5.4) 1.4 (.13) lengths greater than 5 feet.

Ground bed with backfill.

Ground bed with backfill permits joining in series.

Ground bed without backfill.

Mild saline or deep well without backfill.

Severe ground, deep well or sea water without backfill.

Sea water with high current discharge per anode.

Each end enlarged to 1¹/₂ in. (38 mm) dia. with cored opening for joining.

Uniform 1¹/₂ in. (38 mm) dia. with cored opening both ends for joining.

One end only enlarged to 2 in. (51 mm) dia. with cored opening for cable connection.

Each end enlarged to 2 in. (51 mm) dia. with cored opening for cable connection.

Uniform 2 in. (51 mm) dia. with cable connections on one end only.

Each end enlarged to 3 in. (76 mm) with cored opening for joining.

Same as type M but cored opening on one end only for cable connection.

One end only enlarged to 4 in. (102 mm) dia. with cored opening for cable connection.

Uniform 4¹/₂ in. (114 mm) dia. with cored opening each end. Permits two cable connections, if required.

APPROXIMATE WEIGHT EACH

lbs (kg)

170

Table 8.3 Tubular High Silicon Cast Iron Anodes. (Corrpro Companies Inc.)

Fresh water tanks, deep ground beds, or standard ground beds.

Deep ground beds or standard ground beds.

Severe ground, deep well without backfill or sea water.

Seawater with high current discharge per anode or severe ground bed.

Fresh water tanks, deep ground beds, or standard ground beds.

Seawater with high current discharge per anode or severe ground bed.

Fresh water tanks, distributed systems in ground trenches.

Fresh water tanks, deep ground beds, or standard ground beds.

Severe ground, deep well without backfill or sea water.

Fresh water tanks, deep ground beds, or standard ground beds.

Severe ground,deep well without backfill or sea water.

Deep ground beds or standard ground beds.