Graphite anodes have been used for impressed current systems since the 1930’s. Although the development is not attributed to a specific application, it probably resulted from the early recognition of carbon as a possible anode material.
Graphite anodes are made from ground petroleum coke mixed with a coal tar pitch binder. The mixture is heated and extruded into cylinders. After extrusion, the cylinders are cooled in special vats, placed in an oven, packed in a mixture of sand and petroleum coke, and heated to approximately 900 degrees Celsius to fully carbonize the pitch binder. The sand-petroleum coke packing material aids heat transfer and supports the anode during its plastic stage. After cooling in a reducing atmosphere, the anodes are stacked in an Atchison or graphite furnace between two electrodes, covered with petroleum coke and an insulating sand layer, and single phase 60 Hz AC is passed through the pile. This process raises the temperature of the anodes to approximately 2600 degrees Celsius and completes the graphitization process.
The produced graphite material used for anodes typically has the following properties:
Maximum resistivity 10 micro ohm-meters
|Mechanical Strength||Compression – 3000 pounds per square inch|
Flexural – 2600 pounds per square inch.
|Thermal Conductivity||88 BUT/hr. ft. F.|
|Porosity||Less than 5%|
|Coefficient of thermal expansion||0.72 x 10-6/F|
Most anode shapes are cylindrical rods. The common sizes used are a 3″ diameter x 60″ length and a 4″ diameter x 80″ length. Square cross section graphite anodes have also been used. Extremely large shapes up to 24″ x 72″ have been used for offshore application.
The produced graphite anode has a porosity of less than 5%. The anode life is improved significantly by filling the pores with an insulating material. This impregnation reduces the tendency for electrochemical activity to occur in the pores of the anode itself. It also acts as a barrier against moisture intrusion which can cause deterioration of the anode and the anode connection. The most common materials used for graphite treatment are wax, linseed oil, or resin. Use of untreated graphite anodes for any application is not recommended.
Paraffin wax has been successfully used for graphite anode treating for many years. The wax material is in a solid form at ambient temperature. Treating is accomplished by heating the wax to over 200F and submerging anodes in the melted wax. Although treatment time can vary with temperature, moisture content, etc., complete impregnation of 4″ diameter rods can normally be accomplished in a 24 hour exposure. After cooling, the wax within the anode solidifies and remains stable under most environmental conditions. Because the wax is a solid at normal temperatures, there is no tendency for the material to leach out of the anode.
Linseed oil has also been widely used as an anode impregnant. The normal treatment procedure involves submersion of anodes in heated linseed oil in an autoclave under pressure conditions. Typically, the anodes are placed in the treatment vessel and a vacuum is drawn to remove all air from the anode pores. Preheated double boiled linseed oil is introduced into the vessel until the anodes are completely covered. The vessel is then pressurized and temperature maintained until complete impregnation is achieved. This process normally takes 2 to 4 hours. Since the oil is liquid at normal temperatures; this treatment material will have a tendency to leach or ooze out of the anode over a period of time. This effect is visible through the oil film on the surface of the treated anode.
For extremely severe service applications, graphite anodes can be treated with a phenolic resin material. Phenolic resin sets up very hard. Typical properties of the graphite anode are only slightly affected by the resin treating except for a 40% increase in flexural strength. Anodes are surfaced to remove any skin layers and placed in an autoclave. A vacuum is drawn to remove air from the pores in the graphite. While vacuum is maintained, resin is pumped into the autoclave. After all anodes are completely submerged with the liquid resin, pressure is applied to ensure filling the pores with resin. Excess resin is drained from the autoclave and anodes are heat treated to polymerize or cure the resin within the graphite pores. Finally the anode surface is again surfaced to remove surface resin that could electrically insulate the anode from its environment. Proper impregnation with resin requires specialized handling equipment. In addition, there are some toxicity problems with the resin components. As a result, resin impregnation is normally only performed by the graphite manufacturer.
Each graphite anode is normally provided with an individual cable of varying length. There have been numerous methods and procedures for connecting cable to graphite anodes. These range from a simple tamped lead connection to threaded metallic connectors. One of the methods most commonly used is a lead ferrule which is sized to the hole drilled in the anode. The ferrule is soldered to the anode cable and inserted in the hole. The ferrule is then expanded by a pneumatic or hydraulic tool which imposes a longitudinal force of up to 1800 pounds on the ferrule. This method results in connections with pull-out strengths exceeding that of the cable.
Graphite anodes can be end connected or center connected. End connections are made by drilling a 6″ to 8″ deep hole from one end. Holes can be easily drilled with hand tools. Center connections are accomplished by drilling a hole to the longitudinal center of the anode from one end. This procedure requires more sophisticated gun drill type tools to maintain the hole in the center of the anode.
Following cable connection, the annular space around the cable must be filled with a high quality electrical sealant. Common sealants are asphaltic electrical potting compounds. Care must be exercised to insure the compound is at the proper pouring temperature and that there are no voids or air pockets within the cavity. Anode caps such as epoxy or heat shrinkable caps are commonly used for additional protection.
Graphite anodes can be prepackaged in steel canisters with carbonaceous backfill. Common canister sizes are 8″ x 72″, 8″ x 84″, 8″ x 96″, 10″ x 84″, 10″ x 96″, 12″ x 84″, and 12″ x 96″.
Published values of graphite consumption range from 0.25 pounds per ampere-year to 5 pounds per ampere-year. Where oxygen evolution is the primary anode reaction, anode treatment should decrease consumption rate by at least 20%. Where chlorine evolution is the primary reaction, treatment should decrease consumption rate by at least 50%.
In free flowing seawater and in some other applications where chlorine is the primary gas evolved at the anode, the graphite consumption rate should be in the 0.5 pound per ampere year range. In neutral soil or fresh water service, consumption rates may increase to 2.0 pounds per ampere year. Consumption rates are significantly lowered by surrounding the anode with a carbonaceous backfill. The decrease in consumption can be in the order of 75%. A design consumption rate of graphite in a coke breeze backfill is 1 lb/Amp-Year. The recommended maximum current density is 0.50 amperes per square foot in a coke breeze backfill.
Graphite is one of the most commonly used impressed current anode material for underground applications. Underground applications include deep, shallow vertical, or horizontal ground beds with carbonaceous backfill.
Operation of anodes at higher than recommended outputs can cause an extremely low pH environment at the anode surface; resulting in a breakdown of the coal tar pitch binder. When this occurs, large sections of graphite can “slough” off the anode. Premature failures of untreated anodes have been reported as a result of water penetration through the body of the anode to the metallic lead wire connection. Electrolytic current flow between the connector and the anode will cause corrosion of the connector; resulting in connection failure. Some early failures of graphite anodes occurred prior to anode installation as a result of thermal expansion of the anode connector and/or the connection sealing compound. These failures occurred under conditions that resulted in temperatures in excess of 140 F. The majority of anode fabricators now use methods and materials that eliminate this problem.
The use of carbonaceous backfill materials is highly recommended with graphite anodes. Accelerated corrosion rates can occur when the oxygen evolution reaction predominates. Carbonaceous backfills can act as an extended anode; minimizing the effects of increased consumption rates.