History
The origins of cathodic protection date to the days of Sir Humphry Davy with the use of sacrificial
anodes on ship hulls. Virtually all of the early efforts in cathodic protection were related to sacrificial anode systems. In one of the first documented attempts to use impressed current, Thomas Edison tried to apply current onto ship hulls in the 1890's. He had trouble with selection of suitable anode materials and power supplies. It wasn't until the 1920's that the commercial use of impressed current systems appears to have begun.
The development and use of impressed anode materials has gone through three general phases.
Prior to World War II, the principal anode materials were iron, steel, and carbon. After World War II, graphite and cast iron anodes were developed. The 1960's brought on the development of
dimensionally stable anodes. These anodes include the precious metals and ceramic anode materials. The development of new anode materials continues. Even today however; many of the early anode materials are still in widespread use.
Reactions
Anodic reactions occur at the surface of anodes in a corrosion cell. Although there are several possible reactions, gas evolution is the primary oxidation effect of impressed current systems. The two primary anodic reactions in impressed current systems are chlorine evolution and oxygen
evolution. Chlorine evolution occurs when an anode is in the presence of chloride ions. This reaction will predominate in seawater and high chloride environments.
The chlorine evolution reaction is:
2CL Cl2 + 2e
-Chlorine gas then reacts with water to form hypochlorous and hydrochloric acid.
Oxygen evolution occurs in low chloride ion concentrations or when sulfate ions are present. This occurs in underground applications where chloride ion depletion and restriction of ion migration allows the oxygen evolution reaction to dominate.
The oxygen evolution reactions are:
2H2O O2 + 4H+ + 4e
-&
2SO4 + 2H2O 2H2SO4 + O2 + 4e
-These anodic reactions decrease the pH of the solution in the vicinity of the anode. Anodic consumption of coke carbon particles also contribute to lowering pH of the anode environment.
Anodic environments with a pH as low as 1.0 have been observed. In order to be effective, anode materials must be resistant to acid attack.
Anode Material
Effective impressed current anodes should possess the following qualities:
• Good electrical properties
• Mechanically tough
• Economical
• Easily formed into useable shapes
• Low consumption rates through wide range of environments
Any material possessing these properties could be used as an impressed current anode. Materials that have been utilized in commercial applications are:
• Steel
• Graphite
• Cast Iron
• Platinum
• Mixed Metal Oxide
• Conductive Polymer
• Lead
• Magnetite
Steel
The first known appearance of iron or steel as an anode were installations of iron "wastage plates" in the early 1900's in condensers and boilers. Although unintentional, steel acted as an anode on some early DC traction systems. Many of the corrosion failures experienced on these systems were due to DC current discharge from the rails. Probably the first planned use of steel as an anode was in the 1930's. Scrap steel was commonly used, either in the form of old railroad rails or used pipe.
Steel anodes can take many forms. Scrap materials include buried structures which have been abandoned in place; such as pipelines or well casings. Scrap pipe, tubing, or railroad rails are commonly used. Any shape is capable of use; however, massive shapes are more conducive to practical use.
A major problem in the use of steel as an anode is maintaining electrical connections. Multiple connections are typically used. Methods of protecting the connections and maintaining electrical continuity includes coating the structure in the vicinity of connection and continuous coating strips
along the length of the anode. The consumption rate of steel is approximately 20 pounds per ampere year. Complete consumption of anode material is not typically achieved because of non-uniform corrosion and the difficulty of maintaining electrical connection. There is no established maximum recommended current density.
Steel can be used in horizontal, vertical, or deep groundbeds, with or without carbonaceous backfill.
With proper application, steel will perform well as an anode material. The major disadvantages of steel as an anode material are:
• Anodic corrosion product films may build up on anode surface, increasing the resistance to earth. This effect may be partially overcome by installation in carbonaceous backfills.
• Preferential corrosion may occur in the area of the connection.
• Maintaining electrical connections.
• Large mass requirements.
Although most people would consider the use of steel as an anode as outmoded; there are operators who currently use steel in groundbeds with successful results.
Graphite
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:
Electrical Resistivity
Maximum resistivity 10 micro ohm-meters Mechanical Strength Compression - 3000
pounds per square inch Flexural - 2600 pounds per square inch.
Density 99.26 pounds/ft.3 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.
Treatment
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.
Fabrication
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".
Design Parameters
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.
Applications
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.
Cast Iron
Iron containing a high silicon percentage was developed in the early 1900's. The cast material was extremely hard and brittle. It was first seriously considered for impressed current anode application in the early 1950's. It was introduced as an anode material in 1954. A subsequent modification to the alloy in 1959 produced better anode performance characteristics. This alloy consisted of the addition of 4.5% chromium. This anode material has been widely used and accepted in the industry.
High silicon chromium cast iron is a solid, non-porous material. This alloy consists of a matrix of silico-ferrite in which the majority of the carbon is in the form of graphite flakes at grain boundaries.
Adding chromium results in eliminating graphite.
The produced cast iron material used for anodes typically has the following mechanical properties:
Electrical
Resistivity: Maximum resistivity 72 micro ohm-cm
Mechanical Strength:
Compression - 100000 pounds per square inch Flexural - 15000 pounds per square inch.
Coefficient of
thermal expansion: 0.72 x 10-6/F
The standard metallurgical composition of cast iron anodes conforms to ASTM Standard A518-86 Grade 3 as follows:
Silicon:
14.20-14.75%
Chromium: 3.25-5.00%
Carbon: 0.70-1.10%
Manganese: 1.50% maximum Copper: 0.50% maximum Molybdenum: 0.20% maximum
This alloy is cast by several methods including sand mold casting, chill-casting, and centrifugal casting. A variety of anode shapes and sizes are available. The most common anode shapes are cylindrical tubes and solid bars in lengths up to 84", diameters from 1" to 6", and weights up to 280 pounds. The standard length for the solid bar anodes is 60". The standard length for tubular shapes is 84".
Fabrication
Each cast iron anode is normally provided with an individual cable of varying length. Cast iron anodes are provided in both end-connected and center-connected configurations. The solid bar anodes are cast with a hole at one end to accommodate a connecting cable. Center-connections are used for cylindrical tube shapes. There have been numerous methods and procedures for connecting cable to cast iron anodes. The most common connector for solid anodes is a poured and tamped lead
connection in the cast hole. Center-connected anodes utilize a one or two piece lead assembly attached to the interior center of the anode.
Following cable connection, the annular space around the cable is 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. Cast iron 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".
Design Parameters
The reported consumption rate is between 0.2 and 1.2 pounds per ampere-year. The controlling factor appears to be the environment. Manufacturer recommendations for anodes surrounded by carbonaceous backfill is 0.7 pounds per amp-year. Current densities should be limited to
approximately 1 ampere per square foot.
Applications
High silicon cast iron anodes are widely used in underground applications in both shallow and deep groundbeds. Although the performance is improved with coke breeze; its use is not critical. This material is also widely used in freshwater and saltwater environments.
The performance of cast iron as an anode is dependent upon the formation of a thin layer of silicon dioxide on the surface of the anode. Oxidation of the alloy is necessary to form this protective film.
Silicon-chromium cast iron is highly resistant to acid solutions. It does not perform particularly well in alkaline environments or in the presence of sulfate ions.
There have been some reports of early failure when silicon iron anodes are exposed to environments in which both sulfate and chloride ions are present. Other cases are reported where these anodes increase significantly in resistance when exposed to drying conditions. It is thought that this condition interferes with the formation of the conductive silicon dioxide film.
Platinized Titanium / Niobium
The first published results on the use of platinized titanium as anode were in 1958. Further
development of the anode material has resulted in the use of superior substrates other than titanium.
Its use has gone through several phases; however, it is recognized for its superior anodic properties.
Platinum is an excellent anode material due to its high conductivity and low consumption rate.
However, because of its high cost, it is not economical to use platinum by itself. Platinum is made practical for use by cladding or electroplating a thin layer of platinum over a lower cost substrate. This also extends the effective anode surface area. The substrate must also have the ability to form an insulating oxide film under anodic conditions. The two substrate materials most commonly used are titanium and niobium.
Titanium and niobium both form insulating oxide films when exposed to anodic conditions. Titanium is less expensive; however, it has a much lower breakdown potential than niobium. The titanium oxide is reported to break down at anodic potentials in the 10V range. The niobium film is resistant to breakdown up to 80V. Niobium is also a much better electrical conductor than titanium. Niobium is normally used with a copper core. This reduces the cost and also provides a much better electrical conductivity.
Platinum coated anodes are available as rod, wire, sheet, tube, strip, and mesh. Rod and wire sizes normally range from 0.031 inches to 1". Platinum thicknesses range from 25 micro-inches to 1000 micro-inches.
Connection to platinum coated anodes depends upon the anode shape. Wire type anodes normally use a soldered connection. Rod anodes generally have a drilled, threaded connection to the substrate material.
The mechanism of deterioration of a platinum based anode is consumption of the platinum coating.
Rate of consumption is controlled by many factors, primarily environment and current density. The consumption rate of platinum in seawater is approximately 8 mg/A-yr. In fresh and brackish waters,
consumption is 2 to 3 times greater at low current densities (10 A/sq. ft). At high current densities,
consumption is 2 to 3 times greater at low current densities (10 A/sq. ft). At high current densities,