1
TECHNICAL DATA GENERAL Corrosion Definitions:
Anode - An electrode at which oxidation of the surface or some component of the solution is occurring. Practically, this is the electrode at which corrosion occurs. Ampere - The practical unit of electrical current equal to that produced by one volt applied across a resistance of one ohm. Ampere-hour - The unit quantity of electricity equal to that produced by one ampere of current in a one hour period. Cathode - An electrode at which reduction is occuring. Practically, this is the electrode at which protection occurs in a cathodic protection system. Corrosion - The deterioration of material, usually metal, from a reaction with its environment. Current Density - The applied electrical current per unit area. Electrolyte - The common environment with which both a cathode and anode are in contact. Practically, this is the soil or water to which a metal structure is exposed. Galvanic Anode - A metal that provides cathodic protection when connected to other metals, as a result of its relative position in the Galvanic Series. Galvanic Series - A listing of metals in order of reactivity when exposed to an electrolyte. Impressed Current Cathodic Protection - A cathodic protection system derived from the application of external electrical energy from sources such as common electrical power, thermoelectric generators, and solar panels. Ohm - The practical unit of electrical resistance equal to the resistance of a circuit in which a potential difference of one volt produces a current of one ampere. Sacrificial Cathodic Protection - A cathodic protection system derived from the internal electrical energy developed by coupling to more reactive metals such as aluminum, magnesium, and zinc. Volt - The practical unit of electrical potential difference required to produce a current flow of one ampere across a resistance of one ohm.
2
Introduction to Corrosion
Introduction Corrosion is usually defined as the deterioration of a metal or its properties caused by a reaction with its environment. Most metals occur naturally in the form of oxides and are usually chemically stable. When exposed to oxygen and other oxidizing agents, the refined metal will try to revert to its natural oxide state. In the case of iron, the oxides will be in the form of ferrous or ferric oxide, commonly known as rust. Metallic corrosion generally involves the loss of metal at a particular location on an exposed surface. Corrosion occurs in various forms ranging from a generalized attack over the entire surface to a severe concentrated attack. In most cases, it is impossible or economically impractical to completely arrest the corrosion process; however, it is usually possible to control the process to acceptable levels. The U.S. government funded a detailed study of the annual cost of corrosion in 1975. The total cost of metallic corrosion to the U.S. economy was estimated at 4% of the GNP ($70 billion dollars). Of that, approximately 30% was defined as avoidable. Adjustment based on today's current economy puts total current costs in the $300 billion range with over $100 billion of that avoidable.
Corrosion Process 3
Metallic corrosion is caused by the flow of direct current from one part of the metal surface to another. This flow of direct current causes the loss of metal at the point where current discharges into the environment (oxidation or anodic reaction). Protection occurs at the point where current returns to the metal surface (reduction or cathodic reaction. The rate of corrosion is proportional to the magnitude of the corrosion current. One ampere of direct current removes approximately twenty pounds of steel in one year. Where corrosion occurs and to what extent depends upon the environment to which the metal is exposed.
Four conditions must be met for corrosion to occur. Elimination of any of the four conditions will halt the corrosion reaction. • Anode - the oxidation reaction occurs here. Current discharge into the environment and metal loss are associated with this reaction. • Cathode - the reduction reaction occurs here. Current acceptance and metal protection are associated with this reaction. • Electrolyte - the environment to which both the cathode and the anode are exposed. The electrolyte must have the capacity to conduct electrical current through the flow of ions. • Metallic path - the anode and the cathode must be connected via a metallic connection that conducts electrical current flow through the flow of electrons.
Causes of Corrosion Corrosion is a natural process. The primary driving force of corrosion is based upon the transformation of iron from its natural state to steel. The refining of iron ore into steel requires the 4
addition of energy. Steel is essentially an unstable state of iron and corrosion is the process of iron returning to its natural state. The energy used in the refining process is the driving force of corrosion. Corrosion cells are established on underground pipelines for a variety of causes. A primary cause of corrosion is due to an effect known as galvanic corrosion. All metals have different natural electrical potentials. Where two metals with different potentials are connected to each other in a common environment, current will flow causing corrosion to occur. The coupling of steel to a different metal, such as copper, will cause a corrosion circuit to be established. Direct coupling of copper to steel will cause the steel to corrode much faster than normal. Another form of this is the coupling of rusty pipe to new, clean steel. The natural difference in potential causes the new steel to corrode more rapidly than the old steel. Other causes of pipeline corrosion cells include the effect of different soil types, oxygen availability, stray current interference and microbiological growth.
Two other unique causes (and sometimes related) are stress and hydrogen. • Stress Corrosion occurs when even a very small pit forms in a metal under stress. The concentrated stress either deepens and extends the pit, or cracks the protective film which tends to form. Under continued exposure to the corrosive medium and stress, the crack extends by alternate corrosion and stress failure. • Hydrogen Embrittlement and hydrogen attack results when atomic hydrogen penetrates into the grain boundaries of steel producing microcracks, blistering and loss of ductility. The atomic hydrogen combines into molecules and results in blistering and laminations.
Forms of Corrosion Corrosion exhibits itself in a number of ways. A brief description of some of these is provided below. 5
•
• •
• •
General Corrosion is the most common form of corrosion. It exhibits itself in an overall attack of the metal surface with no apparent concentrations. An example is the effect of atmospheric corrosion. Pitting Corrosion results in a localized, concentrated attack and has the appearance of holes or craters. Crevice Corrosion occurs in shielded areas where stagnant corrosive electrolyte accumulates. This type of corrosion occurs under bolt heads, gasket surfaces, and overlapping metal connections. Erosion-Corrosion is a combination of electrochemical and mechanical damage that occurs in environments of high fluid velocities or mechanical movement between two metals. Selective Leaching results in one constituent of an alloy being selectively removed, leaving a porous replica of the original alloy. An example is the dezincification of brass or bronze and the graphitization of cast iron where iron is removed selectively, leaving a replica composed of carbon or graphite.
Control of Corrosion The five general methods used in the control of corrosion are coating, cathodic protection, material selection, environmental modification, and design practices. Control of underground corrosion is 6
primarily achieved by two methods: coating and cathodic protection. An effective external coating can provide corrosion protection to over 99% of the exposed pipe surface. The protective coating is usually applied to the pipe or tank before burial. The coating serves to electrically insulate the metal from the soil. If the metal could be completely isolated, then the establishment of corrosion cells would be prevented and no corrosion current would flow. However, no coating can be considered a perfect coating. Damage to the coating as a result of handling, transportation, installation, thermal stresses, and soil stresses will eventually create defects or "holidays" that expose the underlying steel to the environment. Cathodic protection is an electrochemical technique for preventing corrosion of a metal exposed to an electrolyte. The process involves application of DC electrical current to the metal surface from an external source. By forcing the metal surface to accept current from the environment, the underground metal becomes a cathode and protection occurs. The external source can use outside AC power through a rectifier and groundbed or by attaching sacrificial metals such as magnesium or aluminum to the structure to be protected. It is used extensively in preventing corrosion to underground and submerged steel structures; such as pipelines, production well casings, and tanks. Effective application of cathodic protection can provide complete protection to any exposed areas for the life of the structure. The combination of an external coating and cathodic protection provides the most economical and effective choice for protection of underground and submerged pipelines. For bare or ineffectively coated existing pipeline systems, cathodic protection often becomes the only practical alternative for corrosion protection. Cathodic protection is a mandated requirement of federal and state regulations governing underground transmission pipelines, gas distribution systems, and underground fuel tanks. These requirements include installation, monitoring, and maintenance of cathodic protection systems.
Introduction to Cathodic Protection
Introduction 7
Cathodic protection is an electrochemical technique for preventing corrosion of a metal exposed to an electrolyte. The process involves application of DC electrical current to the metal surface from an external source. The external source can be either a commercial power source or through connection to sacrificial metals such as magnesium or aluminum. It is used extensively in preventing corrosion to underground and submerged steel structures; such as pipelines, production well casings, and tanks. Effective application of cathodic protection can provide complete protection to any exposed areas for the life of the structure. The combination of an external coating and cathodic protection provides the most economical and effective choice for protection of underground and submerged pipelines. For bare or ineffectively coated existing pipeline systems, cathodic protection often becomes the only practical alternative for corrosion protection. Cathodic protection is a mandated requirement of federal and state regulations governing underground transmission pipeline, gas distribution systems, and underground petroleum tanks. These requirements include installation, monitoring, and maintenance of cathodic protection systems.
Process of Cathodic Protection Cathodic protection essentially means the reduction or elimination of corrosion on a metal surface by forcing the metal to become a cathode. The two general types of cathodic protection systems are impressed current and sacrificial. Both types of systems can effectively transfer the corrosion reaction (oxidation) from the metal surface to an external anode. If all exposed parts of a structure become cathodic with respect to the electrolyte, corrosion of the structure is eliminated.
Sacrificial Anode Cathodic Protection Sacrificial cathodic protection occurs when a metal is coupled to a more reactive (anodic) metal. This connection is referred to as a galvanic couple. In order to effectively transfer corrosion from the metal structure, the anode material must have a large enough natural voltage difference to produce an electrical current flow. 8
Effective application of cathodic protection can provide complete protection to any exposed areas for the life of the structure. The combination of an external coating and cathodic protection provides the most economical and effective choice for protection of underground and submerged pipelines. For bare or ineffectively coated existing pipeline systems, cathodic protection often becomes the only practical alternative for corrosion protection.
Three metals are commonly utilized for cathodic protection of steel. The selection of the anodic metal is dependent upon resistivity and electrolyte. A general application guide for these metals are: • Magnesium - soil and freshwater applications • Zinc - low resistivity soils and saltwater • Aluminum - saltwater and limited freshwater applications An advantage of sacrificial anode systems is the flexibility in application. Anodes can be installed in a variety of applications and configurations. No outside power is required for cathodic protection to be effective. Another advantage is the minimal maintenance required for these systems to function. Disadvantages of sacrificial anode systems include the limited protection current available and limited life. Sacrificial anodes are subject to rapid corrosion (consumption) and require replacement on a regular basis. Typical design life of a pipeline system anode is five to ten years. Impressed Current Cathodic Protection Impressed current cathodic protection involves the application of an external DC current through longlasting anodes. A typical source of power for an impressed current system is AC power converted to DC by a rectifier. In order to be effective, impressed current anodes must be designed for long life at high current output. This requires selection of materials with very low corrosion (consumption) rates. The typical expectation of impressed current anode life is over twenty years. Anode materials that have proven to 9
be suitable for impressed current systems include treated graphite, high silicon cast iron, mixed metal oxide, and to a lesser extent platinum and magnetite. Anodes are normally installed in grouped configurations in the electrolyte. These groupings (both horizontal and vertical) in an underground application are called groundbeds. The groundbeds are connected to the power by a positive cable to the power source. A negative cable connects the power source to the structure.
Advantages of impressed current systems include the unlimited current opportunities and longer life. Impressed current systems are typically installed where the structure to be protected is large, requiring higher levels of current. Disadvantages include the requirement for an outside power source and higher maintenance requirements. Outside power might come from sources such as commercial AC converted to DC through a rectifier, thermoelectric generator, or solar panels. A significantly higher monitoring and maintenance effort is required by comparison to sacrificial anode systems.
Cathodic Protection Applications Cathodic protection can be effectively applied to most steel structures in consistent contact with a corrosive electrolyte. Commonly protected structures include buried pipelines, underground tanks (UST's), aboveground tank bottoms (AST's), well casings, internal surfaces of tanks and treating vessels, and off-shore structures.
10
•
•
•
•
•
•
Underground Pipelines are the primary market for cathodic protection. Both sacrificial and impressed current systems are used. Federal and state regulations require cathodic protection for most petroleum or gas pipeline systems. Underground Storage Tanks (UST's) used for fuel are now required by EPA to either have functional cathodic protection systems or to be of a non-corrosive material. Both types of systems are widely used. Aboveground Storage Tank (AST's) bottoms can be protected from soil-side corrosion with cathodic protection. Most major tank operators include cathodic protection in their corrosion control program. Unique problems involved with tank applications include the difficulty of distributing current uniformly over the tank bottom and monitoring the effectiveness of the systems. Production Well Casings usually require impressed current systems due to higher current requirements. The economics of cathodic protection are excellent until production volumes decline and fields near the end of their effective life. This application of cathodic protection is common; but tends to be concentrated in established fields with known corrosion history. Internal surfaces of tanks and vessels are commonly protected by cathodic protection systems. With some exceptions, most of these utilize sacrificial anodes. Possible applications range from heater-treaters, heat exchangers, water storage tanks, and hot water heaters. Offshore structures such as production platforms, docks, and pipelines are almost always protected with cathodic protection systems. Sacrificial anode systems with aluminum anodes are the most common application. Cathodic Protection Criteria
Cathodic protection is considered to be effective when active corrosion is transferred from the metal surface to the installed anode. The effectiveness of the transference can be determined by electrical measurements. Industry accepted criteria for effective protection using these measurements are fully described in various NACE International publications including the Standard Recommended Practice "RP0169-96 Control of External Corrosion on Underground or Submerged Metallic Piping Systems". Of the available techniques, the primary measurement used by industry to determine effectiveness of protection is known as the -0.850 volt, pipe-to-soil potential criteria. This technique measures the voltage difference between the protected structure and a copper-copper sulfate electrode placed in or on the electrolyte near the structure. If the voltage difference is more negative than -0.850 volts, then the structure is considered protected. If the value is more positive than -0.850 volts, the structure is either unprotected or only partially protected. Normal values of unprotected steel in soil typically range from -0.500 to -0.700 volts with respect to copper sulfate. The interpretation of pipe-to-soil potentials requires consideration of the effects of measurement errors such as IR drop in the soil between the pipe and the electrode. The most common consideration of this effect is through an "instant off" measurement obtained by interrupting current sources. Other techniques include potential measurement at permanent "coupon type test stations". Design Considerations There are four primary questions to be answered when designing a cathodic protection system. • • •
How much current is necessary? What source of direct current should be used? How should installation be designed? 11
•
How can effectiveness be measured?
Limitations of cathodic protection must be recognized during the design process. Cathodic protection will be effective only on metal surfaces in continual contact to the electrolyte. Above-ground structures will not be protected. The distribution of current to desired areas becomes difficult in congested or remote areas. Examples include multiple pipeline right-of-ways and storage tank bottoms. Multiple variables should be considered during the design process. Two primary factors include the effectiveness of an external coating and resistivity of the electrolyte. The existence of a high quality coating minimizes current requirements of a structure and greatly influences the design. Electrolyte resistivity affects the choice between sacrificial and impressed current as well as the type of groundbed. Other design factors include foreign system interference, power availability, maintenance requirements, and economics.
Practical Galvanic Series Metal
Volts
Commercially pure magnesium
-1.75
Magnesium alloy (6% AL, 3% An, 0.15% Mn) -1.6 Zinc
-1.1
Aluminum alloy (5% zinc)
-1.05
Commercially pure aluminum
-0.8
Mild steel (clean & shiny)
-0.5 to -0.8
Mild steel (rusted)
-0.2 to -0.5
Cast Iron (not graphitized)
-0.5
Lead
-0.5
Mild steel in concrete
-0.2
Copper, brass, bronze
-0.2
High silcon cast iron
-0.2
Mill scale on steel
-0.2
Carbon, graphite, coke
+0.3
Typical potential normally observed in neutral soils and water, measured with respect to copper/copper sulfate reference electrode. Copyright © 1991 by NACE International. Reprinted from Corrosion Engineers Reference Book with permission by NACE. All rights reserved.
Galvanic Anode Technical Data mA/ft2
mA/m2
Sulfuric acid pickle (hot)
35,000
380,000
Sea water - Cook Inlet
35 - 40
380 - 430
Environment
12
mmm North Sea
8 - 15
90 - 160
mmm Persian Gulf
7 - 10
80 - 110
mmm U.S. - West Coast
7-8
80 - 90
mmm Gulf of Mexico
5-6
50 - 60
mmm Indonesia
5-6
50 - 60
Soil
1-3
10 - 30
0.1
1
0.003
0.03
Poorly coated steel in soil or water Well coated steel in soil or water
Very well coated steel in soil or water 0.0003 or less 0.003 or less Copyright © 1991 by NACE International. Reprinted from Corrosion Engineers Reference Book with permission by NACE. All rights reserved.
Comparison of Reference Electrodes
Reference Electrode Copper/Copper Sulfate
To convert reference potential Structure/electrolyte potential Voltage measurements to the equivalent equivalent to –0.850 volts with Difference potential with respect to respect to copper/copper sulfate copper/copper sulfate 0
-0.850 volt
Silver/Silver Chloride
-50 mv
-0.800 volt
Subtract 0.050 volts
Standard Calomel
-70 mv
-0.780 volt
Subtract 0.070 volts
-1100 mv
+0.250 volts
Subtract 1.10 volts
Zinc
Comparison of different electrodes @ 25° C
Resistance and Resistivity Approximate Resistivity of Water
13
Where: T is ppm dissolved solids R is resistance in ohm-cm. Resistance of a Single Rod Anode to Earth(1)
Resistance of Multiple Vertical Anodes to Earth (1)
(1)
Derived from H.B. Dwight Equations Rough Indications of Electrolyte Corrosivity vs. Resistivity Ohm - Cm
Corrosivity
Below 100
Extremely corrosive
100 - 1,000
Very corrosive
1,000 - 2,500
Corrosive
2,500 - 7,500
Moderately corrosive
7,500 - 15,000
Mildly corrosive
Over 15,000
Progressively less corrosive
SACRIFICIAL CATHODIC PROTECTION
Galvanic Anode Properties Anode
Efficiency Energy Capability Consumption Rate Potential vs CuCuSO4 (%) (Ah/lb) (lb/Ay) (volts)
14
Zinc
95
335
23.5
-1.10
Aluminum (Al, Zn, In)
85
1150
6.5
-1.10
Magnesium (H-1 Alloy)
50
500
8.7
-1.45
Magnesium (High Potential Alloy)
50
500
8.7
-1.70
Magnesium Anode Review Introduction Magnesium anodes are the primary sacrificial anode used for underground cathodic protection. There is a great deal of technical information and many publications detailing the application of magnesium anodes. This review is intended to provide summary information and brief guidelines in the selection of anodes. Magnesium Alloy Two alloys are commonly used for cathodic protection applications. These alloys are generically known as High-Potential Magnesium and H-1 alloy. High potential alloys are produced from primary magnesium refined from saltwater. This alloy provides the maximum output with open circuit voltage exceeding -1.70 volts relative to copper sulfate. H-1 alloy also known as AZ-63 is produced from secondary magnesium sources through recycling facilities. This alloy provides a much lower driving voltage in the range of -1.40 volts relative to copper sulfate. The selection of alloy requires consideration of current requirements, soil resistivity, and costs. In most cases where current demand requires the use of multiple anodes, high potential alloy is the most economical choice. Standard chemistry requirements for the two alloys are: High Potential Anode Chemistry Per ASTM B843 Industry Standard for high potential magnesium anodes Aluminum
0.01% max
Manganese
0.50 - 1.3%
Copper
0.02% max
Silicon
0.05% max
Iron
0.03% max
Nickel
0.001% max
Others, each
0.05% max
Magnesium
Remainder
Element
H-1 Magnesium Alloy Chemistry Content % Grade A Grade B* Grade C 15
Aluminum
5.3-6.7
5.3-6.7
5.3-6.7
Zinc
2.5-3.5
2.5-3.5
2.5-3.5
Manganese
.15 Min.
.15 Min.
.15 Min.
Silicon
.10 Max.
.20 Max.
.30 Max.
Copper
.02 Max.
.05 Max.
.10 Max
Nickel
.002 Max
.003 Max
.003 Max
Iron
.003 Max.
.003 Max.
.003 Max.
Other Impurities
.20 Max.
.20 Max.
.30 Max.
Magnesium Remainder Remainder Remainder * The H-1 alloy supplied is Grade B.
Fabrication Magnesium anodes are generally provided with a low resistivity backfill surrounding the anode. The backfill material provides a uniform environment for the anode and enhances performance by extending life and lowering the local resistivity. The prepared backfill consists of a mixture of 75% gypsum, 20% bentonite, and 5% sodium sulfate. Gypsum is primarily a filler material, bentonite absorbs and retains moisture, and sodium sulfate lowers backfill resistivity. The backfill is contained around the anode using cloth bags. The packaged anode is installed directly into the soil and then connected to the structure by a wire. Anodes are normally provided with ten (10) foot of #12 AWG wire.
Current Output Current output is controlled by three factors: • Soil resistivity - Current output increases as soil resistivity decreases. Generally magnesium anodes are installed in relatively low resistivity soils. Economic application decreases significantly in soil resistivities exceeding 5,000 ohm-cm. Practically, magnesium anodes are not effective above 10,000 ohm-cm. 16
•
•
Anode surface area - Current output is proportional to surface area. As the surface area increases, current output increases. Increased surface area is usually achieved by increasing the length of the anode. Alloy potential - High potential anodes have open circuit potentials approximately 20-25% higher than H-1 alloy. (-1.75 volts versus -1.40 volts). This creates a higher anode current and results in a lower total anode requirement.
Anode current output is calculated using Ohm's Law: I = current in amperes V = potential differential in volts R = anode resistance in ohms The voltage difference is the protected potential of -0.85 volts subtracted from the open circuit potential of the anode. For high potential anodes, the difference is -0.9 volts. For H-1 alloy, the difference is -0.55 volts. Resistance to earth can be calculated by Dwight's Equation:
R = resistance in ohms = soil resistivity in ohm-cm L = anode length in feet d = anode diameter in feet Tefankjian, in his article "Application and Maintenance of Control Facilities" offers a simple process for determining anode current output with consideration of coating, potential, and multiple anode installation. The process involves calculation of anode current for a single high-potential anode installed 10' away from a bare pipeline using the formula:
im = current output in mA P = soil resistivity in ohn-cm f = size correction factor - Table 1 Y = potential correction factor - Table 2 Step 1: For a well-coated pipeline, the constant of 150,000 should be reduced 20% to 120,000. Step 2: Select anode size correction factor from Table 1.
Anode Weight (pounds)
Standard Anodes
Factor (f)
3D3
(packaged)
0.53
5D3
(packaged)
0.60
9D3
(packaged)
0.71
17D3
(packaged)
1.00
20D2
(packaged)
1.60
17
32D3
(packaged)
1.06
48D5
(packaged)
1.09
Step 3: Select potential correction factor from Table 2. P/S
Magnesium
-0.70
1.14
-0.80
1.07
-0.85
1.00
-0.90
0.93
-1.00
0.79
-1.10
0.64
-1.20
0.50
Step 4: Calculate anode current output with formula. Step 5: Adjust for multiple anode installation by application of adjustment factors in Table 3. No. of Anodes in Parallel
ADJUSTING FACTORS Anode Spacing in Feet 5'
10'
15'
20'
2
1.839
1.920
1.946
1.964
3
2.455
2.705
2.795
2.848
4
3.036
3.455
3.625
3.714
5
3.589
4.188
4.429
4.563
6
4.125
4.902
5.223
5.411
7
4.652
5.598
6.000
6.232
8
5.152
6.277
6.768
7.036
9
5.670
6.964
7.536
7.875
10
6.161
7.643
8.304
8.679
As Tefankjian states in his article, the above process should be used only as a guide to estimate current output. Anode Life and Efficiency Life of a magnesium anode is directly proportional to its current efficiency. Magnesium anode alloys have a nominal efficiency of 50%. However, significant variation from this efficiency will directly impact anode life. It is not uncommon for anodes meeting all chemistry requirements to still exhibit efficiencies much lower than the nominal 50%. Anode efficiency should be a critical part of the anode specification. Minimum efficiency should not fall below 45%. In his book "Control of Pipeline Corrosion", Peabody provides a formula for calculation of anode life. 18
L = life in years Wt = weight in pounds I = current in amperes D. A. Tefankjian in his article, "Application and Maintenance of Control Facilities" offers the following formula for calculating anode life.
L = life in years Wt = weight in pounds I = current in milliamperes Table of Current Output and Life Estimates The following table provides estimates of current output and life for a single high potential anode, assuming a well-coated pipeline with an 85% effective use of the metal.
9D3 17D3 20D2 32D5 48D5 Soil Resistivity Output Life Output Life Output Life Output Life Output Life ohm-cm
mA
years
mA
years
mA
years
mA
years
mA
years
1000
122
3.6
163
5
260
4
166
9
208
11
2000
61
7
81
10
130
8
83
19
104
22
3000
41
11
54
15
87
11
55
28
69
34
4000
31
14
40
20
65
15
41
38
52
45
5000
24
18
33
25
52
19
33
47
41
56
6000
20
22
27
31
43
22
28
55
35
67
7000
17
25
23
36
37
26
24
65
30
78
8000
15
29
20
41
33
30
21
75
26
90
9000
14
32
18
46
29
33
18
86
23
101
10000
12
36
16
51
26
37
16
94
21
112
15000
8
54
11
77
17
56
11
141
14
168
20000
6
72
8
102
13
74
8
188
10
224
Design Process Design of magnesium anode systems requires a thorough evaluation of the application with consideration of the important variables. Once these variables are measured or assumed, the process typically is divided into the following steps.
19
•
•
•
•
•
•
Calculate exposed surface area - This requires an assumption of coating quality. A high quality coating can provide protection to over 99% of the total surface area. A conservative assumption of coating efficiency is 95%. Calculate total current requirements - This requires an assumption of current density. The typical range of current density for steel in common soils is 1 to 3 milliamperes per square foot of exposed surface. A conservative assumption is 2 milliamperes per square foot. Calculate anode current output - Using previous discussed formulas, calculate anode output considering the effect of coating, soil resistivity, alloy selection, and anode spacing. Several anode sizes should be evaluated. Select anode size based on life - Once anode current outputs have been calculated, the appropriate anode size can be selected by evaluating the projected life. Typical design lives range from 10 to 20 years. Calculate total anode quantity - Divide the total current requirement in amperes by the calculated anode output in amperes. The result is the estimated total number of anodes required. Determine anode location and configuration - This is primarily based on local site conditions and an even distribution of current from the anodes.
Design Process Design of magnesium anode systems requires a thorough evaluation of the application with consideration of the important variables. Once these variables are measured or assumed, the process typically is divided into the following steps. •
•
•
•
•
•
Calculate exposed surface area - This requires an assumption of coating quality. A high quality coating can provide protection to over 99% of the total surface area. A conservative assumption of coating efficiency is 95%. Calculate total current requirements - This requires an assumption of current density. The typical range of current density for steel in common soils is 1 to 3 milliamperes per square foot of exposed surface. A conservative assumption is 2 milliamperes per square foot. Calculate anode current output - Using previous discussed formulas, calculate anode output considering the effect of coating, soil resistivity, alloy selection, and anode spacing. Several anode sizes should be evaluated. Select anode size based on life - Once anode current outputs have been calculated, the appropriate anode size can be selected by evaluating the projected life. Typical design lives range from 10 to 20 years. Calculate total anode quantity - Divide the total current requirement in amperes by the calculated anode output in amperes. The result is the estimated total number of anodes required. Determine anode location and configuration - This is primarily based on local site conditions and an even distribution of current from the anodes Bare Pipe Chart Soil Resistivity in ohm-cm
1K 2K 3K 4K 5K 6K 7K 8K 9K 10K
Anode Recommendation
48# 48# 32# 17# 17# 9# 9# 9# 9# 9# 20
Nominal Output in milliamperes
208 104 55 40 33 20 17 15 14 12
Nominal Life in years
11 22 28 20 25 22 25 29 32 36
Pipe inch
O.D. inch
Surface Area Ft2/L.F.
Current Reqmt. mA/L.F.
Nominal Anode Spacing in feet
2
2.275
0.60
1.19
175 87 46 34 28 17 14 13 12 10
4
4.50
1.18
2.36
88 44 23 17 14
8
7
6
6
5
6
6.625
1.73
3.47
60 30 16 12 10
6
5
4
4
3
8
8.625
2.26
4.51
46 23 12
9
7
4
4
3
3
3
10
10.75
2.81
5.63
37 18 10
7
6
4
3
3
2
2
12
12.75
3.34
6.67
31 16
8
6
5
3
3
2
2
2
14
14.00
3.66
7.33
28 14
8
5
5
3
2
2
2
2
16
16.00
4.19
8.37
25 12
7
5
4
2
2
2
2
1
18
18.00
4.71
9.42
22 11
6
4
4
2
2
2
1
1
20
20.00
5.23
10.47
20 10
5
4
3
2
2
1
1
1
22
22.00
5.76
11.51
18
9
5
3
3
2
1
1
1
1
24
24.00
6.28
12.56
17
8
4
3
3
2
1
1
1
1
26
26.00
6.80
13.61
15
8
4
3
2
1
1
1
1
1
28
28.00
7.33
14.65
14
7
4
3
2
1
1
1
1
1
30
30.00
7.85
15.70
13
7
4
3
2
1
1
1
1
1
32
32.00
8.37
16.75
12
6
3
2
2
1
1
1
1
1
34
34.00
8.90
17.79
12
6
3
2
2
1
1
1
1
1
36
36.00
9.42
18.84
11
6
3
2
2
1
1
1
1
1
Calculated values based on current density of 2 milliamperes per square foot.
Coated Pipe Chart
21
Soil Resistivity in ohm-cm
1K
2K
Anode Recommendation
48# 48# 32# 17# 17# 9# 9# 9# 9#
9#
Nominal Output in milliamperes
208 104 55 40 33 20 17 15 14
12
Nominal Life in years
11
36
22
3K 4K 5K 6K 7K 8K 9K 10K
28 20 25 22 25 29 32
Pipe inch
O.D. inch
Surface Area Ft2/L.F.
Current Reqmt. mA/L.F.
Nominal Anode Spacing in feet
2
2.275
0.60
0.06
3494 1747 924 672 554 336 286 252 235 202
4
4.50
1.18
0.12
1766 883 467 340 280 170 144 127 119 102
6
6.625
1.73
0.17
1200 600 317 231 190 115 98 87 81
69
8
8.625
2.26
0.23
922 461 244 177 146 89 75 66 62
53
10
10.75
2.81
0.28
739 370 196 142 117 71 60 53 50
43
12
12.75
3.34
0.33
623 312 165 120 99 60 51 45 42
36
14
14.00
3.66
0.37
568 284 150 109 90 55 46 41 38
33
16
16.00
4.19
0.42
497 248 131 96 79 48 41 36 33
29
18
18.00
4.71
0.47
442 221 117 85 70 42 36 32 30
25
20
20.00
5.23
0.52
397 199 105 76 63 38 32 29 27
23
22
22.00
5.76
0.58
361 181 96 69 57 35 30 26 24
21
24
24.00
6.28
0.63
331 166 88 64 53 32 27 24 22
19
26
26.00
6.80
0.68
306 153 81 59 49 29 25 22 21
18
28
28.00
7.33
0.73
284 142 75 55 45 27 23 20 19
16
30
30.00
7.85
0.79
265 132 70 51 42 25 22 19 18
15
32
32.00
8.37
0.84
248 124 66 48 39 24 20 18 17
14
34
34.00
8.90
0.89
234 117 62 45 37 22 19 17 16
13
36
36.00
9.42
0.94
221 110 58 42 35 21 18 16 15
13
Calculated values are based on current density of 2 milliamperes per square foot. Calculated values based on a coating efficiency of 95%
22
Underground Anode Installation Instructions Galvanic anodes are installed on underground pipelines to provide localized cathodic protection for that segment of line. The amount of protection derived from an anode is dependent upon several variables including coating quality, line size, and soil resistivity. Underground anodes are generally installed in a packaged form, with the anode surrounded by a prepared backfill. The installation process is generally the same for all anode sizes. A preferred method of installation is with the anode connected to the pipe through a test station. This configuration permits monitoring of anode performance. The other method of installation is to directly connect the anode wire to the pipe. Anodes may be installed either horizontally or vertically. A minimum separation distance of 5' is desirable to maximize the performance of the anode. This separation can be achieved either horizontally or vertically depending on local conditions. In most cases, anodes should be installed no shallower than pipe depth. In dry soils, additional depth may enhance anode performance by reaching lower resistivity soil. Wetting the anode with approximately five gallons of water after installation will activate the anode faster and provide initial current output data.
IMPRESSED CURRENT CATHODIC PROTECTION Impressed Current Anode History The origins of cathodic protection date to the days of Sir Humphry Davy with the use of sacrificial 23
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 2SO4 + 2H2O
O2 + 4H+ + 4e& 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 24
•
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 25
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 0.72 x 10-6/F expansion 26
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 27
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 28
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 Compression - 100000 Strength: pounds per square inch Flexural - 15000 pounds per square inch. Coefficient of 0.72 x 10-6/F thermal expansion:
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% 29
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. 30
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, 31
consumption is 2 to 3 times greater at low current densities (10 A/sq. ft). At high current densities, consumption is much higher. The use of platinum is now primarily limited to water environments. Its predominant use is probably in fresh water tank applications; with secondary applications such as condenser water boxes, reinforced concrete, process equipment, docks, etc. Anode manufacturers indicate that platinum can be successfully used underground, both in surface beds and deep anode beds. However, most operators experience has been negative. There have been numerous reports of anode failures when installed underground. The primary failure mechanism is felt to be excessive consumption in small areas and loss of substrate oxide. This could be a result of non-uniform electrolyte resistivities and/or non-linear current distribution in the anode conductor core. Platinum has performed well in water applications. It is probably the most widely used impressed anode material in fresh water applications such as storage tanks and condensers.
Mixed Metal Oxide Mixed metal oxide anodes have been used in groundbeds since the early 1980's. The anodes were originally developed in Europe in the 1960's for use in the chlor-alkali industry. This material consists of a high purity titanium substrate with an applied coating consisting of a mixture of oxides. The titanium serves as a support for the oxide coating. Titanium functions as a "valve metal" which form thin, self-healing, adherent oxide films which are acid resistant and resist the passage of anodic current. The oxide is formed on the titanium substrate by thermal decomposition of precious metal salts that have been applied onto the substrate. Four configurations of these anode materials have been used for underground applications. These are tubular, wire or rod, mesh or net, and strips. Common tubular anode sizes are 1.6 cm and 2.5 cm diameter tubes, 50 cm and 100 cm long. Wire/rod anodes range in diameter from 1.5 to 12 mm. Expanded mesh anodes made from thin gauge titanium strip have been used in confined spaces. This form of anode operates at much lower current density, typically less than 0.5 amps per square meter. Anode strips with widths less than 50 mm have been used in confined spaces. Several different connection designs have been utilized. The tubular anodes are attached to a conductor cable using a mechanical or welded center connection. Multiple anodes are often attached to a single cable creating anode strings. Wire/rod anodes and mesh/strip anodes have generally used crimped end connections that are sealed with tape, heat shrink, and epoxy materials. Both tubular anodes and wire/rod anodes are commonly packaged in steel canisters ranging in length from 3' to 10' and in diameter from 2" to 6". The anodes are surrounded by fine carbonaceous coke breeze backfills in these containers. Anode properties include high electrical conductivity, low consumption rate, and a high surface area to catalyze oxidation reactions. Consumption rates of the anode oxide are on the order of 1 mg per amp year for a chlorine evolution environment such as seawater. For an oxygen evolution environment, consumption rates are on the order of 5 mg per amp year. 32
Maximum recommended current densities for underground application with a coke breeze backfill material is 100 A/sq. m (9.3 A/sq.ft). This current density relates to a design life of 20 years. Current densities for muds and freshwater may be reduced by over half, depending upon temperature and life. Mixed metal oxide anodes are now commonly used in underground, and water environments. They are also the predominant anode material used for protection of tank bottoms when installed with nonconductive secondary containment liners. There have been some instances of bed drying which have resulted in increased groundbed resistance. There have also been failures with anodes on strings where soil resistivity varied within the groundbed. This may be more of a concern in deep anode bed applications. Variations in resistivity can result in widely varying currents among the anodes. This has lead to accelerated consumption of anodes that were operating at current densities exceeding manufacturer's recommendation. Generation of chlorine has led to attack of the standard cable insulation used for tubular anodes. In these cases, a dual extrusion cable material is utilized. Conductive Polymer This anode material has been available since the early 1980's. The anode material consists of a continuous semiconducting polymer material extruded on a copper wire. The active anode component is carbon contained in a polymer matrix. It is a flexible wire-like anode and is provided on continuous rolls. The material has an outer diameter of 1/2" with an inner core of #6 AWG stranded copper wire. The anode is also provided prepackaged in carbonaceous backfill. The package is a nylon sheath containing the anode and backfill, provided on continuous rolls. Connections of the inner copper core to a main cable are normally with mechanical crimped connectors. The ends of the anode and connections are normally sealed with manufacturer-provided heat shrink enclosures. The manufacturers recommended maximum current density for the conductive polymer anode is 16 milliamperes per lineal foot, when installed underground. When this material is used in long-line parallel applications, the attenuation of current in the conductor must be considered. Typically, a heavy gauge parallel copper cable is installed with the anode and multiple connections are made at regular spacings along the anode length. This material is used extensively for long line anode system installed parallel to pipelines in areas where coating has deteriorated or where sub-surface conditions do not permit efficient current distribution from conventional current sources. It is normally installed with a surrounding carbonaceous backfill. Conductive polymer anodes have also been installed in confined areas such as between tank bottoms and non-conductive secondary containment liners. Conductive polymer anodes can provide uniform low-current density output over their entire lengths. Reports have generally been very good. There are some reports of failures in areas where the wirelike anode was not installed in carbonaceous backfill and accelerated consumption of the carbon occurred. These cases have almost all been related to excessive current discharge in low resistivity wet areas such as creek or stream crossings.
33
Application Table The selection of an impressed current anode should be based upon a thorough evaluation of the application. Items of consideration include environment, current requirements, life requirements, space, and economics. There is probably no anode material that is optimum or even capable of effectively meeting the requirements of every situation. The anode materials developed within the past 30 years certainly expand the arsenal of the corrosion engineer. However, for underground application; the use of materials such as graphite and high silicon cast iron still far exceed those of the newer materials. IMPRESSED CURRENT ANODE APPLICATION CHART Anode
Soil with Coke Soil without Coke Freshwater Saltwater
Steel
X
Graphite
X
HISI Cast Iron
X
X
X
X X
X
Platinum
X
X
X
X X
Mixed Metal Oxide
X
X
Conductive Polymer
X
X
34
Anode Comparison Table Anode
Environment Consumption Rate lb/amp-year
Design Current Density
Size
amps/SqFt
Steel Graphite
HISI Cast Iron
Platinum
20.0
None
Soil
1.0
0.5
Freshwater
2.0
NR
Saltwater
0.5
2.0
Soil
0.7
1.0
Freshwater
1.0
2.5
Saltwater
0.5
2.5
Soil
NR
NR
Freshwater
8 mg/A-yr
50
35
Weight Surface Max* pounds Area Design Sq Ft. Output
(soil) Amps
3" x 60"
30
4.0
2.0
4" x 80"
72
7.3
3.6
1.5" x 60"
27
2.0
2.0
2" x 60"
44
2.6
2.6
2" x 60"
60
2.7
2.7
3" x 60"
110
4.0
4.0
4.5" x 60"
220
5.5
5.5
2.2" x 84"
50
4.2
4.2
2.6" x 84"
64
4.9
4.9
3.8" x 84"
95
7.0
7.0
4.8" x 84"
122
8.8
8.8
Mixed Metal Oxide
Conductive Polymer
Saltwater
8 mg/A-yr
50
Soil
5 mg/A-yr
10
Freshwater
5 mg/A-yr
10
Saltwater
1 mg/A-yr
50
Soil
16 ma per LF
Freshwater
3 ma per LF
Saltwater
NR
* based on coke breeze backfill
36
1.6 x 50 cm
NA
0.275
2.5
1.6 x 100 cm
NA
0.56
5.0
2.5 x 50 cm
NA
0.44
4.0
2.5 x 100 cm
NA
0.88
8.0
