Galvanic anodes system design


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Design of Cathodic Protection Systems

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Galvanic anodes system design

  1. 1. Galvanic AnodeCathodic Protection System Design by James B. Bushman, P.E. Principal Corrosion Engineer Bushman & Associates, Inc Medina, Ohio USAThe contents of this white paper including all graphics are protected by copyright of Bushman & Associates, Inc. (B&A), Medina, Ohio USA and may not be duplicated or distributed without the express written permission of B&A. It may be reproduced as a single copy for study and review by any person who downloads the document from B&A’s Internet Web Site. BUSHMAN & Associates, Inc. CORROSION CONSULTANTS P. O. B o x 4 2 5 M e d i n a, O h i o 4 4 2 5 6 P h o n e: ( 3 3 0 ) 7 6 9 - 3 6 9 4 Fax: ( 330)769-2197
  2. 2. DisclaimerEvery effort has been made to ensure that the information contained within this paper is accurate and reliable.However, neither B&A or the author shall not be liable in any way for loss or damage resulting from use of thisinformation or for violation of any federal, state, or municipal regulation with which it may conflict. This B&A technical paper represents the opinion of the author. Its use does not preclude anyone, whether he hasadopted procedures from the paper or not, from manufacturing, marketing, purchasing, or using products, processes, or procedures not in conformance with the procedures recommended in this paper. Nothing contained in this B&A technical paper is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or protecting anyone against liability for infringement of Letters Patent. This paper represents the thoughts and opinions of the author and should in no way be interpreted as a restriction on the use of better procedures or materials. Neither is this technical paper intended to apply in all cases relating to the subject. Unpredictable circumstances may negatethe usefulness of this technical paper in specific instances. Neither B&A or the author assumes no responsibility for the interpretation or use of this technical paper by other parties.Users of this B&A technical paper are responsible for reviewing appropriate health, safety, environmental, andregulatory documents and for determining their applicability in relation to this technical paper prior to its use. ThisB&A technical paper may not necessarily address all potential health and safety problems or environmental hazardsassociated with the use of materials, equipment, and/or operations detailed or referred to within this technical paper.Users of this B&A technical paper are also responsible for establishing appropriate health, safety, and environmentalprotection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance withany existing applicable regulatory requirements prior to the use of the information contained within this technicalpaper.CAUTIONARY NOTICE: B&A technical papers are subject to periodic review, and may be revised or withdrawnat any time without prior notice. B&A requires that action be taken to reaffirm, revise, or withdraw this technicalpaper no later than five years from the date of initial publication. The user is cautioned to obtain the latest edition.Purchasers of B&A technical papers may receive current information on all technical papers and other B&Apublications by contacting Bushman & Associates, Inc. at 6395 Kennard Road, Medina, OH 44256 USA Phone:330-769-3694.This paper is protected by copyright of Bushman & Associates, Inc. A single printout of this paper may be used bythe individual who downloaded the document from B&A’s website and shall not be further duplicated in any formfor distribution, training, publication or for any use other than the edification of the individual who downloaded thedocument. Page 2 of 10 ©Bushman & Associates, Inc
  3. 3. Galvanic Anode Cathodic Protection System Design Portions of the following were excerpted from the Appalachian Underground Corrosion Short Course "Advanced Corrosion Course" text that was edited and revised for applicability to this course by James B. Bushman, P.E. Bushman & Associates, Inc. P.O. Box 425, Medina, Ohio,44256 Phone: (330) 769-3694 – Fax: (330) 769-2197Galvanic anodes are an important and useful to the protected structure. Oncemeans for cathodic protection of installed, very little maintenance isunderground storage tank systems, pipelines required for the life of the anode. Theand other buried or submerged metallic anode of a galvanic anode system isstructures. The application of cathodic not subject to the same degree ofprotection utilizing galvanic anodes is electrical or mechanical malfunction asnothing more than the intentional creation of that of an impressed current system.a galvanic electrochemical cell in which two • Efficient and non-interfering – Thedissimilar metals are electrically connected relatively low, and normally wellwhile immersed in a common, electrically distributed, current output of theconductive electrolyte. In the "dissimilar galvanic system can result in a moremetal" cell, the metal higher in the constant current density at theelectromotive series (or more "active") protected structure. This minimizesbecomes anodic to the less active metal and over protection and wasted consumed during the electrochemical The low current output reduces thereaction. The less active metal receives possibility of interference to asome degree of cathodic protection at its minimum. The advantages mentionedsurface due to the current arriving from the enable the galvanic cathodic protectionanodic metal. The design of a galvanic system to be utilized efficiently in acathodic protection system involves variety of applications, such as:consideration of all factors affecting theproper selection of a suitable anode material a. For well-coated undergroundand its physical dimensions, placement, and storage tanks and pipingmethod of installation. b. In rural areas and offshore whereADVANTAGES AND LIMITATIONS power is not availableThere are several important advantages to c. For supplemental protection,using galvanic anodes: such as at pipeline crossings • No power source is required – Due to d. In isolated corrosive areas ("hot the fact that the protective current is spots") generated by the electrochemical e. In highly congested, urban areas reaction between the metals, no where current distribution and externally supplied power is required. interference present problems • Installation and maintenance cost is f. On electrically discontinuous reduced – Normally, galvanic anodes structures have the advantage of not requiring g. Well coated pipelines additional right-of-way purchase since the anodes are usually installed close Page 3 of 10 ©Bushman & Associates, Inc
  4. 4. Galvanic Anode System Design edited by James B. BushmanHowever, the galvanic anode system is not The efficiency is dependent upon the alloy;without limitations. The difference in the therefore, it is important that once the properpotential of the anode and cathode alloy has been selected, the material(protected structure) that causes the purchased meets the alloy specifications.protective current to flow is normally quite The next two tables list some typical alloysmall. The small potential difference, or specifications in common usage."driving potential," results in very limitedcurrent outputs, especially in high soil The following elements, most commonlyresistivity areas. This fact severely limits the present in magnesium, affect the efficiencyeconomic use of galvanic systems on: of magnesium anodes used for cathodic • Large structures protection in soils: • Poorly-coated structures • Aluminum – Significant effectsAVAILABLE ANODE MATERIALS outside ranges shownThe most commonly used materials forgalvanic anodes on buried structures are • Manganese – Controls to some degreealloys of magnesium and zinc. the negative impact of iron by surrounding the iron particles duringWhen the anode alloy is placed in the casting solidificationelectrolyte for the protection of a structure, acertain amount of the current is generated • Nickel – Detrimental to efficiencydue to the self-corrosion of the anode. Thecurrent efficiency is a measure of the actual • Copper – Detrimental to efficiencycurrent available for cathodic protection ofthe primary structure expressed as a • Iron – Detrimental to efficiency, butpercentage of the total current generated. can be controlled to some degree byBecause the anode corrosion rate is directly larger amounts of manganeseproportional to the current output delivered,the efficiency is an important consideration • Silicon – Detrimental above 0.1in the selection of anode material. The percenthigher the efficiency is, the more useableenergy per pound of material purchased. • Zinc – Only slightly detrimental in H-1 Alloy Hi- Hi- higher amountsCharacteristic (AZ-63) Potential Purity Mag. Alloy Mag. Alloy Zinc • Other (lead, tin, beryllium) – Minor Solution -1.55 -1.80 -1.10 impurities that do not significantly potential to Cu-CuS04 ref. affect anode efficiency in amounts cell commonly found, but can be Faradaic 8.8 8.8 23.5 detrimental above these limits Consumption Rate The following two tables provide industry Current 25-50 50 90+ standard alloy elements for both magnesium efficiency (%) and zinc anodes commonly used in cathodic Actual 250-500 500 360 amps-hrs/lb protection applications. Deviation from Actual 35-17.5 17.5 26.0 these alloy specifications can result in lb/amp/year anodes that suffer from pacification, inter- Page 4 of 10 ©Bushman & Associates, Inc
  5. 5. Galvanic Anode System Design edited by James B. Bushmangranular corrosion deterioration and • Reduces self-corrosion of the anode byexcessive consumption rates. promoting a uniform corrosion attack, thereby improving efficiency Common alloy specifications - Magnesium The most commonly used anode backfill Grade Grade mixture is 75 percent gypsum, 20 percent Grade Hi-Pot. “B” “C”Element Mg (%) “A” Mg Mg Bentonite clay, and 5 percent sodium Mg (%) sulfate. This mixture is selected because, (%) (%) 0.010 5.3 - 5.3 - over the wide range of soils likely to be Al 5.0 - 7.0 max 6.7 6.7 encountered, it has shown the best success in 0.50 - 0.15 0.15 0.15 achieving the desired characteristics. Due to Mn 1.30 min min min the solubility of backfill components, the 2.5 - 2.O - Zn 0 2.5 - 3.5 3.5 4.0 backfill tends to "condition" the adjacent 0.10% 0.30% 0.10% soil for several feet. Si 0 max max max ANODE SELECTION Cu 0.02 0.02 0.05 0.10 Ni 0.001 0.002 0.003 0.003 After considering the available materials, Fe 0.03 0.003 0.003 0.003 one must make a suitable selection. The 0.05% criterion for selection is, as one would each or 0.30% 0.30% 0.30% Other 0.03% max max max expect, an analysis of performance versus max tot. cost. The performance of an anode is Mg Balance Balance Balance Balance measured by the following criteria: Common alloy specifications - Zinc • Anode life – Life is a function of three factors: weight, current output, and Hi-Purity Zinc Hi-Amp Zinc (ASTM B418-67 efficiency. Longer life is achieved (Mil-A 18001) through heavier weight, lower currentElement for Seawater Type II) Use Only Primarily for output, and high efficiency. Underground Use (Percent %) Percent (%) • Current output – Current output is Al 0.1 - 0.3 0.005 max governed by electrolyte resistivity, Cd 0.025 - 0.06 0.003 max anode resistance to electrolyte, and Fe 0.005 max 0.0014 max alloy potential. Higher current output Pb 0.003 max 0.003 max Zn Remainder Remainder is achieved through lower resistivity, lower resistance to electrolyte, andSHAPES, SIZES, AND BACKFILL higher alloy potential.Galvanic anodes are offered in a wide The costs involved with the installation andvariety of standard shapes and sizes, and operation of galvanic anodes can bemay also be ordered in custom sizes. categorized as follows:The use of a prepared anode backfill • Material costs--This is based on alloy,accomplishes the following effects: backfill, and anode size. Generally, the • Stabilizes anode potential heavier the anode, the lower the cost per pound of material. More efficient • Prevents anode polarization, enhancing anode material results in a lower cost current maintenance per ampere hour of current delivered. • Lowers anode-to-earth resistance, • Installation costs--The installation cost increasing current output would not be expected to vary greatly Page 5 of 10 ©Bushman & Associates, Inc
  6. 6. Galvanic Anode System Design edited by James B. Bushman on a per anode basis regardless of the actual amount of current required. Current alloy or size of anode selected. testing can be performed utilizing temporary Therefore, consideration of installation “ground bed” of one or more driven metallic costs normally involves an rods energized by a test rectifier or storage investigation of the number of anodes battery. required. The temporary ground bed is energized and • Maintenance costs--The cost of its effect upon the structure is measured. maintenance normally involves only Utilizing proper measurement techniques, the periodic testing of the cathodic the current output is adjusted until the protection system, which would not be selected criterion for protection is achieved substantially affected by the type of with the least amount of current. One or anode selected. This cost is usually more such temporary ground beds may be neglected in the selection process. required to analyze sections of the structure, especially if the physical characteristics ofPRE-DESIGN CONSIDERATIONS the structure vary significantly. One mustThe primary consideration in the design of remember that the resistance to ground ofthe galvanic system is the efficient the driven rods is likely to be much higherdistribution of sufficient current to achieve than permanent buried anodes; therefore, thecathodic protection. Due to the limited range driving voltage required in the test is notof voltages available the problem of indicative of the actual driving voltageachieving the desired current becomes one requirement.of regulating the resistance of the electrical DESIGN CALCULATIONScircuit. The electrical circuit that governs the currentThe most important (and least controllable) output of a galvanic anode is depicted in thefactor affecting the circuit resistance of next figure.underground galvanic cathodic protectionsystems is soil resistivity. For a small Galvanic Anode Electrical Circuit Componentsstructure, such as an isolated, very wellcoated buried tank, it is often more Gradeeconomical to overdesign rather than Ia Rwperform field testing. On the other hand, it is Connecting Wir eimperative that testing be conducted for apoorly coated tank structure. The number oftest points to be considered will vary from Ea Raf Rcp Rpg Rcgstructure to structure and will depend on the Rcf Ec Protected Structurevariation of the resistivity measurements and Anode Backfill containingthe physical characteristics of the structure. 75% Gypsum, 25% Bentonite and 5% Sodium SulphateAreas of predominantly uniform resistivity Galvanic Anode Ingotwill require less frequent measurements thanareas of varying resistivity. where: Ea = potential of anodeIf the tank structure for which the galvanicanode design is intended exists, current Ec = potential of cathoderequirement tests should be performed in Ia = anode currentorder to more accurately determine the Page 6 of 10 ©Bushman & Associates, Inc
  7. 7. Galvanic Anode System Design edited by James B. Bushman Raf = anode film resistance is generally considered to be relatively small when compared to RTAV, the above formula is Rap = backfill resistance often reduced to the following simplified form: Rcf = cathode film resistance E −E Rcg = cathode-to-earth resistance I = A P R A TAV Rpg = backfill-to-earth resistance Rw = resistance of connecting wire This theoretical expression will normallyRcf is usually negligible in value, compared result in a conservative value of current forwith the other resistive components, whereas anodes in backfill that are installed in theRaf and Rap are constant for a given anode soil. In addition, it is time-consuming toin a given backfill. Rcg, the cathode-to- calculate the various resistive factors, andelectrolyte resistance, is heavily dependent often certain. assumptions must be madeon the quality of the structure coating, being that result in an approximate currentnearly negligible for bare structures. calculation. The output of magnesium andTherefore, Rpg, Rcg, and Rw are the zinc anodes has been fairly well documentedsignificant and variable components which under varying conditions, and many graphs,must be considered. charts, and tables have been prepared based on actual outputs. These references provideRTAV, the total resistance of a vertically a simplified and reasonably accurateinstalled anode in the electrolyte can be determination of anode output underapproximated by H. B. Dwights equation: conditions normally encountered in the design of cathodic protection systems for 0.00521   8 L   pipelines, buried tanks, etc. One of the R = p ln  − 1 L   d   TAV widely used references has been prepared by D. A. Tefankjian. He developed a set of equations for the output of an anode at awhere: polarized structure potential of -0.85 volts versus a Cu-CuSO4 reference electrode.RTAV = Resistance of vertical, rod shapedanode Correction factors are then applied to adjust the result for various shapes and structurep = resistivity of electrolyte, potentials:L = length of anode rod Imb = 150,000 FY/pd = diameter of packaged anode Izb = 50,000 FY/pOnce the total anode resistance has been Imc = 120,000 FY/pcalculated, the current output of the anodecan be calculated in accordance with Ohms Izc = 40,000 FY/pLaw: where: E −E I = = amperes A P Imb = current output for magnesium anode R +R +R +R +R A on bare structure in milli-amperes AF AP PG CF WSince RAF + RAP + RPG is equal to RTAV Izb = current output for zinc anode on barecalculated above and since RCG + REF + RW structure in milli-amperes Page 7 of 10 ©Bushman & Associates, Inc
  8. 8. Galvanic Anode System Design edited by James B. BushmanImc = current output for magnesium anode Note: Anodes are installed vertically. on coated structure in milli-amperes Driving voltage correction - Table (y)Izc = current output for zinc anode on Structure coated structure in milli-amperes Potential Std. Hi-Pot Zinc (vs. Cu- Mag. MagP = soil resistivity in ohm-centimeters CuSO4)F = factor from anode shape table -0.70 1.21 2.14 1.60Y = factor from driving voltage table -0.80 1.07 1.36 1.20Anode shape correction - Table (f) -0.85 1.00 1.29 1.00 Anode -0.90 0.93 1.21 0.80 Weight Packaged -1.00 0.79 1.07 0.40Alloy Factor (lbs.) Dimensions (F) -1.10 0.64 0.93 n/aMg 3 3" x 3" x 4.5" .53 -1.20 0.50 0.79 n/aMg 5 3" x 3" x 7.5" 0.60Mg 9 3" x 3" x 13.5" 0.71 The equation assumes a minimum resistivityMg 9 2.75" x 2.75" x 26" 1.01 of 500 ohm-centimeters and a distance 1.5" x 1.5" x 72" between anode and structure of 10 feet. ItMg 10 ingot, 4" x 78" 1.71 Package can be seen immediately from the tables that 1.6" dia. x 10 increasing the surface area of the anodeMg 15 extrusion, 6” x 10’ 2.61 (especially length) or use of a high potential Backfill alloy has the effect of increasing resultantMg 17 4" x 4"x 17" 1.00 current output, assuming other factors are 2" x 2" x 72" ingot,Mg 18 1.81 equal. 5" x 78" Package 2.5" x 2.5" x 60" For example, compare the current output ofMg 20 ingot, 5" x 66" 1.60 17-pound standard alloy, high-potential Package 1.3" dia. x 20 alloy, and 20-pound (2" dia. x 60")Mg 20 extrusion, 6” x 20’ 4.28 magnesium anodes. Assume a well coated Backfill structure, a soil resistivity of 3000 ohm- 2" dia. x 10 centimeters, and an anticipated structure-to-Mg 25 extrusion, 8” x 10’ 2.81 soil potential of 0.85 volt. BackfillMg 32 5" x 5" x 21" 1.06 Standard 17# H-1 Alloy Magnesium Anode 3.75" x 3.75" x 60"Mg 40 ingot, 6.5" x 66" 1.72 120,000(1.0)(1.0) I = = 40mA Package MC 3000 3" x 3" x 72 ingot,Mg 42 1.90 6" x 78" Package Standard 17# High Pot. Magnesium AnodeMg 50 8" dia. x 16" 1.09Mg 50 5" x 5" x 31" 1.29 120,000(1.0)(1.29) 1.4” x 1.4” x 36” I = = 51.6mA 3000 MC Zn 18 ingot, 5” x 42” 1.68 Package 2” x 2” x 30” ingot, Long 20# H-1 Alloy Magnesium Anode Zn 30 1.44 5” x 36” Package 1.4” x 1.4” x 72” 120,000(1.60)(1.0) Zn 36 ingot, 5” x 78” 1.81 I = = 64mA 3000 MC Package 2” x 2” x 60” ingot, Zn 60 1.72 Anodes may be connected in parallel, in 6.5” x 66” Package order to achieve a higher total current output Page 8 of 10 ©Bushman & Associates, Inc
  9. 9. Galvanic Anode System Design edited by James B. Bushmanat a given location. Unfortunately, the Multiple anode adjusting factorsoutput of two anodes in parallel which are (Vertically Installed Anodes)buried less than 30 feet apart (center to Anode Spacingcenter spacing) is not quite equal to the sum (in Feet)of the current from two separate anodes of No. ofthe same size. Anode s in 5 10 15 20 25The closer together the anodes are spaced, Bankthe more the current output is restricted 2 1.84 1.92 1.95 1.97 2.03because the current from one anode tends tobe opposed by the current output from 3 2.46 2.71 2.80 2.85 3.02adjacent anodes. To determine the 4 3.04 3.46 3.63 3.71 4.01approximate current output of a multiple 5 3.59 4.19 4.43 4.56 4.98anode ground bed, multiply the single anode 6 4.13 4.90 5.22 5.41 5.96current previously calculated by the 7 4.65 5.60 6.00 6.23 6.91appropriate adjusting factor found in the 8 5.15 6.28 6.77 7.04 7.85table below. 9 5.67 6.96 7.54 7.88 8.82The table is calculated for 17-pound 10 6.16 7.64 8.30 8.68 9.75packaged anodes installed vertically inparallel. For approximate calculations, it is 11 6.76 8.41 9.14 9.56 10.75good for any size anodes. 12 7.30 9.12 9.93 10.40 11.71For a more exact calculation, an adjusting 13 7.83 9.83 10.72 11.23 12.68factor may be determined from the 14 8.37 10.54 11.51 12.07 13.64following equation (based upon the E.D. 15 8.91 11.25 12.30 12.91 14.61Sunde formula for resistance to earth of 16 9.44 11.96 13.09 13.75 15.57multiple anodes). This equation is provided 17 9.98 12.68 13.89 14.58 16.54immediately following the table developed 18 10.51 13.39 14.68 15.45 17.50by Mr. Tefanjian. 19 11.05 14.10 15.47 16.26 18.47 N 20 11.59 14.81 16.26 17.10 19.43 MA = ADJ 2 L(ln 0.656 N ) 1+   8L   S ln  − 1 To determine the approximate current output   d   of six 17-pound standard alloy anodes spaced on 10-foot centers in 3000 ohm-Where: centimeter soil with a structure potential of MAADJ = Multiple Anode Adjusting (-)0.85 volts, it was determined earlier that Factor the current output of a single 17 pound N = number of anodes in parallel anode under these same conditions = 40 milli-amperes. L = length of the anode in feet From the Multiple Anode Adjusting Factor d = diameter of the anode in feet Table, select 4.90 from the 6 anode row and S = spacing, center-to-center in feet the 10’ column. Page 9 of 10 ©Bushman & Associates, Inc
  10. 10. Galvanic Anode System Design edited by James B. BushmanTherefore the output of the six anodes = The utilization factor accounts for a(40)(4.90) = 196 ma. reduction in output as the surface area of the anode decreases with time, limiting theHaving arrived at an anode configuration anode output. This factor is usually assumedthat will produce the required current output to be 0.85. The equation may then beis not sufficient in itself. An examination of reduced to simpler form by substituting thethe estimated life of the anodes must be constant factors:undertaken in order to determine whetherthe design will provide protection for a For magnesium:reasonable period of time. The followingexpression may be used to calculate theestimated life of the anode: 48.5W L = I M Anode Life = [Faraday Consumption Rate (Ampere For zinc: Hours/Pound)/No. of Hours per Year] x Anode Weight (lbs) 32.5W L = x Anode Efficiency x Utilization Z I Factor/Anode Current in Amperes where: W = Anode metal weight in pounds I = Current output in milli-amperes LM = magnesium anode life, years LZ= zinc anode life, years The expected life of the cathodic protection system should be consistent with the design life, use, and maintenance of the protected structure. Page 10 of 10 ©Bushman & Associates, Inc