Various methods are available for determining whether the structure to be protected is being effectively protected through the application of anodic protection. The technical basis for corrosion and anodic protection is electrochemical. Electrochemical methods of determining the effectiveness of cathodic protection systems are the most widely used criteria for establishing the adequacy of the protection. In addition to electrochemical methods, inspections to determine the actual condition of the structure being protected can be used to determine whether or not effective protection has been achieved in the past. If there is no attack of the protected system in an aggressive environment, then the protective system has been functioning adequately. For buried or submerged systems where access is restricted, the electrochemical criteria are the most widely applied.

The primary organization issuing criteria for evaluation of the effectiveness of cathodic protection installed on various structures is the National Association of Corrosion Engineers (NACE). NACE has issued Technical Standards known as Recommended Practices for corrosion control. Contained in several of these Recommended Practices are electrical criteria to be used to evaluate the effectiveness of cathodic protection systems. The three standards which pertain most to the structures and systems in operation at installations are as follows:

RP0169 Recommended Practice for Control of External Corrosion on Underground or Submerged Piping Systems

RP0285 Control of External Corrosion on Metallic Buried, Partially Buried, or Submerged Liquid Storage Systems

RP0388 Impressed Current Cathodic Protection of Internal Submerged Surfaces of Water Storage Tan

Table 2: Stand Number and Title.

For submerged and buried structures, criteria based upon the electrochemical potential of the surfaces of the structure to be protected are the most widely used criteria for determining whether or not the structure is being effectively protected. In making these electrochemical potential measurements, as shown in Fig. 1, a high impedance voltmeter is used to measure the difference in potential between the structure and a reference electrode placed in contact with the electrolyte. For buried structures, the copper/copper sulfate reference electrode is the reference electrode most commonly used for this purpose. For structures submerged in seawater, the silver/silver chloride reference electrode is commonly used. Other reference electrodes can be used when appropriate. Potential readings obtained using any given reference electrode can be related to readings obtained with other reference electrodes (see Table 3). In order to assure that the potential readings obtained are properly interpreted, the reference electrode used should always be noted. Readings should be reported as “XX.XX Volts versus a YYY” where YYY is the type of reference electrode used to measure the structure potential. As these potential measurements are most commonly used to measure the potential of buried pipelines, they are commonly called “pipe-to-soil potentials,” even though they may refer to the wall of a water storage tank in contact with potable water. The more precise term for these measurements is “structure-to-electrolyte potential.” In order to determine whether or not a given surface is being adequately protected, structure-to-electrolyte potential measurements are taken at various locations surrounding the structure. Based upon a combination of corrosion theory, experimental and laboratory tests, and, more importantly, upon actual field experience with a large number of protected structures, criteria for interpreting these structure-to-electrolyte potentials have been developed. Refer to the latest version of the appropriate Recommended Practice for more information on the use of these criteria.


Failure rate analysis: Corrosion damage, as measured by frequency of system failure, usually increases logarithmically with time after the first occurrence of corrosion failure. When effective cathodic protection is applied to a structure which has experienced corrosion damage, the frequency of failures will be significantly reduced. However, due to the presence of existing corrosion damage, the failure rate will not immediately be reduced to zero. Mechanical damage and previously undetected corrosion related damage may still result in failure, but if effective cathodic protection is achieved, corrosion failures should cease after a period of 1 or 2 years. Accurate failure records should be kept for both protected and unprotected systems in order to determine the need for cathodic protection and the effectiveness of installed systems. A typical failure rate analysis is shown in Fig. 2.

Nondestructive testing of facility: Periodic evaluation of the condition of the protected system can be used to determine the adequacy of the cathodic protection system installed on the structure or to establish the need for protection. If the surface of a structure is accessible or is exposed for repairs, alterations, or specifically for the purposes of inspection, visual inspection may be used to evaluate the need for protection or the effectiveness of cathodic protection applied to the structure. Signs of corrosion such as the presence of corrosion products, pitting, cracking, reduction in physical size, or other evidence of deterioration should be noted. A variation of visual inspection is the installation of small metal samples, or coupons, electrically connected to the structure at various critical points on the structure. Periodic removal and evaluation of these samples, including visual observation and weight loss measurements, can be used to infer the corrosion activity affecting the structure being monitored.

Consequences of under-protection: If the measured potentials of a structure are not as negative as required by one or more of the applicable criteria for cathodic protection, some corrosion of the structure may occur. However, the corrosion of the structure will be reduced in proportion to the amount of current supplied. When only parts of the structure do not reach the desired criteria, those areas will corrode at a rate inversely proportional to the current they receive. When partial protection occurs, corrosion is reduced in those areas receiving partial protection but is not totally stopped. When protective currents are totally interrupted, corrosion will usually return to a normal rate after a short period of time.

Consequences of overprotection: In addition to being wasteful of anode material or electrical power, excess potentials can cause disbondment of protective coatings and can cause hydrogen embrittlement of certain types of steels, especially high-strength steels. Excess cathodic protection potentials can result in the generation of hydrogen gas. When the cathodic protection potential reaches the polarized (instant-off) potential of -1.12 V , with respect to a copper/copper sulfate reference electrode, the generation of hydrogen gas will occur. When hydrogen gas is generated, it is often trapped between the coating and the surface and causes blisters and disbonding of the coating. Electrolyte can subsequently fill the gap between the coating and the metal and, as the coating is an electrical insulator, sufficient current will occur. Coating disbondment is a particular problem in water tanks, where potentials should not be more negative than -1.1V. In soil environments when high quality coatings are used, disbondment is seldom encountered at potentials less negative than -1.6 V (current on) or -1.12V polarized (instant-off) potential. The hydrogen produced when cathodic protection currents are excessive can also result in the reduction of the ductility of steel. This is particularly true for high strength steels (in excess of 130,000 pounds per square inch (psi) yield strength).


In order to provide the increased structural lifetime and reliability intended, cathodic protection systems must be monitored and maintained. Economic analysis, made at the time when cathodic protection was selected as a means of corrosion control, should have included the cost of periodic monitoring and maintenance.

Although anodic protection systems often have design lives in excess of twenty (20) years, the effectiveness of cathodic protection systems usually changes with time. The consumption of both sacrificial and impressed current anodes can result in decreased anode output which results in inadequate protection. Deterioration of cable insulation or connections can result in increased circuit resistance with similar effects. Rectifier output may be reduced by aging of the diode stacks, or may be completely interrupted by electrical failure. The corrosion environment may change if there is a change in drainage patterns or the area around an anode is paved, reducing local soil moisture content. Construction of additional structures or modification to existing structures in the area may result in interference.

In order for a cathodic protection system to be effectively monitored and maintained, the parameters used in the design of the system and the “as-built” configuration of the system must be known.

• Drawings:“As-built”drawings of the cathodic protection system and the structure being protected should be available as should drawings of other structures in the area which might cause interference problems. The cathodic protection system drawings should include, as a minimum, the location and configuration of all test stations, the location and type of all anodes and rectifiers, and the location of all connections and insulating flanges. These drawings should be periodically updated to show any changes made to the cathodic protection system, the structure being protected, or nearby structures.

• System data: The following system design parameters should be recorded and kept with the system drawings in order to properly monitor and maintain the cathodic protection system.

• Design Potentials: The desired potentials used in the design of the cathodic protection system should be indicated. In some cases, different criteria may be used to establish minimum protective potentials at different locations on the same structure.

• Current output: The design current outputs of the rectifiers or galvanic anodes in the system should be recorded. This data is most important in the initial system check-out but may also be used to evaluate discrepancies in structure-to-electrolyte potential readings.

• System settings and potential readings: The initial system settings and potential readings should be recorded. Potential readings taken both at the time of initial system adjustment and during periodic monitoring should be recorded in order to detect trends in the readings. Changes in potential readings are often more important than the actual values themselves in determining the cause of improper system operation.

• Rectifier instructions: In order that all rectifiers in the system can be properly maintained, adjusted, and repaired, instructions for the rectifiers must be retained. An original copy should be retained in the maintenance files and a copy should be kept within the rectifier enclosure for field reference.

The operation and maintenance of galvanic anode cathodic protection systems should include visual inspections, whenever possible; the measurement of current and potential for buried systems and water storage tanks; and the replacement of anodes and adjustment of anode output.

Visual inspection may be made of both the protected structures and of the galvanic anodes. Inspection of the protected structure tells if corrosion is occurring and if corrosion control methods are effective. Inspection of galvanic anodes tells whether the anodes are, in fact, sacrificing themselves to protect the structure and if the anodes need to be replaced.

Structures should be inspected for damage to coatings and for corrosion damage. The Natural Gas Pipeline Act (40 CFR, Parts 190-195) contains recommended procedures for inspection of gas lines. Usually, only structures such as the interiors of hot water tanks, chillers, and heat exchangers may be scheduled for visual inspection. Underground structures should be inspected as the opportunity arises. Inspection reports must be maintained in a permanent file. Buried pipelines and tanks must be inspected by the corrosion engineer when excavations are made for repairs, extension, or the installation of other facilities. Excavations for the specific purpose of examining buried structures are seldom justified. When inspecting structures, one should look at both the type and extent of corrosion and the condition of any coating which may be present. The surface of the uncoated structure should be cleaned down to bare metal by using a wire brush, scraper, or chipping tool on selected spots. The appearance of the clean metal should be reported, as well as the depth and extent of any pits or grooves which may be noted. If it is necessary to remove the applied coating of wrapped pipe for inspection purposes, the coating should be properly repaired before the backfill is replaced.

Visual inspection of cast iron structures is necessary. Remove any graphitization. Visually inspect for breaks. Find the origin of a break. Looking for porosity, wall thinning, other defects, load concentration, corrosion, and again, graphitization.

Anodes. Galvanic anodes used in hot water tanks, water storage tanks, and mechanical equipment such as water chillers, heat exchangers, and evaporative condensers should be visually inspected whenever the equipment is down for repairs or inspection. If necessary, replacement of galvanic anodes should be made during the down period. A record of the inspection report should be kept in the corrosion engineer’s file. Buried galvanic anodes cannot be visually inspected.

Since the components of buried galvanic anode systems cannot be visually inspected on a routine basis, the operator and engineer must rely on electrical measurements to indicate the condition of the system. The routine measurements to be made are structure-to-soil (electrolyte) potential, anode-to-soil potential, and anode-to- structure current.

It is desirable to take a series of electrical measurements on a newly installed buried galvanic anode system to determine the initial level of cathodic protection. After the initial series, measurements should be made after six months and one year of operation. This will enable the corrosion engineer to identify deficiencies and program corrective action. After the first year of operation, measurements should be made at least annually, unless conditions indicate more frequent testing.

The primary measurement to be made is that of structure-to- soil potential since this determines the adequacy of protection. Fig. 1 shows the basic arrangement for making the measurement. The following spacing of potential measurements must be used initially and annually.

• In congested areas, structure-to-soil potentials must be measured at service risers and at points over the main farthest from the anodes.

• Potential measurements over long pipelines must be made over the main at points farthest from the anodes. The maximum interval between potential measurements should be 500 feet. The measuring and recording of structure-to-soil potentials can be used to determine the level of protection and the trend of changes in protection. Gradual changes in potential are indicative of anode depletion, changes in soil resistivity, or coating degradation. Sudden changes indicate broken leads, interference, shorted insulation, or changes in piping.

The measurement of anode-to-soil potential serves as a check of lead continuity, anode condition, and location (peak potential occurs directly over anode). Where test stations are provided, the measurement should be made at the same time as that of structure-to-soil potential.

The rate of anode metal loss is directly dependent on the rate of current flow between the structure and the anode. This current flow is measured to give an indication of proper anode operation and to allow calculation of anode life. Where test stations are provided, the measurement should be made at the same time as that of structure-to-soil potential.

It may be cheaper and easier to install new anodes than to troubleshoot and repair damage. Breaks in long collector wires may be located with cable and pipe locators. When structure- to-earth potentials are too low because of depleted anodes, new anodes should be added to provide continuous protection of the structure.

Galvanic anodes are routinely used for protection of the waterside of mechanical equipment such as condensers, hot water tanks, evaporative condenser plants, fire truck tanks, or other water carrying equipment. Although anodes are available in many shapes and sizes, generally they can be grouped as blocks, rods, or ribbons. The general pattern of metal loss is such that rod-shaped anodes may “neck down” near the point of attachment or suspension and a large part of the anode may fall off. For this reason, long slender anodes usually have an iron core which, being itself protected by the anode material, does not corrode, and holds the pieces of anode together, both physically and electrically, until all of the anode material is sacrificed. Block-type anodes, on the other hand, can be bolted in place. Fig. 3 shows a typical block-type anode installation for protecting a condenser head. Note that the stud is insulated from the anode and electrical connection is made through a resistance washer.

Washers are available in 1⁄4, 1⁄2, and 1 ohm resistances. When replacing those anodes, use a washer with the same resistance as originally installed. Fig. 4 shows a method of installing sacrificial anodes in hot water tanks. Domestic hot water tanks usually use vertical rods or sectionalized anodes. The sectionalized anodes are made of magnesium sections on flexible wire. The wire can be bent to allow installation of long anodes where headroom is low. Generally, equipment type anodes should be replaced with similar items. Do not change anode material or anode shape without getting the corrosion engineer’s approval. Different anode materials establish different cell potentials and levels of protection. Changes in anode shape change the physical area protected.

Installing and replacing anodes in soil includes excavation, placing and backfilling, restoring the electrical connection to the test station or structure, and testing and adjusting the output.

Anodes are normally installed in holes augered near to the structure to be protected. Since current output is largely determined by soil resistivity, it is desirable to make each hole as deep as possible in an attempt to reach permanent moisture. Whenever possible, anodes should be installed at a lower elevation than the protected structure so that, should the ground water level go down, the anode will remain in conductive moist soil even though the soil around the structure has dried out. In case rock is met, the anode may be placed horizontally above the rock. The anode should be installed at least as deep as the bottom of the structure and a sufficient distance away to give current throwing power (the distance along the pipe that protection is achieved). Anodes attached to coated pipe should be installed at least 2 feet away from the pipe. Anodes installed on a bare structure should be installed at least 5 feet away.

To simplify installation and reduce costs, prepackaged anode units have been developed. The anodes are prepackaged in standard anode backfill and must always be used for galvanic systems installed in soil. The backfill serves as a good conductor to allow electricity to flow between the anode and the soil. Installations using galvanic anodes should maintain adequate bench stock of the most appropriate size for the local soil resistivity. Anode installation requires placement of the prepackaged unit in the hole. An additional advantage of the prepackaged unit is that the anode may be installed horizontally and will still be properly centered in the chemical backfill. Horizontal installation may be needed over rock or where only a thin layer of low-resistivity soil is available. Magnesium anodes must be carried as a standby level item where bench stock will not be maintained because of the consumption rate.

Prepackaged anodes are normally shipped in an outer plastic- lined paper bag containing one or more anodes depending on weight. The outer plastic-lined paper bag must be completely removed and discarded before installation of the anode. Only one anode should be installed in an augered hole. The anode should be lowered into the augered hole by grasping the neck of the cloth bag or using a fabric or rope sling. The anode lead wire must never be used to lower the anode into the hole. The hole must be backfilled in 6-inch layers and each layer must be well tamped around the anode. Care must be exercised not to strike the anode or lead with the tamper. If immediate testing is desired, water may be added only after backfilling and tamping has been completed to a point which completely covers the anode. Approximately 5 gallons of water may be poured into the hole. After the water has dissipated, backfilling and tamping may be completed to the top of the hole.

It is good corrosion control practice to install a magnesium anode in the open hole that has been excavated for repair of a leak if potentials show the level of cathodic protection is below recommended criteria. Installation should be at the maximum distance from the pipe that the hole will allow.

When anodes are installed in a low resistivity electrolyte, the current output may be too high. This excess current may adversely affect structures constructed of metals such as aluminum, zinc, or lead and will cause the anodes to be used up too quickly. If it is necessary to limit the anode current output, a resistor must be installed in the lead between the anode and the structure to prevent the excess current. The actual current needed should be determined by the corrosion engineer. The best way to adjust anode current is by trial, using a piece of nichrome wire as the resistor. Using a movable clip, determine the exact length of nichrome wire needed to properly limit the current, and permanently install that much nichrome wire in the anode structure circuit between the anode lead and the pipe lead. The nichrome wire may be carefully coiled on a pencil and the coil placed inside the test station after removing the pencil. If bare nichrome wire is used, care must be taken not to short the turns of the coils to each other or to the test station.

Test stations are necessary for measuring cathodic protection system performance. It is desirable to install all of the anodes on a collector wire through a test station. If this is not possible, one of every ten anodes should be connected to the pipe through a test station. The overall plan of test stations should be determined by the corrosion engineer. The two types of test stations used are discussed in paragraph 4-20.

Above grade stations may be made from a pipe or a weatherproof terminal box and a short piece of electrical conduit fastened to an upright steel or wood post. The post should extend into the ground 2 or 3 feet or more to give adequate protection against damage from moving equipment. It may also be desirable to extend the post above the box and paint it a conspicuous color so that it is visible at all times.

Prior to any excavation work in an area, all underground wires should be located and staked out, to avoid damage. Plastic warning tape must be installed in the ditch above the anode lead wires. The warning tape should be located 6 to 8 inches below the surface. The warning tape replaces the wooden board as a protective warning for direct buried lead wires.

Since cathodic protection systems are electrical, it is important that the current be allowed to flow where it is needed without unnecessary loss or restriction. The two usual causes of loss and restriction are high-resistance connections and poorly insulated connections. Connections to the structure will be exothermic welded or brazed and then insulated from the electrolyte. Conductor splicing should be avoided where possible. Permanent conductor splices should be accomplished by exothermic welding or crimped pressure connection with proper compression tools.

The simplest and best method of attaching leads to structures and getting a good electrical connection is the exothermic welding process, such as Thermit™ welding. Exothermic welding is a means of permanently fastening copper conductors to steel or iron structures or to another copper conductor. Powdered copper oxide and aluminum are burned to produce heat and molten copper. The molten copper flows over the conductor and the structure, permanently welding them together. The equipment is light and portable, and no outside source of heat or power is needed. The exothermic welder must be of the proper size for the pipe and wire size encountered. Fig. 5 shows a sectionalized view of an exothermic welding apparatus. Underground splices usually can be adequately insulated by three wraps of insulating electrical tape.

The completed exothermic weld and exposed copper wire must be coated with an exothermic weld cap filled with mastic (see Fig. 6). The cap must not be installed until after the weld has cooled sufficiently to prevent the mastic from melting or burning.