100 mV CRITERION APPLICATION
By Joseph Pikas NACE CP & Corrosion Specialist
NACE RP0169 states that external corrosion control of steel and cast iron piping may be achieved if there is “a minimum of 100 mV of cathodic polarization between the structure surface and a stable stable reference electrode contacting the electrolyte.” Measurement of cathodic polarization with the cathodic protection (CP) system ene rgized or polarization decay when the CP system is turned off are both acceptable methods of evaluating conformance to this criterion. Despite widespread acceptance of this protection criterion by the corrosion control industry, most CP practitioners prefer to evaluate their CP system effectiveness against the -850 mV potential criterion. criterion. The simplicity of a single potential measurement versus multiple measurements or continuous observation of cathodic po larization buildup or decay is the deciding factor for operators concerned with monitoring costs. However, aging coating systems and increasing use of IR free or instant off polarized potential measurements have resulted in increased use of the 100 mV polarization criterion. In a recent industry survey survey of 75 CP practitioners/users, practitioners/users, the number of companies employing the 100 mV polarization criterion has increased 300% (from 26% to 76%) in the last ten years. While most of the the survey respondents report using the polarization criterion for only a small percentage of their system (11% on average), the trend towards more widespread use in locations where it offers advantages of users is clearly established. The following discussion assumes familiarity with NACE TM0497-97, “Measurement Techniques Related to Criteria for Cathodic Protection of Underground or Submerged Metallic Piping Systems”. This report is intended intended to summarize the circumstances where where the 100 mV polarization criterion is most often utilized, as well as identify some of the sensitivities and problems associated with testing to this criterion. WHEN TO USE THE CRITERION
The majority of CP practitioners surveyed utilize the polarization criterion only in areas where it is uneconomical or impractical to a chieve the -850 mV CSE potential po tential criterion. However, there may be circumstances where a polarization criterion is preferred for technical reasons. These rationale are categorized as follows:
-850 mV CSE is Impractical:
Large bare structures such as tank bottoms, bare (uncoated) pipelines, and structures where older coatings have significantly deteriorated such as on older pipe systems make the -850 mV criterion impractical to achieve without addition of massive amounts of current to achieve compliance with the criterion. Simple application of depolarization measurements will often reveal that a structure with polarized potentials more positive than -850 mV CSE have depolarized potentials in the order of -300 mV to -600 mV. Attempts to achieve -850 mV CSE polarized potential are therefore unnecessarily conservative and expensive. Native Potentials are Unusually Low:
The -850 mV polarized potential criterion has been arrived at in order to afford assurance of corrosion protection in most situations. It is therefore conservative and in some locations the structure will be protected with much less current. Well drained, aerated soils (such as sand) are known to be associated with extremely low native structure potentials. . In this circumstance, the 100 mV polarization criterion is an appropriate criterion for CP designers. Native Potentials are Unusually High:
In cases where native structure potentials range between -750 and -800 mV CSE, often associated with wet, low resistivity, low oxygen, acidic or sulfate reducing bacterial containing soil or high pipe temperatures, simply attaining -850 mV CSE may not meet the intent of RP-0169. Clause 6.2.2.2.2 states “In some situations, such as the presence of sulfides, bacteria, elevated temperatures, acid environments an d dissimilar metals, the criteria in Paragraph 6.2.2.1 (-850 mV) may not be sufficient”. CP practitioners who encounter such rest potentials during surveys are advised to use the 100 mV polarization criterion. IR Free Potentials are Unattainable due to Non-Interruptible Current Sources:
The two most common situations that render the -850 mV CSE polarized potential criterion difficult or impossible to evaluate are (pipelines) where sacrificial anodes are directly connected without test stations, or where foreign c ompany rectifiers which influence the protected pipeline may not be interrupted. The existences of these noninterruptible current sources mean that simple interruption of the impressed current sources will not ensure that IR free polarized potentials are reliably obtained. Coupon test stations may be used under these conditions to establish protection in accordance with the 100 m polarization criterion. Coating Preservation:
Many pipeline systems are comprised of sections with greatly varied coating efficiencies. Protecting bare pipe to -850 mV CSE in these instances can result in very high polarized
potentials on the well coated parts of the protected structure leading to accelerated coating damage. Several pipeline operators cite coating preservation as a significant factor in their decision to use the 100 mV polarization criterion. The True Criterion:
Because the polarized survey involves an interrupted on/off survey and a second depolarized survey some time later, the additional cost of the polarization survey must be balanced against the additional cost of CP to assure one of the first two NACE criteria. Many will perform an on potential or an off potential survey first and then, when adding incremental cathodic protection is undesirable, proceed to perform a polarization survey to determine whether or not 100 mV or more of polarization exists. IMPLEMENTATION
In addition to the test methods outlined in TM0497-97, the following comments are provided: Polarization Growth vs. Decay:
Application of any monitoring procedure should be practical. It is recommended that new, unprotected structures have a complete native state (static) potential survey performed prior to initial application of cathodic protection. This will ensure that unusually high native potentials are considered appropriately, and provide an excellent baseline for a polarization survey. This survey will necessarily be conservative for two reasons: a) the rest potential of the protected structure will typically become more positive as the system ages, and b) if monitoring of the cathodic polarization is terminated when 100 mV of polarization is achieved a steady state level of polarization (which would be more cathodic) may not yet be achieved. For existing facilities, it is most practical to establish the level of polarization through monitoring of depolarization. This test will also be conservative, because the native potential may not yet be achieved once 100 mV has been ob served and the CP systems re-energized. It is important to observe basic guidelines for monitoring either polarization or depolarization: Seasonal Variations in Native Potentials:
Seasonal variations in native (free corrosion) potential vary geo-climatically, with structure age and CP history. These variations can change the native potential by 100 mV or more. It is therefore critical that the native potentials be taken within a short time of the polarized potential. For these reasons the depolarized potential should be taken every time the 100 mV criterion is evaluated in order to correctly determine the amount of polarization and confirm a 100 mV shift.
Because the polarized potential and native potential may fluctuate seasonally, a conservative approach is to perform the 100 mV criterion evaluation when polarization is the lowest. Polarization tends to decrease when moisture around the structure increases, and the structure temperature increases. These cycles are typically seasonal in nature and may or may not occur at the same time depending upon geo-climatic location and pipeline operating characteristics.
Long Line Currents:
Long line currents flowing during a depolarized survey can make the 100 mV criterion non-conservative. Long line currents arise when cathodic protection is switched off, as during a depolarized survey. They are caused when current flows between regions of the pipe with different potentials (anodic and cathodic regions) caused by many factors, most notably soil variations. When such current flows, the measured potentials will be distorted somewhat. In regions of active, corrosive soil, the very negative native potential will appear a little more positive due to these long-line currents. Since the amount of polarization is calculated by subtracting this slightly lower potential from the polarized potential, the total amount of polarization will appear greater than the actual amount of polarization at this location. Since this location corresponds with the more corrosive soil on the pipeline, it may be prudent to increase the amount of polarization in these regions to more than 100 mV or to use other tools such as cathodic protection coupon test stations or in-line inspection data to assure corrosion protection. High pH Susceptibility:
RP0169 advises that “caution is advised against using polarized potentials less negative than -850 mV for cathodic protection of pipelines when operating pressures and conditions are conducive to 1 stress corrosion cracking….” The November 1996 SCC report by the NEB lists in Table 3.1 (pg. 16) that high pH SCC can occur in the range of -600 mV to -750 mV. More recent research suggests that this range is also a function of pipe temperature and may be a little smaller than indicated in the 1996 report. The Importance of Reference Electrode Placement
TM0497 recommends identifying “the location of the electrode to allow it to be returned to the same location for subsequent tests.” This is critical where soil composition is highly variable and/or has high resistivity, as reference electrode placement variations of 50cm or less can significantly affect the potential measurements. It is recommended that the ground be temporarily marked with flags or paint between measurements. Permanent markings are also useful for subsequent surveys.
Current Interruption Strategies
To implement the 100 mV polarization criterion, the current sources affecting the structure are interrupted. This is difficult, if not impossible, on galvanic cathodic protection systems where the anodes are connected directly to the structure. In this type of situation, cathodic protection coupons are being used. This is accomplished by connecting the coupon to the structure so that it is protected with the structure. The protected current to the coupon is interrupted and the polarization measured on the coupon. On impressed current cathodic protection systems, all current sources affecting the structure are interrupted. This will include the impressed current sources on the system, bonds to other systems, and impressed current sources on other systems that may be influencing the potential on the system under test. Typically, interrupting two or three impressed current sources either side of the section being tested is sufficient. When interrupting more than one current source, the current interrupters are usually synchronized for ease of data interpretation. Special current interrupters with accurate clocks are manufactured for this purpose. There are also current interrupters that use the signals from GPS satellites to synchronize and thus control the interruption cycle. To minimize the depolarization of the structure during testing, the time the current is off is minimized compared to the on time. A duty cycle of 75 to 80 percent current on is often used. An example would be eight seconds on and 2 seconds off. The interrupter can be either a slow cycle (8/2 seconds) or a fast cycle (800/200 milliseconds). Some people believe that there is less depolarization of the structure during the off time using the fast cycle interruption. Interrupters can be installed anywhere on the current source that is convenient. They are often installed in one of the DC output leads, the AC input leads, or across the tap settings of rectifiers. The location is often dictated by the current and voltage interruption rating of the interrupter. When placed in the DC circuit, the interrupter adds resistance in the circuit and therefore decreases the current output for the rectifier. The location of the interrupter can sometimes have an effect of the shape of the waveform after interruption and thus affect how long after interruption one should wait before taking the off potential. Oscilloscopes are often used to monitor the DC waveform to ensure that all interrupters are synchronized and to determine when the off potential should be recorded. Other DC current sources such as TEG’s, solar cells, etc are usually interrupted by placing the current interrupter in one of the DC leads. Polarization characteristics
Polarization of a structure is time dependent. A plot of polarization versus time will generally result in an exponential curve. The shape of this polarization curve is dependent on the time constant of the structure. The time constant is a function of the
total current applied, the amount of bare metal, the size of the structure, the environment and other factors. Polarization times can range from a few seconds to days to weeks. One means of determining the polarization time is to use a recording voltmeter to generate a plot of the polarization curve. The time for a structure to polarize after current is applied may be considerably different form the time for the structure to depolarize after the current is removed. Measurement of the polarization on a structure requires the measurement of the instant off potential. This measurement requires the installation of current interrupters on the current sources influencing the structure. The instant off potential can be obtained by interrupting all the current sources simultaneously or by interrupting each current source one at a time and using superposition to determine the actual instant off potential. Depending on the location where the interrupter is installed and the resistance, inductance and capacitance of the circuit, interruption of the DC current can cause spiking in the instant off potential and thus erroneous data. Polarization testing can be performed on a system protected by galvanic anodes if the current from the anodes can be interrupted. Isolated directly connected galvanic anodes do not have to be disconnected if there are sufficient other current sources to achieve the required amount of polarization. The amount of polarization of a structure is proportional to the current applied. The depolarized potential or native potential of a structure will decrease in magnitude with time and polarization, with the depolarized potential decreasing approximately in direct 2 proportion to the amount of polarization. Monitoring Practices
The 100 mV polarization criterion can be met by measuring either the amount of polarization from native potentials or the depolarization after the cathodic protection system has been in service time to allow for complete polarization. Polarization testing is usually performed by measuring the native o r static potential of a system before any current is applied. The cathodic protection system is then energized and the system allowed to polarize until the on potential stabilizes. On and instant off potentials are then measured with the polarization being the difference between the instant off and the native potential. Coupons can also be used to measuring the amount of depolarization. Depolarization testing is usually performed by measuring the on and instant off potentials, turning the current sources off, and allowing the system to depolarize. After the system has depolarized, the potential is again measured. The amount of polarization is the difference between instant off potential and the depolarized potential. Depolarization may take from a few minutes to a number of weeks. . Once the 100 mV criterion has been established at a location, verification is usually performed on a periodic basis. . One way to accomplish this is by repeating the complete
polarization testing. Another is to use either the on or instant off potential reading obtained during the initial testing as a measure of the effectiveness of the criterion if there have been no significant changes in the system being protected. Retesting time frames for performing a complete polarization test usually vary from one to five years with some going longer. Retesting is usually driven by changes in the system that is being protected. These changes can include changes in rectifier current output, changes in the amount of material being protected, changes in cathodic system configuration, environmental changes, etc. Advantages and Disadvantages Cost of survey
The cost to perform a survey to demonstrate the 100 mV shift criterion is greater than the –850 mV criterion. There is duplicate effort required in that the areas have to be surveyed twice. In addition, the cost of analysis is more, since the interpretation of the data is more involved. The areas that meet the –850 mV criterion may not be resurveyed to demonstrate 100 mV shift criterion in order to reduce the cost. Since the 100 mV criterion is often applied to older pipe systems, some compa nies perform close interval surveys of the on, instant off and depolarized potentials. This also adds additional cost. Additional cost in some areas is due to the time required to depolarize, in some cases requiring 30 days or more to fully depolarize. Conversely, performing a 100 mV polarization survey can reduce costs by reducing the amount of remediation necessary to demonstrate compliance to criteria, as well as allow work to go on a more practical schedule, since compliance has been demonstrated, and prompt remedial action timeframes are not required to be met. Cathodic protection coupons can reduce the cost of obtaining the polarization shift data. The 100 mV shift criterion is not be valid for use on systems with the interconnection of different metals (e.g. steel pipe connected to copper grounding system) In addition, for galvanic cathodic protection systems, a depolarized survey may not be practical to perform. Cost of current
Facilities that meet the 100 mV shift criterion have less current required in most soil conditions. Applying the criterion is almost always less expensive than recoating or installing additional cathodic protection current. Reduction of cathodic protection current should always be performed following complete depolarization. This may be necessary where the cost of current is extremely high, for
example solar, engine generator, wind, thermo-electric generation, or other sources of cathodic protection current. Bare pipelines can rarely meet the –850 mV potential criterion, and other criteria are usually more expensive to evaluate.
Level of Protection
The 100 mV shift criterion is a more technically correct criterion than potential criteria. There are concerns about the possibility of high pH SCC. Some regulatory agencies do not accept the 100 mV shift criterion. Annual demonstration of compliance can be expensive if the entire survey is required to be performed annually. The 100 mV shift criterion can reduce the possibility of overprotection, including cathodic disbondment and hydrogen embitterment, and reduce the risk of stray current. Effects of Heat and Bacteria
Hotter environments and the present of bacteria tend to increase the amount of polarization required to obtain protection. In these cases, more than a 100 mV of polarization may be required. IR Drop Errors
IR drop errors can be a problem in the interpretation of some of the cathodic protection criteria. Due to using the instant off potential in the 100 mV polarization criterion, IR drop errors are not a problem. Stress Corrosion Cracking (SCC)
SCC can be a problem for some pipelines operating in certain environments. One of the variables that affect the susceptibility of a pipeline to SCC is the instant off potential. The instant off potential of pipelines protected using the 100 mV criterion may fall in the range of potential that are susceptible to SCC. It may be necessary to increase value of polarization where SCC is suspected such as underneath shielded high dielectric type coatings. Note: For Additional Information see NACE TM0497-97, “Measurement Techniques Related to Criteria for Cathodic Protection of Underground or Submerged Metallic Piping Systems”.
