4. UNDERSTANDING AGEING OF UNDERGROUND PIPING AND TANK SYSTEMS
4.10. Mitigation methods
4.10.1. Cathodic protection systems
CP can be applied to steel and other metals that are immersed in an electrolyte (soil or water). It can be used to minimize or eliminate all forms of corrosion, is recommended for buried steel pipelines and tanks and can extensively reduce life cycle costs of the protected equipment.
CP is based on the decrease of corrosion potential by application of a current (either externally impressed or deliberately derived from dissimilar metals). Protection is achieved when the potential of local cathodes is electrically polarized to be more negative than the local anodes. The system should be operated so that CP criteria are met at every location in the system.
CP is normally applied in accordance with a consensus standard recognized by national authorities. European standard EN 12954 [44], for example, has sample guidelines on specific requirements for buried or immersed metallic structures.
There are three methods of applying the required electrical current: sacrificial anode systems, impressed current systems and combination or hybrid systems.
4.10.1.1. Sacrificial anode cathodic protection
Sacrificial anode (SACP) or galvanic CP systems are those in which protective current is provided by a metal of more electronegative corrosion potential than the protected item.
Such systems can provide protection to small areas due to their limited current capabilities. Anodes are typically zinc, magnesium or aluminium (Fig. 78) and are generally limited to soil resistivities of less than 50 000 Ω cm [12]. Sacrificial anodes in buried applications are typically magnesium or zinc. Aluminium is not
TABLE 18. SAMPLING OF UNDERGROUND PIPING AND TANK ISSUES IN NUCLEAR POWER PLANTS (cont.) PlantProblem areaRemedial measure implementedReferences Vermont Yankee USA (2010)Two reported underground piping system leaks which released tritiated water into the environment (January and May). Causes were inadequate construction and housekeeping practices employed when the
Advanced Off-Gas Building was constructed
in the late 1960s and early 1970s, and when a drain line was added in 1978; and inef
fective monitoring and inspection of
vulnerable SSCs that eventually leaked radioactive materials into the environment.
Sources identified, pipes repaired, contaminated soil excavated and nearly 1.1 ML of tritium-contaminated groundwater extracted from the site
[182, 192]
Hatch USA
(2011)
Elevated tritium outside of condensate storage tank. Source of leak was from condensate transfer line.
Use of transfer piping terminated[172, 192]
Hatch USA
(2011)
25.4 cm (10 in) safety related service water piping found with pinhole leaks during ultrasonic testing (UT). Soft, low alkaline water and high levels of dissolved oxygen provided a corrosive environment.
Temporary repair made with metal plate until full replacement possible. Increased inspections and long range plan implemented.
[172] Summer-1 USA (2011)
Leak from radioactive liquid waste pipe. Leak had been retained within exterior CS guard pipe until it filled and overflowed a containment structure. Degradation was caused by MIC.
Yoloy piping (see Section 4.2.1.6) replaced with stainless steel with no guard pipe.
Additional system changes and leak detection added. Additional borescope inspection of similar piping.
[1] Wolf Creek-1 USA (2012)
Poor backfill installation related to grounding cable installation impacted on emer gency service water piping (requisite amount of backfill for ESW piping removed). Both ESW trains were deemed inoperable.
Site ground penetration permit/excavation procedures reviewed. Gap analysis of fast-track modification processes performed.
Oconee-1 USA
(2012)
Pits discovered (some through wall) on buried diesel fuel oil storage tank. Most likely cause was MIC. No detectable leaks to ground were noted.
Changed regular inspection intervals. Review of chemistry control of tank oil (perhaps including addition of a biocide).
[179]
St. Lucie USA (2014)Blockage in the site’s storm drain system caused water to back up within the emergency core cooling system pipe tunnel. Water
entered the reactor auxiliary bay through two degraded conduits that lacked internal flood barriers. Four additional conduits discovered during extent of condition review
.
Installing qualified internal water seals on all affected conduits[89]
FIG. 78. Zinc, aluminium and magnesium sacrificial anodes and graphite and mixed metal oxide impressed current anodes (courtesy of Corrpro Companies Inc.).
practical for buried installations (because of problems associated with keeping it electrically active with good efficiency), but is commonly used in specialized marine applications.
External power sources are not required for sacrificial anodes. Anodes are directly connected to pipe (typically through a test post) and buried relatively close (usually less than 3 m) to the structure at depths at least equal to the structure (Fig. 79). Typical designs allow for 10–15 year service lives of installed anodes before replacement is necessary. The systems may be used on a temporary basis for new or existing pipelines until a powered impressed current becomes available.
Both horizontal and vertical installation of galvanic anodes is possible; however, vertical beds are generally preferred. Test posts allow for monitoring levels of CP and anode effectiveness. If anodes are connected directly without a test post it becomes impossible to take effective measurements.
In some applications mixed metal oxide (MMO) coated titanium wire or ribbon form can be buried in trenches during construction and run adjacent to the pipeline for its entire length. Sometimes two wires opposite from each other are required to prevent shielding [199].
Using specially prepared anode backfill can help to stabilize anode potential, prevent anode polarization, lower anode to earth resistance (increasing current output) and improve efficiency by reducing anode self-corrosion [200].
4.10.1.2. Impressed current cathodic protection
Impressed current cathodic protection (ICCP) systems utilize direct current (normally produced from AC by a transformer rectifier) in conjunction with relatively inert anodes such as graphite, thin coatings of platinum or activated MMOs on metals such as titanium, niobium, lead alloys or silicon-iron (Fig. 78). Such materials have lower consumption rates than those used in typical SACP anodes. In some cases a consumable anode such as scrap iron or steel is used. Such systems are typically not limited by structure size or soil resistivity. Anodes are available in many configurations, including rods, discs, meshes or ribbons.
ICCP systems utilize an outside power supply (rectifier) to control voltage between the buried component and an anode in such a manner that the pipe becomes the cathode in the circuit and corrosion is mitigated.
Equipment required includes an external power source, a transformer to step down incoming AC power, a rectifier to convert AC into DC and a control circuit to adjust current provided to the structure (Fig. 80). If AC power is not available, solar power, thermo-electric generators, diesel generators or windmills may be used.
Many anodes are typically required to provide desired design life and are buried some distance from the structure in an ‘anode bed’. Anode beds can be installed in many configurations, including shallow or linear arrangements
FIG. 79. Sacrificial anode cathodic protection system (courtesy of Cathodic Protection Co. Ltd) [199].
or in deep beds that can be 30 m deep or more. Deep beds accommodate congested areas where shallower beds may interfere with other structures, or pipes may be shielded from the anodes by other buried equipment. Deep installations need to consider any groundwater or aquifer impacts as part of their design, for example, by ensuring surface contaminants do not enter groundwater via any installed anode wells. NACE SP0572 [202] provides more detail regarding these deep bed systems. Linear anodes are long assemblies used extensively for new construction, since they can be readily installed in the same trench as the protected pipe [16].
Special carbonaceous backfills can be utilized to reduce anode consumption and circuit resistance. Such backfills bear most of the consumption resulting from current discharge.
In dry climates anode ‘dry out’ can be an issue. Anode dry out occurs when chemical reactions at the anode–soil interface result in the consumption of water through electrolysis that is not sufficiently replaced by groundwater. The condition can be addressed by an artificial water supply or by reducing CP rectifier current until equilibrium is reached.
In addition to the rectifier and anodes, electrical connections from the rectifier to the pipe and the anode bed are required (Fig. 81). Correct polarity is essential to such connections to ensure that it is the anodes that corrode as opposed to the pipeline structure.
In addition to the equipment providing protection, post mounted, wall mounted or flush-to-grade test stations are needed to monitor CP system effectiveness. Sufficient locations are required to provide a representative sample of pipe-to-soil potentials that are used to gauge CP system performance. Such stations use ‘permanent’ reference electrodes (typically copper sulphate; see NACE TM0211 [203]), voltage drop free coupons and/or corrosion rate (electrical resistance) probes at or near the pipe depth to assess the CP system.
Coupon test stations manufactured of the same material as the protected pipe are often installed at pipe crossings, where copper grounding grids are nearby, where pipe enters a concrete building wall and where testing indicates higher current drain from the CP system. These can provide independent data on corrosion rates and CP system performance. NACE SP0104 [301] (formerly RP0104 [204]) provides guidance in using such coupons with CP systems.
Isolating devices may be used in congested areas to isolate pipework to be cathodically protected from the ground grid and any other piping systems. They can thus reduce the amount of current needed as well as electrical hazards for personnel. Types of devices may include insulating couplings or DC decoupling devices. The DC devices only block DC current flow while still allowing AC currents and lightning to pass through to the ground. The use of isolating devices in areas where piping is congested may result in stray current corrosion. NACE SP0286 [205]
provides guidance regarding the use of insulating devices for cathodically protected pipelines [16].
FIG. 80. Typical impressed current cathodic protection rectifier [201].
4.10.1.3. Pulse cathodic protection systems
Pulse CP is a special application of ICCP systems that gives out short DC impulses to underwater or underground metal structures. It has been used extensively in the oil and gas industry for well casing protection and early water applications dating back to the 1970s. As a rule, voltage is provided for 10% of the time and so the influence of neighbouring metal structures is lowered when compared to traditional CP. Typical pulse frequencies are in the range of 1000 to 10 000 Hz [59].
High-ampere current impulses flow on the metal structure, which gives it a significant charge. Free electrons quickly spread the charge along all metal surfaces. The metal structure has its own induction, which limits the speed of charge movement. Charge in the metal structure, even for complex configurations, is distributed, so that the vector of the intensity of the electric field is perpendicular to the metal surface. In places where coatings are damaged, electric current leakage may occur. This causes protective electrochemical reactions on the border of the metal-electrolyte transition. In longitudinal objects, e.g. pipelines, the phenomenon of self-induction ‘pushes’
the charge further away, providing the best protection to more remote areas. Therefore, the application of pulse CP systems can lead to more uniform structure protection and widen the protected area.
It is possible to select pulse current parameters at which O2–H2 water dissociation occurs and then recombines after interruption of the current pulse. This reduces H2 formation and the probability of coating damage and other damage due to hydrogen stress cracking.
Sample pulse CP systems developed by the Orgenergogaz and GeoLineGroup companies in the Russian Federation are shown in Fig. 82 and have technical parameters described in Table 19.
Pulse CP operates as follows: the electrical field impulses force positive ions to move towards sites of the pipeline with damaged covering, and negative ions (which cause corrosion) to move towards the anodic ground electrode. A layer with a low concentration of negative ions will form near areas of damaged covering. After the impulse stops, negative ions from areas of high concentration diffuse back towards the pipe; however, this process is much slower than the movement of ions in the electric field. Therefore, the pause duration can be set to be longer than the impulse duration (e.g. 10× longer), economizing on the consumed electric power. The next impulse is given before the concentration of the negative ions near the pipe has risen up to a certain appreciable low level.
During ICCP hydrogen ions penetrate partially inside the pipe material. They make a complex combination with iron ions, which form a crystal lattice, and with impurities partially turn into hydrogen molecules which mainly release into the atmosphere. Then a small amount of molecular hydrogen diffuses into the pipe material which gradually makes it brittle.
TABLE 19. SAMPLE PULSE CP SYSTEM PARAMETERS
Technical parameters Powered by
FIG. 81. Impressed current cathodic protection system.
Those processes become much less intensive during pulse CP. According to Faraday’s first law of electrolysis, the mass of evaluated hydrogen is directly proportional to the quantity of electricity transferred through an electrolyte. During pulse CP, the quantity of electricity is many times less. The recombination process reduces the concentration of hydrogen near the pipe surface even more. Therefore, there should be no hydrogen absorption in pipelines by pulse CP, or it should occur much more slowly in comparison with ICCP systems.
Figure 83 illustrates a surface area of electrodes made from thermo-expanded graphite (external sides), which were situated in electrochemical cells under action by impulse and DC currents. The photo on the right distinctly shows surface swelling, indicating gas release inside of the electrode under DC action. The electrode removed from the electrochemical cell through which the impulse current was passed did not indicate any swelling. This experiment shows that gas release is much lower under pulse current CP than under ICCP.
Pulse CP systems can thus potentially (when compared to traditional ICCP) be smaller and more efficient (in terms of power savings, increased electrode life and fewer repairs), and provide better protection.
TABLE 19. SAMPLE PULSE CP SYSTEM PARAMETERS
Technical parameters Powered by
electric line ~220 V Powered by gas energy
Output voltage (V) 24 24
Pulse current (А) Up to 80 Up to 35
Duration of impulse (s) 0.8–1.2 0.8–1.2
Duration of pause (s) 4.5–14.5 4.5–14.5
Consumption of power (Wt) 500 90
Overall dimensions (mm) 430 × 180 × 350 870 × 550 × 450
Weight (kg) 10 85
Service life (years) 10 10
FIG. 82. Pulse cathodic protection (CP) systems: (left) pulse CP station powered by electric line; (right) pulse CP station powered by autonomous electricity source (courtesy of Rosenergoatom).
4.10.1.4. Hybrid systems and cathodic protection comparison
Hybrid systems consist of a mixture of sacrificial anodes and externally powered impressed current anodes.
Sacrificial anodes provide protection when the ICCP system is shut down for inspection and maintenance, or at difficult areas called ‘hot spots’.
In most cases, ICCP systems are designed to deliver relatively large currents from a limited number of anodes, while SACP systems are designed to deliver relatively small currents from a larger number of anodes. At nuclear power plants, because piping is connected to the copper grounding grid, most CP current will flow to other structures. Thus current requirements will be large and the ICCP method is almost always used. Unless the anodes are used for ‘hot spot’ protection, the SACP method of protection should only be used for coated and electrically isolated pipes. Table 20 provides a comparison of the two methods.
4.10.1.5. Buried tank cathodic protection
External surfaces of buried tanks can be protected by SACP or ICCP systems. Blanket systems that protect multiple tanks can also be installed. Isolation from other metallic structures, including grounding systems, is often necessary. The large ground grids typical at nuclear power plants make the current levels required for CP of underground tanks significantly higher than those required to protect isolated systems. ICCP systems are thus more typically used at nuclear power plants.
Tanks that contain water or a water phase (e.g. oil or petrochemical tanks with an aqueous phase) can be protected by internal CP. Water tanks can be protected by SACP or ICCP (depending on electrolyte resistivity) and oil tanks by SACP. SACP systems typically have anodes welded or bolted directly to the structure, while ICCP systems have anodes suspended from the roof, drilled through the tank walls or suspended from eyelets mounted in the tank [199]. Biocides may be added to fuel oil to reduce potential MIC.
ICCP anode cable tails need to be routed back to the power source, normally located in a non-hazardous area.
In particular, for ICCP systems it is recommended that permanently installed monitoring systems be installed to ensure adequate levels of protection.
Factory-fabricated and field installed SACP and ICCP systems are available for new USTs. SACP systems for new USTs consist of coatings with high dielectrical qualities, electrically isolated fittings and sacrificial anodes.
Pre-engineered USTs are increasingly available with CP already incorporated (see Fig. 15 in Section 4.2.5).
TABLE 20. COMPARISON OF CP CHARACTERISTICS [206]
Sacrificial anode system Impressed current system
No external power required External power source required
Simple design More complex design
Fixed driving voltage Adjustable driving voltage Limited current (10–50 mA typical) Larger current (10–100 A typical) Typically used with electrically
isolated, well coated structures May be used with electrically isolated structures and grounded structures that are coated or bare
Typically used in soils with lower
resistivity May be used in low resistivity or high resistivity soils
Lower maintenance Higher maintenance
Does not cause stray current corrosion Stray current corrosion can be generated on isolated structures that are in close proximity to the anode ground bed
FIG. 83. Thermo-expanded graphite electrodes: (left) after action by pulse current; (right) after action by DC (courtesy of Rosenergoatom).
Guidelines regarding the design, installation and testing of CP systems for USTs and piping can be found in API RP 1632 [207], NACE SP0169 [208], NACE SP0285 [209], STI R892 [210], STI R051 [211], STI R972 [212]
and in [213]. An assessment guide for determining the suitability of a buried steel tank to be upgraded with CP is available from ASTM [214].
4.10.1.6. Concrete pipe cathodic protection
CP can be effectively applied to PCCP if electrical continuity is established along the line. Electrical continuity is accomplished by a combination of small gauge shorting straps installed under the prestressing wire prior to wrapping and by installing bonding jumpers across each pipe joint as the pipe is installed. Bonding jumpers are connected to steel components of each pipe length. ICCP systems should be used with caution when providing CP for prestressed concrete pipe due to the possibility of overprotection. Overprotection of the prestressing wires may cause hydrogen embrittlement, leading to their failure. SACP cathodic protection systems can be used to minimize this potential for embrittlement.
NACE has prepared a specific guide, RP0100 for CP of PCCP pipe [215], which can be referred to for more detail.
4.10.2. Coating application