GUIDELINES FOR AVOIDING GALVANIC CORROSION IN NUCLEAR...

17th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors August 9-13, 2015, Ottawa, Ontario, Canada

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GUIDELINES FOR AVOIDING GALVANIC CORROSION IN NUCLEAR PLANT PIPING SYSTEMS

George J. Licina1, Heather F. Jackson1 1Structural Integrity Associates, Inc, 5215 Hellyer Avenue, San Jose, CA, 95138, United States

ABSTRACT

Nuclear power plants continue to replace above-ground carbon steel pipe (bare or lined) with more corrosion resistant alloys (CRAs) for various reasons. Such modifications include replacement of above-ground cement lined carbon steel with corrosion resistant alloys such as 6% molybdenum or high molybdenum duplex stainless steels or replacement of unlined carbon steel service water piping near flow control devices with more erosion and erosion-corrosion resistant stainless steel alloys.

In essentially all such cases, as the piping is “alloyed up”, plants connect the dissimilar metals (i.e., the original carbon steel and the new erosion or corrosion resistant material) using flanges and insulating kits to avoid galvanic corrosion at the point of connection. In piping systems that are electrically isolated, such an approach works well. However, in nuclear plants, where all buried piping, structures, etc. are commonly grounded, the common grounding can “defeat” the insulating kit, merely changing the shortest electron path from the shortest distance between the carbon steel and the corrosion resistant alloy to a path through the plant grounding grid.

This paper will describe the results from an EPRI1-sponsored project �[1] to provide plants with guidelines for preventing galvanic corrosion in above-ground dissimilar metal piping systems. The project considered the requirements and unique design features of piping, including buried piping, in a nuclear power plant.

Keywords: galvanic corrosion, cathodic protection, insulating kits, grounding

1. BACKGROUND Nuclear power plants continue to replace above-ground carbon steel pipe (bare or lined) with more corrosion resistant alloys (CRAs) for various reasons. Several examples are described briefly below. Galvanic coupling of the carbon steel and CRA piping, even after an insulating flange has been installed between the two metals, continues to be an issue.

One plant example involved a design modification to remediate cavitation after a throttling valve. The modification replaced a short section of carbon steel pipe that contained a flow control orifice plate upstream of a butterfly valve with a similarly configured 6% molybdenum stainless steel (6% Mo) pipe to avoid that metal loss. The above-ground 10 inch fresh water service water system (SWS) piping provides emergency diesel generator (EDG) cooling. Both the carbon steel portions and stainless steel portions of the line have multiple pipe hangers attached to the concrete walls, ceiling, and floor.

A second plant example involved replacing all above-ground SWS piping, cement lined carbon steel from original construction, with 6%Mo stainless steel for improved corrosion resistance. The below ground pipe will remain cement lined carbon steel with no modifications.

1 Acronyms are listed in the NOMENCLATURE section


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1.1 Insulating Kits In most cases, like the examples described above, plants connect the dissimilar metals (i.e., the original carbon steel and the new erosion or corrosion resistant material) using flanges and insulating kits (Figure 1) to avoid galvanic corrosion at the point of connection and into the original carbon steel piping. In piping systems that are electrically isolated, such an approach works well. However, in nuclear plants, where all buried piping, structures, etc. are commonly grounded, the common grounding can “defeat” the insulating kit, merely changing the shortest electrically conductive path from the shortest distance between the carbon steel and the corrosion resistant alloy to a path through the plant grounding grid.

1.2 Objective The unique design features of nuclear plant piping, including buried piping, has produced galvanic corrosion in above-ground dissimilar metal piping systems despite the installation of insulating kits. The purpose of the guideline is to provide examples of designs and design modifications that may be susceptible to galvanic corrosion and that can benefit from installation of appropriate hardware and to provide guidance on selection of insulation or isolation approaches, installation of hardware to avoid galvanic corrosion, and testing and maintenance of those systems.

2. POWER PLANT DESIGN FEATURES

2.1 Power Plant Design and Grounding Power plant buried piping, unlike cross-country transmission piping or many other piping systems, is commonly grounded. The common grounding is done for personnel safety (e.g., there are numerous motor-operated valves, potential for lightning strikes, etc.). Common grounding also helps to eliminate or minimize the risk of stray current corrosion, at least from piping “owned” by the utility. The commonly grounded pipe does require that a significantly greater amount of cathodic protection (CP) current is used since a large fraction of the CP current is dispersed to, in effect stolen by, the grounding grid, building rebar, etc. as opposed to all of that current providing corrosion protection. In general, at least in the non-power plant world, common grounding and cathodic protection are considered to be mutually exclusive practices.

Power plant piping is supported by pipe hangers, either from above or below, and those pipe hangers are ultimately electrically attached to some part of a building and the grounding grid, as shown in Figure 2. Walls, ceilings, and floor are built from reinforced concrete in essentially all power plant structures. As noted above, that reinforcement is ultimately connected to the ground. The total effective resistance of an above-ground plant pipe will be determined by the design and construction of pipe hangers and the effective electrical resistance of the concrete (which will primarily be a function of the length of the path through the concrete, the concrete’s moisture content and the conductivity of that pore water).

2.2 Effective Resistance Between Dissimilar Metal Pipes In the following discussion, it is assumed that a dissimilar metal pipe (e.g., a CRA) will be used to replace a carbon steel pipe and that the two metals will be connected by an insulating flange kit. The discussion then focuses on the overall resistance between the CRA and carbon steel pipes through the hangers, building, and the site ground as opposed to the very high resistance that is or should be provided by the insulating kit.

The first consideration of the effective resistance between an above-ground pipe and the site ground deals with the method of attachment of hangers. For example, pipe hangers may be attached to the building using Hilti bolts or the equivalent, where the objective is to avoid the rebar. Alternatively, pipe hangers may be attached to the building via “welded to embed” connections. In the former case, the overall resistivity between the dissimilar metal pipes through the grounding grid will be a function of the


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resistance between pipe and hangers (hangers may or may not be electrically insulated from the pipe), the resistance of the hangers themselves (generally low), the “concrete path” between each hanger and one or more rebars, and the resistance of the concrete in that path.

Resistivity of concrete will be a function of the water content and wet vs. dry surfaces, aging of the concrete (e.g., does it dry out over time or gain moisture), cracking (including the sizes of cracks), and the different types of concrete within the plant (e.g., different aggregates, different water content, more or less entrained air, aggregate sizes). Resistivity is clearly a strong function of the free water content of the concrete and the nature of the water that may be present. For example, if ground water provides the major source of wetness, the conductivity of that water will strongly influence how much more conductive (how much less resistive) the concrete becomes as water intrudes into the concrete.

Figure 3 provides an electrical schematic of two above-ground pipes, connected by an insulating flange but also electrically connected through the site ground.

Most of the resistances shown are straightforward except that each RCi is comprised of the resistance of the concrete path to reinforcement, which will be a function of:

the distance between the end of the hanger (e.g., H1x) and the reinforcement, the resistance of the reinforcement in the concrete, the resistance of the concrete path between the reinforcement and another hanger

(e.g., RCi for that path), and the resistance of that second hanger to the pipe (e.g., R2x)

Obviously, for hangers that are connected to the reinforcement, the resistance of the concrete path is eliminated.

2.3 Design and Operational Factors Pipe related factors, such as pipe sizes, numbers and types of fittings, lining and lining condition, and flow (e.g., consideration of differences between normally flowing systems, normally stagnant systems, and intermittent flow systems) have essentially no effect on the effective resistance between the dissimilar metal pipes. However, pipe related factors can have a significant effect on the available options to mitigate low resistance paths between the dissimilar metal pipes.

3. DISCUSSION OF ALTERNATIVE APPROACHES

3.1 Insulation and Isolation Approaches Corrosion requires an anode (where dissolution occurs as electrons are lost), a cathode (where electrons are accepted; rates of electron loss and electron acceptance must be equal), an electrolyte, and an electron path.

Galvanic corrosion involves an electron path between dissimilar metals that are in the same electrolyte. That electrical coupling increases the corrosion rate of the more active metal (e.g., carbon steel) as the more noble metal (e.g., stainless steel) provides an area that can serve exclusively as the site for cathodic events, dramatically increasing the rate of cathodic half-reactions, increasing the overall corrosion rate, and focusing all of the metal loss to the more active metal.

Figure 4, Figure 5, and Figure 6 are examples of Evans diagrams that illustrate these effects. These types of plots are also often plotted in potential vs. current (rather than current density) space to incorporate the effects of surface area.

Evans diagrams are generated by adjusting the electrochemical potential of the material of interest (relative to a reference electrode, saturated calomel electrode, SCE, in the case of the examples shown


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below) and measuring the amount of current required to achieve that change in potential. The anodic and cathodic curves (positive slope and negative slope, respectively) are generated. The intersection of those two curves defines the corrosion potential and the corrosion current. The corrosion current is directly proportional to the corrosion rate. The slopes of the anodic and cathodic curves also illustrates the metal’s polarization characteristics (e.g., X mV/decade of corrosion current).

By overlaying the Evans diagrams for two different metals, each determined independently, the net anodic and cathodic curves and the corrosion potential and corrosion currents for the couple can be determined as shown in Figure 6.

Figure 4 shows that carbon steel exposed to a nuclear plant’s service water system environment (fresh water) exhibited a corrosion potential (also often called the open circuit potential) of -585 mV (vs. SCE) and a corrosion rate, determined from the measured current density, of 1.23 mpy. Those values are very consistent with commonly published values for potential (e.g., as given in a galvanic series) and corrosion rate �[2]. Figure 6 shows that a stainless steel/carbon steel couple will exhibit a much higher corrosion rate than the uncoupled carbon steel.

3.2 Key Attributes of Proper Isolation A plot like Figure 6 can be extended to provide an estimate of how much resistance between the carbon steel and the stainless steel would be required, independent of the approach to achieving that resistance. The blue arrow shows that if the anodic and cathodic branches for the stainless steel were shifted as shown, the intersection of the stainless steel’s cathodic branch with that of the carbon steel would occur at the same current density (corrosion rate) as existed with no coupling. Maintaining the assumption that the galvanic effect as shown in Figure 6 would exist even for an electrical resistance of 1 to 10 ohms (i.e., greater than the 0 ohms often assumed for galvanic couples), increasing that resistance by a factor of approximately 2200 (the ratio of current densities between the Ecorr/icorr intersection for the heavy green dashed lines for the Type 316 stainless steel and Ecorr/icorr intersection for the black dashed line that is drawn to illustrate the shift) could be sufficient to “cancel” the galvanic effect (Figure 7, blue arrow). As shown in the sample computations above, resistance through the hangers, concrete, and the grounding grid cannot necessarily be counted on to sufficiently isolate a couple like that shown in Figure 7 to avoid galvanic corrosion: the effective resistance through that long path can be significantly less than what is required to have the two metals corrode independently of each other.

It is possible to achieve proper isolation using insulating flanges, however, the common grounding that exists in a nuclear power plant as shown in Figure 2 can effectively defeat the insulation provided by that insulating kit. In situations where the above-ground piping configuration has sufficient room to install a short carbon steel pup piece and two insulating kits, it was possible to properly insulate the dissimilar metal joint. The following discussion is from an actual seawater cooled nuclear power plant example [4].

The sequence of materials in the piping system then went:

AL6XN® (6% molybdenum stainless steel) with flange

Carbon steel pup piece with flanges on both ends

Carbon steel original piping with flange

The short carbon steel pup piece enabled the contractor to properly install an insulating flange between the AL6XN and the pup piece AND to install another insulating flange between the pup piece and the original piping (i.e., two insulated joints) to achieve electrical isolation that was not compromised by hangers and grounding grid (Figure 8). The background on prior failures of the carbon steel/AL6XN joint and successes achieved by using two insulating flanges are provided in [4].


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Monolithic isolation joints (MIJ), a stand-alone section of “pipe” that is comprised of two “pup pieces” of the pipe size of interest that are connected by a factory-manufactured isolating joint that consists of fiberglass epoxy isolation rings, rubber seals on the “hub” side of the joint (Figure 9), offer a similar approach to that of insulating kits. However, all fabrication steps associated with insulation are done at the factory and the power plant owner can simply make similar metal welds of the two pup pieces that match the piping installation on each side of the joint. Such joints are hydrostatically tested at the factory and are then electrically tested to a minimum electrical resistance of 25 M-ohms (25,000,000 ohms) and a minimum breakdown voltage of 5 kV at 50 Hz for 1 minute.

MIJs offer greater ease of installation, and concerns with alternate electrical paths (at least within the flange assembly as a result of installation), flange leakage, gasket aging, etc. are eliminated vs. insulating flanges. GPT industries, one manufacturer of the monolithic flange joints, claims that these joints are cheaper than insulating flanges. While manufacturer claims that such joints are cheaper than insulating flanges are difficult to comprehend, it would be reasonable to expect that installation of an MIJ (i.e., two similar metal welds between materials that the plant is qualified to weld) may be more likely to assure proper installation than would a flanged insulating kit installed in the field. Further, the overall lifetime cost of MIJs could be significantly less than that of insulating flange kits.

While MIJs are almost certainly more expensive up front, they do offer several key advantages:

Leak tightness was built in and checked at the factory

The resistance of the dissimilar metal joint is NOT a function of site installation

Site installation will be via welding of similar materials (e.g., carbon steel/carbon steel on one end and stainless steel/stainless steel on the other)

However, concerns with short circuiting through the common ground has the same potential for defeating isolation of the MIJ as it does with insulating kits. The use of two MIJs, done in the same way that the two insulating kits were used as described above, can make MIJs an option for avoiding galvanic corrosion in above-ground, commonly grounded power plant piping.

3.3 Isolation vs. Insulation Figure 10 is a schematic diagram of a coated insert spool that reduces or eliminates galvanic corrosion of the more active of the two dissimilar metals. The approach shown in Figure 10 achieves that isolation by increasing the length of the electrolyte path such that the resistance of that path increases the effective resistance of the overall corrosion cell to the point that the corrosion of carbon steel is not influenced by the presence of the stainless steel. In effect, the electrolyte path, one of the four necessary conditions for corrosion to proceed, has been dramatically reduced or effectively eliminated.

As described in [5], the US Navy has evaluated methods for isolating seawater piping from galvanic corrosion that are essentially identical to the approach shown in Figure 10. In Navy applications, Alloy 625 (UNS N06625) was considered to be a superior piping material to the 70-30 copper-nickel (UNS C71500) that had historically been used. However, there was a definite expectation that the 70-30 could suffer from galvanic corrosion when coupled to the Alloy 625. As in other industries, the Navy’s initial selection for avoiding galvanic corrosion was the use of electrical isolation at the interfaces. Bimetallic rubber insert sound isolation couplings (RISICs), a standard commercially available product, and non-conductive hoses were included in the original design. As design progressed, it became clear “that RISICs and hoses cannot effectively electrically isolate the dissimilar metals because of the significant number of alternate paths for electrical shorting of the dissimilar metals. Such paths include pipe hangers, electrical safety grounding straps, electrically actuated pump and valve electrical grounds, and metal-braided hydraulic lines. Unintentional, intermittent grounding is also possible through the


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inadvertent placement of metal ladders, tools, and steel wool pads used for cleaning.” (emphasis added). Those issues are essentially identical to the issues faced by nuclear plants as discussed in Section 2.

A proposed solution to problems associated with full-time electrical isolation was proposed for evaluation. That alternative method used short, electrically isolated (but not ID coated) piping sections between the dissimilar metals. That approach would reduce galvanic corrosion by “increasing the electrical resistance of the seawater path through which the galvanic current must flow.” [5]

The study performed in [5] involved ten foot (3m) lengths of 2-inch (51mm) diameter 70-30 copper-nickel and Alloy 625 tubing that were connected either by a direct connection, through an RISIC or through a separator piece of 70-30 copper-nickel or Alloy 625 that was one, three, or ten feet long. Tubing with thicknesses and inside diameters that matched those of 2 inch SCH 40 pipe was procured for the test. Seawater at a low flow (


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a separator one foot or longer. With no separator, the potential at the interface is approximately -40 mV (Ag/AgCl), an 85 mV shift. With an RISIC only, the potential still shifts (i.e., is affected by the coupling to the Alloy 625) by more than 50 mV.

With 6 feet per second flow, the potential ranges at the interface vary less than for low flow (the purple ovals are smaller), and the effects of separator length continue to reduce the galvanic effect at the interface as the length of the separator increases. For example, with no separator, the potential at the interface is -40 mV (Ag/AgCl) vs. the freely corroding potential of approximately -150 mV (Ag/AgCl), a shift of more than 100 mV. If the interface is defined as occurring at 10 feet, neither manufacturer’s RISIC reduced the amount of shift due to galvanic coupling, however, if the interface is defined to be at 11 feet when either RISIC is in place, the RISICs reduce the potential shift due to coupling by approximately 40 mV. With a one foot long separator, the galvanic potential at the interface shifts are no different than from that with no separator or an RISIC. With a three foot separator, the potential shift is reduced by up 60 mV (but is nil for all but the Alloy 625 separator). With a ten foot long separator, the potential shift is reduced by 30 mV.

Table 1 shows that a reduction of integrated currents (hence corrosion rate) of greater than 50% was achieved for uninsulated separators that were approximately 60 diameters long. For an uninsulated separator of approximately 18 diameters length, the corrosion rate was reduced by nearly 50% after 6 months; nearly 40% after one year.

Post-test examination of the separators revealed that the 70-30 copper-nickel separators were corroded, most severely near their interface with the Alloy 625, but with attack persisting some distance into the separator as was expected. The ten foot long 70-30 copper-nickel test pipe (i.e., that was to be protected by the separator) also exhibited corrosion nearest the interface with the separator and along a portion of its length from that interface commensurate with the level of protection provided by the separation as illustrated by the electrochemical potentials and integrated currents.

Although the seawater experiments done by and for the Navy did not specifically test the hypothesis of whether insulation between either or both ends of the separator was actually necessary, the results clearly show that insulation between either end of the joint between the separator and either pipe had minimal effect. For example, the continuity of potential changes in the one foot length upstream and downstream of each connector shows that the insulator had no real effect.

Application of this approach to avoid galvanic corrosion between commonly grounded pipes offers a number of advantages in a power plant environment by eliminating the need to purchase and install an insulating flange (i.e., a regular flanged connection would work), no need for special (and numerous) hanger modifications that would defeat an insulating flange, elimination of concerns with temporary effects (e.g., casual grounds), and no need to add a DC decoupler to provide DC isolation while maintaining AC grounding.

Gartland, et al. [6] also faced the problem of galvanic corrosion in seawater piping systems. Typical materials pairs involved included carbon steel (typically cement lined) and titanium or stainless steel or highly alloyed stainless steels (e.g., 6% Mo grades) with copper-nickel piping. They addressed the problem in several ways including isolating flanges or insulated spool pieces. They noted that “electrical isolation is almost always ruined by electrical paths between the pipe parts through supports, pumps, etc.”

Insulated spools that Gartland, et al. used involved relatively short pipe sections, often made of a non-metallic material or a metal with a thick coating on the inside to provide a much longer ionic path rather than electrical insulation. They noted specifically that if an uncoated metal piece is used, electrical isolation flanges are required at both ends, implying that when a non-metallic or well coated insert was used for insulating spool pieces that electrical insulation at the end of the spool was not required.


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Figure 13, calculated corrosion profiles for stainless steel/carbon steel couples in seawater, showed that for equally sized stainless and carbon steel components that:

• Galvanic corrosion with no intermediate spool resulted in a corrosion rate on the carbon steel >10 times that of the uncoupled rate at the interface

• An insert two diameters long decreased that rate to


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Isolating spools, based upon increasing the electrolyte path, with or without electrical insulation between the dissimilar metal pipes, may consist of:

Coating applied along a defined length of the more noble metal

An ID coated spool fabricated from the more noble metal, with or without insulating kits on either end

Long uncoated metallic spool of the more noble metal or the more active metal (with that spool regularly inspected and considered to be disposable)

Table 2 provides a summary of the results from Appendix A of [1] along with results from other examples discussed here. The summary table lists the required insert length to reduce peak corrosion rate (i.e. at the dissimilar metal interface) to less than 150% of the uncoupled rate (i.e., a 50% corrosion penalty). The required lengths to achieve mitigation of galvanic corrosion are only common for seawater. Agreement between the very different approaches is actually very good.

The minimum recommended insert lengths for a coated or non-conductive spool for fresh and brackish water are as shown in Table 2. The recommended minimum insert length for seawater applications is 25 pipe diameters. Note that the bottom row of Table 2 is for a spool that is electrically isolated on both ends and that presents a different approach than all of the other rows in Table 2. An insert length of 2 to 5 diameters (i.e., freshwater or brackish water) should be readily accommodated in all system designs where dissimilar metal piping will be coupled to carbon steel or other non-corrosion resistant alloy piping. In the case of seawater systems, a 25 pipe diameter insert can be very long. For example, 25 pipe diameters in a 30 inch diameter piping system is 60 feet (probably three spools) long.

Local CP could be an effective approach to mitigate galvanic corrosion in post-construction dissimilar metal joints. However, the need to insert a third material (either a sacrificial anode or a non-corroding anode for an impressed current system) does introduce a foreign material into the piping system that could become loose. While local CP offers advantages, the methods described above are considered to provide more practical solutions at this time.

5. CONCLUSIONS Original construction materials coupled to more corrosion resistant alloys will create galvanic couples.

Insulation kits have often been used in cases where dissimilar metal piping is connected. Insulation kits have their own issues including fragility concerns with the insulators and sealing ability of such flanges. Power plant experience to date has shown that testing during installation of insulating flanges has been sporadic and that regular testing of resistance values after insulation has been nil.

Unique features of the nuclear plant such as common grounding of all systems and structures can defeat even well installed insulating kits.

Alternatives to insulating kits include:

Completely insulated spool pieces (requires two insulating flanges and no electrical connections between the spool piece and the plant), a length of approximately four diameters

Isolation that addresses the ion path rather than the electron path via:

o Long uncoated metal separators, insulated on both ends o ID coated metal separators (may or may not be electrically insulated on both ends) o Coating applied to the ID of the more noble pipe o Lengths of separation provided by the insert pieces will range from two diameters or

more in fresh water, with a range from 8 diameters to 60 diameters in seawater; where the


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required length is driven by the specific dissimilar metal pair and whether the corrosion on the more active metal is general (a shorter length is required) or is localized.

Specific recommendations, shown in decreasing order of preference are:

1. Coated or non-conductive insert as discussed above with detailed lengths shown in Table 2. The noble member in the galvanic couple (e.g., stainless steel, 6% Mo stainless steel) should be coated: the more active member (carbon steel, coated carbon steel, etc.) should not serve as the coated insert. No insulating flange is required.

2. A spool that is electrically isolated with a properly installed insulating kit on both ends and that has no hangers or other supports to the building.

3. Monolithic Isolation Joint. The required connections to the system will be similar metal welds and no on-site assembly and installation of insulating flanges will be required. However, the MIJs will have the exact same susceptibility to being defeated by the common grounding that insulating flanges do. It is also be possible to install two MIJs (one CS/CS; the other SS/SS); a combination of items 2 and 3.

4. Insulating kits. While this is the legacy approach, difficulties with insulating kits, both with sealing and with achievement or maintenance of high electrical resistance, have been stated regularly by site personnel. Such kits can be difficult to install, are often installed incorrectly, and they can and often will be defeated by the common plant ground.

6. REFERENCES [1] Guideline for Preventing Galvanic Corrosion in Above-Ground Dissimilar Metals Piping

Systems, EPRI 300200319, Palo Alto, CA, May 2014. [2] Service Water Piping Guideline, EPRI-1010059, Palo Alto, CA, September 2005. [3] Advance Products & Systems, Flange Isolating Gasket Kits. [4] S. Paul, Galvanic Corrosion Challenges and Solutions: AL6XN Interconnections to Carbon Steel

and Other Alloys, EPRI Buried Piping Integrity Issues Group, July 2013. [5] H.P. Hack, W. Wheatfall, Evaluation of Galvanic and Stray Current Corrosion in 70/30 Copper-

Nickel/Alloy 625 Piping Systems, CARDIVNSWC-TR-61-94/15, Naval Surface Warfare Center, Bethesda, MD, July 1995.

[6] P.O. Gartland, et al., How to Prevent Galvanic Corrosion in Seawater Piping Systems, CORROSION/96, Paper No. 496, NACE International.

[7] J. Keldsen, Why Some Flange Isolation Kits Fail, Pipeline and Gas Journal, July 2012. [8] B. Krantz, Galvanic Effects Accelerate Crevice Corrosion of Type 316L Stainless Steel Flanges

Coupled to 6% Mo Alloy”, Materials Performance, Volume 38, No. 6, 1999, 74-76.

7. NOMENCLATURE EPRI Electric Power Research Institute

mpy mils per year (1 mpy = 0.0254 mm/year)

Ecorr Corrosion potential, V or mV

icorr Corrosion current density, A/cm2 or mA/cm2


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Table 1. Summary of Current Measurements for Different Separator Materials and Lengths (data from [5])

Table 2. Summary of Required Insert Lengths for a Coated or Non-Conductive Spool

Environment Insert Length, pipe diameters Comments Source

Fresh water ~2 Slightly less for larger diameter pipes Appendix A of [1]

Brackish water – low conductivity 3

Strong function of coupled corrosion rate

Appendix A of [1]

Brackish water – high conductivity 5

Strong function of coupled corrosion rate

Appendix A of [1]

Seawater 10-40 Higher in smaller diameter piping Appendix A of [1]

Seawater 18-60

(50% reduction in integrated currents, per Hack. Non-coated spools)

[5]

Seawater 8 (general corrosion); 60 (localized corrosion)

General corrosion effects in 6%Mo stainless steel vs. carbon steel or Cu-Ni for general corrosion; vs. Cu-Ni only for localized corrosion

[6]

Seawater 4-5

Rule of Thumb of isolated spool with insulating kits on each end

[4]

70-30Cu-Ni I625 70-30Cu-Ni I625 70-30Cu-Ni I625 70-30Cu-Ni I6250 524 524 430 4301 403 380 23% 27% 430 472 0% -10%3 314 246 40% 53% 261 207 39% 52%

10 287 184 45% 65% 189 132 56% 69%

0 873 873 950 9501 710 786 19% 10% 873 1032 8% -9%3 644 541 26% 38% 650 547 32% 42%

10 537 455 38% 48% 435 349 54% 63%

@ 1 year

Sepa

rato

r Le

ngth

6 fps @ 6 mos

Integrated Currents, mA-d

Reduction of Integrated Currents

Separator Material Separator Material

@ 1 year

@ 6 mosLow Flow

Separator Material

Sepa

rato

r Le

ngth

Integrated Currents, mA-d

Reduction of Integrated CurrentsSeparator Material


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Figure 1. Examples of Flange Isolating Kits [3]


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where HPx = Hanger “x” on Pipe P, IF = Insulating Flange

Figure 2. Schematic of Above-ground Piping

where RHi = Resistance of the Hanger “i”,

RCi = Resistance of the Concrete Path at Hanger “i”,

RG = Resistance of the Grounding Grid,

RIF = Resistance through the Insulating Flange, and

Reff = Total Effective Resistance Between Pipes 1 and 2

Figure 3. Electrical Resistances of Commonly Grounded Above-ground Dissimilar Metal Pipes


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Figure 4. Plot of Electrochemical Potential vs. Log of Current Density for Carbon Steel (data from a fresh water plant’s service water system; straight line simplification of anodic/cathodic branches)

Figure 5. Plot of Electrochemical Potential vs. Log of Current Density for Stainless Steel (data

from the same fresh water plant’s service water system as shown in Figure 4)

Plant Service Water

-1000

-800

-600

-400

-200

0

200

400

1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06

Icorr, A/m2

Pote

ntia

l, m

V vs

. SC

E

CS AnodicCS Cathodic

1.23 mpy

Plant Service Water

-1000

-800

-600

-400

-200

0

200

400

1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06

Icorr, A/m2

Pote

ntia

l, m

V vs

. SC

E

304SS Anodic304SS Cathodic316SS Anodic316SS Cathodic

0.029 to 0.053 mpy


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Figure 6. Plot of Electrochemical Potential vs. Log of Current Density for the Carbon Steel-

Stainless Steel Galvanic Couple, Generated from Data in Figure 4 and Figure 5

Figure 7. Plot of Electrochemical Potential vs. Log of Current Density for the Carbon Steel-

Stainless Steel Galvanic Couple, with “shift” required (blue arrow to reach dashed black line) to achieve no galvanic effect.

Plant Service Water

-1000

-800

-600

-400

-200

0

200

400

1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06

Icorr, A/m2

Pote

ntia

l, m

V vs

. SC

E

CS AnodicCS Cathodic304SS Anodic304SS Cathodic316SS Anodic316SS Cathodic

237 to 444 mpy1.23 mpy

0.029 to 0.053 mpy

Plant Service Water

-1000

-800

-600

-400

-200

0

200

400

1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06

Icorr, A/m2

Pote

ntia

l, m

V vs

. SC

E

CS AnodicCS Cathodic304SS Anodic304SS Cathodic316SS Anodic316SS Cathodic

237 to 444 mpy1.23 mpy

0.029 to 0.053 mpy


16

Figure 8. Carbon Steel/AL6XN Dissimilar Metal Joint: Two Insulating Flanges (Carbon

Steel/Carbon Steel Spool and Spool/AL6XN) and No Other Connections [4]

Figure 9. Monolithic Isolation Joint (from www.gptindustries.com)

TWO Insulating Flanges; No Hangers


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Figure 10. Dissimilar Metal Joint with Coated Insert Spool


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Low Flow

6 feet per second

Figure 11. Effect of Separator Length on Galvanic Currents on 70-30 Cu-NI and I625 in Seawater [5]

Low Flow

0

100

200

300

400

500

600

700

800

900

1000

0 2 4 6 8 10 12

Separator Length, ft

Inte

grat

ed C

urre

nt, m

A-d

ays

70-30Cu-Ni I625 70-30Cu-Ni @ 1 yr I625 @ 1 yr

6 feet per second Flow

0

200

400

600

800

1000

1200

0 2 4 6 8 10 12

Separator Length, ft

Inte

grat

ed C

urre

nt, m

A-d

ays

70-30Cu-Ni I625 70-30Cu-Ni @ 1 yr I625 @ 1 yr


19

Low Flow

6 feet per second

Figure 12. Effect of Separator Length on Potentials of 70-30 Cu-Ni and I625 in Seawater [5] Purple ovals indicate the range of potentials at the interface

Low Flow

-200

-150

-100

-50

0

50

100

150

200

250

300

0 5 10 15 20 25 30

Distance, ft

Pote

ntia

l, m

V vs

. Ag/

AgC

l

No Separator Aeroquip RISIC I625 - 1 ft I625 - 3 ft I625 - 10 ftCuNi - 1 ft CuNi - 3 ft CuNi - 10 ft

6 feet per second Flow

-200

-150

-100

-50

0

50

100

150

200

250

300

0 5 10 15 20 25 30

Distance, ft

Pote

ntia

l, m

V vs

. Ag/

AgC

l

No Separator Aeroquip RISIC I625 - 1 ft I625 - 3 ft I625 - 10 ftCuNi - 1 ft CuNi - 3 ft CuNi - 10 ft


20

10”/10” Stainless Steel/Carbon Steel Couple

12”/4” Stainless Steel/Carbon Steel Couple

Figure 13. Calculated Corrosion Rate Profiles for Stainless Steel/Carbon Steel Couples in Seawater

with Various Separator Lengths [6].

GUIDELINES FOR AVOIDING GALVANIC CORROSION IN NUCLEAR...

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