Dielectriic Couplings

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By Walter J. Sperko, P.E.

Most engineers understand that the presence o dierent metals in a heating

or air-conditioning piping system may cause accelerated local corrosion,

and that one should avoid using dissimilar metals wherever possible. This is good

thinking, but sometimes availability or cost o materials make it necessary to join

dissimilar metals together to build engineered piping systems. Using dierent

metals in a heating or air-conditioning piping system does not mean that galvanic

corrosion will cause a ailure, nor does it mean that dielectric fttings must be

used at transitions between materials. This article presents guidance on what to

do when dierent metals are used in heating and air-conditioning piping systems.

Basics of Aqueous Corrosion

Four conditions must be present or

corrosion to occur in aqueous solutions:

An anode;

A cathode;

A metallic current path between the

anode and cathode; and

An ionic conducting medium.

Corrosion can be reduced by blocking

current access to the anode or cathode

or both, or by increasing resistance to

current fow in either the current path or

the liquid medium.

Galvanic Couples

The reason that joining two metals

together may cause accelerated corrosion

is that some metals are more easily cor-

roded than others. This goes back to basic

chemistry. All engineering metals react

with their environment, and some are more

reactive than others. The rate o reaction

depends on the environment; when dis-

similar metals are electrically connected

to each other, each interacts with the

environment, and the more reactive metal

takes the brunt o the reaction as shown in

the iron/zinc couple inFigure 1.While corrosion in a couple always

occurs at the anode, there is normally

little i any corrosion at the cathode. The

potential o metals or galvanically driven

corrosion is ranked using the galvanic

series, and the most widely published o

these is or metals in seawater. At the top

o this series1 (i.e., the most cathodic or

corrosion-resistant) is platinum ollowed

by titanium, Hastelloys, 18-8 and other

high-chromium stainless steels, Inconels,

copper-based alloys, tin, lead, cast ironand steel, aluminum and zinc. Magne-

sium is last and is the most anodic metal

on the list. The urther apart two metals

About the Author

Walter J. Sperko, P.E., is president of Sperko

Engineering Services, Inc. and Brazing Dim-

pler Corporation. He is past chairman of the

ASME B31.9, Building Services Piping Com-

mittee and a current member of various other

ASME Pressure Technology committees. He is

also past-chairman of the AWS D10 commit-

tee on Pipe and Tube.

Dissimilar Metals

In Heating and AC

Piping Systems

2 8 A S H R A E J o u r n a l a s h r a e . o r g A p r i l 2 0 0 9

2009, American Society o Heating, Rerigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Journal,Vol. 51, April 2009. This posting is by permission o ASHRAE. Additional reproduction, distribution, or transmission in either print or digitalorm is not permitted without ASHRAEs prior written permission.


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are on a galvanic series, the greater the potential dierence,and the aster the anode will corrode.

Environmental Effects

The galvanic series, however, is not the fnal answer when

determining the potential or galvanic corrosion. The arrange-ment o alloys in any galvanic series depends on the solution,and the specifc corrosion behavior o the metals involved inthat solution. Three examples are:

In Figure 1, current must ow between the metals andthrough the solution. I the uid is nonconductive, gal-vanic corrosion does not occur.

A zinc anode in a steel potable water tank will protect thesteel because it is electronegative to the steel. However,i the water temperature exceeds 180F (80C), reversalo potential occurs, and the steel tank becomes the anodeand the zinc becomes the cathode.2 I this happens, thetank corrodes to protect the zinc, which is not good. Thatswhy magnesium anodes should be used where the watertemperature exceeds 160F (70C).

When aluminum is coupled to steel, the aluminum actsas an anode in chloride solutions such as brackish or sea-water solutions, but it acts as a cathode in potable water.3

Fluid conductivity heavily inuences the overall corrosionrate. Condensation that collects on the outside surace o achilled water line that is indoors in an ofce/school/hospitalenvironment is a comparatively low conductivity uid. Accord-ingly, the corrosion rate due to condensation on the outer suraceo a couple in such a line will be low. That same condensate onan outdoor line in an industrial or seacoast environment will be

contaminated with salt or chemicals and become highly conduc-tive; not only will a couple corrode at a greater rate, but the pipewill rust. Outdoor piping in industrial/seacoast environmentsshould be protectedeven when there is insulation.

Another actor that controls corrosion rate is the presence ocorrosion products on the surace. These corrosion productsorm a barrier that slows the reaction rate and may eectivelystop urther attack. Titanium coupled to steel in highly con-ductive seawater, or example, does not result in acceleratedcorrosion o the steel even though there is a large potential di-erence between titanium and steel. This is because the titaniumorms an oxide barrier that eectively blocks current ow.4This

phenomenon is reerred to as polarization.Some polarization occurs with all metals when oxides and

other corrosion products are ormed on the suraces; the extento polarization controls the rate o corrosion. As a result, eachmetal and each solution has its own electrochemical corrosionbehavior. Characteristically, corrosion rates are initially high,but they decrease as polarization occurs. Polarization occurswith individual metals and with couples.

Wet fre protection systems are an interesting example opolarization combined with a change in the environment. Wetfre protection systems are usually flled with potable water thatis only mildly corrosive and contains some oxygen. The oxygen

attacks the steel surace orming a tight rust flm. Once the oxygen

is used up, corrosion stops,5 including any corrosion associatedwith couples. Periodic testing o a fre protection piping restartscorrosion since new water and oxygen are introduced during thetest. Since the amount o oxygen is small, the extent o corro-sion is usually small, even ater many reflls. Addition o a small

amount o sodium silicate to the water during flling6

wouldinhibit this process, but this is not common practice. The biggestrisk with wet sprinkler systems is the introduction o hydrogensulfde and anaerobic bacteria (requently ound in well water)which do not require oxygen to cause extensive and rapid damage.

Distance Effect

When the uid is highly conductive, such as seawater, gal-vanic eects reach beyond the immediate area o the couple,and attack will occur over a distance. In lower conductivitysolutions, attack occurs near the couple, resulting in knie-lineattack along the edge o the couple7 (Figure 2).

A similar phenomenon (i.e., limited distance o attack) oc-curs when the uid is highly conductive, but the uid is limitedto a thin flm such as a surace condensation flm. When thishappens, corrosion occurs rapidly at the metal interace suraceand may not be observed during casual examinations.The distance between anged aces made o dissimilar

materials aects the corrosion o the anode because the cur-rent through the uid has to ow across the gasket. For low-conductivity solutions, the presence o a nonconductive gasketbetween anges reduces the galvanic corrosion rate because thecurrent encounters more resistance due to the longer path it hasto travel as shown inFigure 3. For high-conductivity solutions,the gasket thickness makes no dierence.

Area Effect

Imagine that two copper plates are joined using steel rivets,and two steel plates are joined using copper rivets. Both areimmersed in seawater. Ater 15 months, the steel rivets willhave corroded away, but the copper rivets will still hold thesteel plates frmly together.8

In both sets o plates, the steel is anodic and will corrodeto protect the copper. Because the surace area o the copperplates is large compared to that o the steel rivets, the rivetswill be consumed protecting the copper plates. Although thesteel plates will rust, there is plenty o steel surace available

to sacrifce itsel to protect the copper rivets.In low-conductivity water, the overall corrosion rate is lower,

and corrosion is limited to the steel immediately surroundingthe copper rivets. Area eect is less o an inuence with lowerconductivity solutions, but most low-conductivity solutions areless corrosive to begin with.

Because o the area eect, thin copper tubes can be used in athick steel tubesheet in a heat exchanger, and the steel tubesheetwill protect the copper tubes rom corrosion. Although thetubesheet will corrode over time, extra thickness o the inex-pensive steel can be provided to compensate.The corrosion rate o couples may be reduced by coating or lining

the members near the couple. Even though it seems that protect-

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ing the part that will be attacked (i.e., theanodic metal) would be a good idea, coatingonly the anode is a bad idea. When onlythe anode is coated, the smallest aw inthe coating results in rapid local corrosion

at that aw due to the area eect, leadingto rapid penetration and leakage. I onlythe cathode is coated, a small aw in thecathode has no signifcant eect, but currentow is reduced due to the coating barrier.Coating o the cathode is usually an eec-tive corrosion mitigation tool when couplesare involved unless hydrogen evolutionoccurs at the cathode; when this happens,the coating will disbond and plug up thesystem. Coatings should be used with care.

When evaluating the relative surace areao a couple, it is the wetted surace areas thataect corrosion. For example, i there aretwo dierent metals in a tank that may bepartially flled at any time, the relative areas o the dierent metalsthat are wetted at any given time control the corrosion rate at thattime. That is, a carbon steel surge tank that has a stainless steel drainplug at the bottom wont suer rom much galvanic attack whenthe tank is ull because there is a large wetted carbon steel (anode)surace. However, it may be attacked heavily when the tank is nearlyempty because the wetted carbon steel (anode) surace is small.

Inhibitors

Corrosion inhibitors reduce the aggressiveness o the water.

A properly inhibited closed-loop hot or cold system should lastthe lietime o a building i its water and/or steam chemistry ismaintained properly.

Inhibitors work by orming a thin flm o oxides or organiccompounds on metal suraces that block the ow o current.When current ow is reduced, corrosion rates are reduced,including corrosion rates or dissimilar metal couples.There are two common metal-based inhibitors: molybdates

and zinc. Molybdates orm a complex oxide flm with iron atthe anode, whereas zinc will orm a zinc hydroxide or carbon-ate flm at cathodes. Both are usually used in conjunction withother inhibitors that will be described later. Chromates are

eective corrosion inhibitors or steel, copper, and aluminum,but concerns over toxicity have largely eliminated their use.

Non-metal inhibitors include phosphates, nitrites, nitrates,azoles, and silicates. Polyphosphates orm a polarizing flm onthe cathodes, whereas orthophosphates combine with calcium toorm an iron phosphate flm on anodic suraces o errous metals.Nitrites orm their own flm on anodic suraces on steels and iron,whereas nitrates protect aluminum. Azoles are copper-specifcinhibitors that support ormation o a natural copper oxide flm,and silicates will protect steel, lead, copper and copper alloys, andaluminum. Silicates are anodic inhibitors used with aluminum,particularly where coolants may contact ood products.

Other inhibitors are organic compounds such as amines

9

that

adsorb on the metal surace and suppress metal dissolution byblocking current ow into the uid. Many organic inhibitorsaect both the anodic and the cathodic reactions and can beused on steam and water systems.

Oxygen corrodes metal, and reducing oxygen in a closedpiping system reduces corrosion o the metals in that system.In neutral or slightly alkaline water, oxygen can be removedby using scavengers such as ascorbic acid or sodium sulfte,10

thereby reducing corrosion rates.Selection o an appropriate water treatment system is a task

or a specialist. The engineer who designs the piping systemshould consult with a reputable water chemistry specialist toselect a program o water treatment and corrosion inhibitionthat is suitable or the materials and temperatures in his system.

The engineer should be sure that the eventual owner/operator othe piping system is cognizant o his responsibility to maintainthe water treatment properly; this cannot be overemphasized.

While maintaining the proper inhibitor levels in closed sys-tems is easy, those systems should be monitored to ensure thatthey stay closed. Engineers should always install metering o

makeup water in closed systems or three reasons:11

Water

Zn+2

Zinc (Anode)Steel (Cathode)Electron Flow

H2 H2

H H H H H

Zn+2 Zn+2 Zn+2

Zn+2

Electrical Couple

Sea Water High Conductivity

Potable Water Low Conductivity

Cathode CathodeAnode Anode

Original

Surface

Potable Water

Current Flow

Bolt

Cathode Anode

Original

Surface

Fi gure 2: Di stance effect with high and low conductivity water.

Figure 1: I ron-zinc couple.

Fi gure 3: Ef fect of a gasket in l ow conductivity water.


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So that the rate o loss can be monitored and remedialaction can be taken to stop unnecessary water losses.

Potable water contains dissolved minerals and 6 to 12milligrams o oxygen per liter (6 to 12 ppm), increasingthe corrosiveness and scaling tendency o the water. Ex-

cessive makeup water leads to increased corrosion rates. Water treatment chemicals in the correct proportions mustbe added to makeup water to replace those that are lostto maintain the efcacy o the inhibitors.

Makeup water should be checked ensure that contaminantsare dealt with properly. Municipal water contains undesirableoxygen and chlorine. These can be eliminated by deaeratingor chemically treating the water prior to pumping it into thesystem. Well water may not contain chlorine, but it can containbacteria that can lead to microbiologically induced corrosion(MIC), and it can contain hydrogen sulfde (H2S), which is anaggressive chemical agent. Biological matter can be treated byfltering, which is ollowed by a biocide. H

2

S can be removedchemically or by steam deaeration.

Inhibitors in open systems (e.g., wet cooling towers) can work,but they must be supplied continuously, and biocides are also re-quired. For open-loop systems, cathodic protection o dissimilar

Copper, Monel, copper-nickel, brass, and bronzealloys;

Steel, iron, and cast iron; Aluminum alloys, although pure aluminum (1100

alloy) is anodic to other aluminum alloys;

Stainless steel, including 11% to 30% Cr alloys andaustenitic stainless steels such as 304, 304L, 316,316L; and

Nickel and nickel alloys. All heating and air-conditioning piping systems handling

water or steam should be protected rom general corrosionby use o appropriate inhibitors. I dissimilar metals are inthe system, an inhibitor system that protects both metalsshould be used. The engineer should consult with a watertreatment specialist to speciy the inhibitor system to beused, and, the owner should be instructed on the importanceo maintaining proper water chemistry. For closed systems, the engineer should provide or meter-ing o makeup water even though it may not seem necessary.

When insulating connections (dielectric couplings) arebeing considered, engineers should recognize that creeprelaxation occurs in the plastic parts and gaskets that are

metal couples by use o a third metal that isanodic to both metals or use o impressedcurrent will stop general and galvaniccorrosion and should be considered orlarge equipment. Dissimilar metal couplesshould be avoided in open-loop pipingwhere cathodic protection o the inside othe pipe is impractical.

Recommendations

Heating and cooling piping systemcorrosion problems can be avoided by: Knowing what materials are in a

system; Knowing when they can cause a prob-

lem due to galvanic coupling; and Selection o an appropriate system o

water treatment.The ollowing apply when two dierent

metals will be used:

Galvanic corrosion does not occurwhen metals are dry. Systems han-dling noncondensing air, nitrogen, orother gases can have dissimilar metals

joined together without concern orgalvanic corrosion.

Galvanic corrosion is not a concern oramilies o similar metals because theirelectrochemical potentials in waterare close to each other. The ollowingamilies o materials are not subjectto galvanic corrosion1when materials

within a amily are coupled:

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standard. Speciy insulating plastic grades that are suit-able or the service temperature.

Dissimilar metals do not cause problems when the wettedsurace area o the more noble metal (cathode) is smallrelative to the wetted surace area o the less noble metal

(anode). Accordingly, stainless steel instrument connections,plugs, etc., in carbon steel piping tanks and similar will causenegligible local corrosion.

Threaded joints between dissimilar metals should be avoidedunless corrosion inhibitors that protect both metals are used.Brazing (with braze metal that is cathodic to both metals)or welding is preerred.

I coatings or liners are used, they should cover the cathodic

metal in preerence to both. Coatings should neverbe usedonly on the anodic metal. When the outside o the joint may become wetted by con-

densation, rain, etc., the pipe that isinstalled outdoors should be protectedby paint or similar coating even iinsulated; this will also protect anydissimilar metal joints rom corrosion.

For potable water systems, dielectricfttings should be used when connectinghot water tanks and heat exchangers andat-grade penetrations along with light-ning protection to protect rom externalcorrosion. The engineer should checkor proper installation (gaskets, sleeves,washers, and o-rings) when dielectricfttings are specifed. Gaskets shouldbe suitable or the service temperature.

Conclusions

Although dierent metals are used in aheating or air-conditioning piping system itdoes not mean that galvanic corrosion willcause a ailure or that dielectric couplingsmust be used. Engineers and owners should

recognize that using dielectric isolators isnot a substitute or proper water chemistrycontrol, and that proper water chemistrycontrol can eliminate the need or trouble-some dielectric fttings, and also ensuresa long service lie or the piping system.

References

1. Fontana, M.G. 1986. Corrosion Engi-neering. N.Y.: McGraw-Hill. p. 43.

2. Corrosion Engineering. p. 44.3. Revie, R.W. 2000. Uhl igs Corrosion

Handbook, Second Edit ion. John Wiley &Sons. p. 144.

4. Corrosion Engineering. p. 45.5. Lane, R.W. 1993. Control of Scale and

Corrosion in Buil ding Water Systems. N.Y.:McGraw-Hill. p. 164.

6. Control of Scale and Corrosion in Buil d-ing Water Systems. p. 203.

7. Uhl igs Cor rosion Handbook, SecondEdition. p. 146.

8. Corrosion Engineering. p. 46.9. Corrosion Engineering. p. 282.

10. Corrosion Engineering. p. 283.11. Control of Scale and Corrosion in Buil d-ing Water Systems. p. 149 50.

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