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Galvanic corrosion of metals:
This article defines galvanic corrosion and explains the galvanic scale, the effects of corrosion on metal roofing, and an explanation of the galvanic scale and causes of corrosion between dissimilar metals in any application.
We explain the
Causes & rates of corrosion between dissimilar metals; tables of the galvanic scale for building materials;
Catalog of corrosion & rust sources in building components.
Here we explain the galvanic scale, the effect of corrosion caused when certain metals are placed in contact, and we provide examples of galvanic corrosion hazards that occur in buildings metal roofing, building electrical components, building plumbing components, and at underground oil storage tanks and oil piping systems. Links at the end of this article provide further detail about rust and corrosion on nearly every building component where corrosion is a particular concern.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
The Galvanic Scale and Its Role in Corrosion of Metals
Galvanic corrosion is the damage to or deterioration of metal components caused by an electrochemical reaction (metallic ion formation and migration) that occurs when two different metals are in contact with one another, principally when they are wet by or submerged in a liquid that acts as an electrolyte. Galvanic corrosion occurs around the point of contact between the two metals. Salts or other ingredients that increase the conductivity of the water or moisture increase the activity of galvanic corrosion.
A common example of corrosion is seen in plumbing systems where a copper water pipe is connected directly to an iron or steel water pipe.
Galvanic corrosion can occur at both metals through the action of an electrical current (galvanic current) that occurs at the anodic and at the cathodic member of the pair of metals. The anode member of the pair of metals gives up metal as ions of that metal move to and are deposited on the cathode member of the pair
The severity of galvanic corrosion that will occur where two metals are in contact depends on several variables including at least the following
Extent of dissimilarity of the two metals that are in contact and the resulting difference in electrode potentials of the pair of metals
Condition of the surface of each metal: presence or absence of a protective film including the film caused by the corrosion product itself. - (North 1970)
Electrolyte properties including ionic species, pH, conductivity, temperature, volume and flow rate.
Effects of the local environment such as moisture/drying cycles, exposure to sun, seasonal variations in temperature, humidity, moisture
Physical, geometric factors such as the areas of each metal, their distance, contact points and position and possibly even orientation - (Jia 2006)
Metallurgical properties of the metals including the alloy mix, heat treatment, mechanical disturbance
Other factors such as microbiological contributors to galvanic corrosion and chemical reactions and reversible electrode potentials - (Borenstein 1994) (Zhang 2011)
Definition of the Galvanic Scale: Tables of Noble to Less-Noble Metals
What is the Galvanic Scale
The galvanic scale (see Table 2-11) ranks a metal’s tendency to react in contact with another
metal in the presence of an electrolyte, such as water or
even moisture from the air
Some sources put graphite and platinum ahead of gold in resistance to corrosion while magnesium, zinc, and aluminum alloys tend to be among the most-easily corroded materials.
Metals at the top of the chart
are called anodic, or active, and are prone to corrode;
metals at the bottom are cathodic, or passive, and rarely
The farther apart two metals are on the chart, the
greater their tendency to react and cause corrosion in
the more active metal. Metals close to each other on the
scale are usually safe to use together.
Relationship of Properties: Anodic vs Cathodic, Corrosion Prone vs Corrosion-Resistant, Least-Noble vs. Most-Noble, Highest Relative Voltage Potential vs Lowest Voltage Potential
All of these term pairs can be used as descriptors of the corrosion resistance of a metal.
Metallic Corrosion Scale Terms
Highest Resistance to Corrosion
Lowest Resistance to Corrosion
Cathode or Most-Cathodic
Anode or Most-Anodic
More-Noble or Most-Noble
Less-Noble or Least-Noble
Low or zero relative potential voltage
Higher relative potential voltage
Actually higher relative potential voltage will be a more negative number where the entire relative potential voltage scale ranges from zero volts (most cathodic) to minus or - 1.65 volts (most anodic)
Magnesium, Zinc, Beryllium, Aluminum Alloys, Cadmium, Mild Steel, Cast Iron
Copper and lead are closer to the middle of this scale;
Metals that have relatively lower resistance to corrosion, when placed touching another metal with higher resistance to galvanic corrosion, will act as the anode and will suffer corrosion.
Example: copper pipe joined directly to mild steel or cast iron pipe will produce corrosion appearing more noticeably on the mild steel or cast iron pipe.
A detailed galvanic corrosion chart ranks various metals plotted against a scale that extends from most-noble or most-cathodic (most resistant to corrosion) metals to least-noble or least-cathodic (least resistant to corrosion).
The galvanic scale can also be expressed in voltage potential of these metals in an electrolytic solution (such as salty water), where the most-noble, most-cathodic, most-corrosion resistant metals have the lowest relative potential voltage (close to zero) while the least-noble, least-cathodic, least-corrosion resistant metals have the highest voltage potential.
"Least-cathodic" is the same as saying "most-anodic" when describing a metal that is most easily corroded.
Galvanic Scale Ordering for Metals Commonly Found in Buildings or Building Mechanical Systems
The metals in these lists are ordered from #1 - least noble, most anodic, most corrosion prone, to higher numbers = more noble, more cathodic, less corrosion prone. Some sources make slight re-orderings of pairs of metals in the lists given below, most likely to account for variations in alloys of metals in specific applications.
Other significant variations include the environment. For example metals on boats in salt water will show a more severe range of corrosion properties than in fresh water or above water.
Plumbing Metals: least-noble to most-noble
Galvanized iron pipe = zinc plated mild steel pipes
Zinc-protected metals: Galvanized Metals: least-protected to most-protected against corrosion by zinc plating
When metals are protected by a zinc coating such as in some roofing applications, (galvanized metals) the galvanic scale ordering changes somewhat from the lists above.
Aluminum with zinc protection
Galvanized steel with zinc protection
Cadmium with zinc protection
Mild steel & wrought iron with zinc protection
Cast iron with zinc protection
Lead-tin solder with zinc protection
Lead with zinc protection
Brass or bronze with zinc protection
Copper with zinc protection
Stainless Steel with zinc protection
Source: "Prevention of Galvanic Corrosion in the Roofing Industry", Shadowcrest Roofing, 265 Pawnee st. Unit C, San Marcos CA 92078, USA TEl: 760-593-0300, retrieved 2015/11/04, original source http://shadowcrestroofing.com/prevention/prevention-of-galvanic-corrosion-in-the-roofing-industry/
Examples of Measures to Control or Prevent Galvanic Corrosion on Building Components
In buildings and on building components or mechanical systems it is sometimes possible to prevent or retard harmful galvanic corrosion of metal components.
The following concepts can guide steps to minimize galvanic corrosion on or in buildings:
Choose metals close together in the galvanic scale: If you must joint different metals (as opposed to using only one kind of metal) when combining the two metals (such as connecting two types of metal piping), pairing metals that are as close together as possible in the galvanic series will minimize galvanic corrosion.
Prevent electrical connections: if two dissimilar metals are to be joined they can be separated by a non-conductive component such as a dielectric fitting on pipes
Use corrosion-resistant connectors: when you must join two dissimilar metals such as copper to iron pipe, a brazed or soldered joint will be more durable than a mechanical or threaded joint
Choose proper relative area or size of joined metals: when you must joint two dissimilar metals, if the less-noble or more anodic metal area is small compared to the higher-noble cathodic metal the corrosion rate will be greater
Use protective coatings correctly: if using an anti-corrosive coating or paint be sure both metals are coated, don't just coat one of them
Use a sacrificial anode where feasible - source: Shadowcrest Roofing, Op. Cit.
Measures to Protect Specific Building Materials from Galvanic Corrosion
Corrosion is controlled if not prevented at electrical connections, such as between wires or between a wire and a connecting lug by the use of an antioxidant paste (photo above). The antioxidant paste itself typically contains a less noble metal than the wire or connector and thus that acts as a sacrificial anode. A less noble metal is one that has less resistance to corrosion or chemical action than the metal to which it is being compared. Examples of high-noble metals that are very corrosion resistant include gold, platinum and silver.
An example of controlled galvanic corrosion is the use of a sacrificial anode in a water heater tank. The galvanic corrosion of the sacrificial anode in the water tank slows corrosion of the water tank body itself.
An example of prevention of galvanic corrosion is the use of dielectric fittings, basically fittings that include either an intermediate metal (brass) or an insulating plastic component between connections of copper to steel water pipes.
With metal roofing or any metal building components, the
safest strategy is not to mix metals that come in direct contact
with one another.
Use aluminum flashing and fasteners
with aluminum roofing, copper flashing and copper nails
with copper roofing, etc. When this is not possible, choose
a second metal that is not likely to lead to galvanic corrosion
or use a physical barrier to separate the two metals.
The Area Effect Determines the Rate of Metal Corrosion
The rate of corrosion is controlled by
the area of the more passive metal. For example, a galvanized
steel nail (active) will corrode quickly if surrounded
by a large area of copper flashing (passive).
If a copper
nail is used in galvanized steel flashing, however, the corrosion
of the steel will be slow and spread over a large
area, so it may not be noticeable. In each case, the active
metal corrodes, and the passive metal is protected.
Galvanic Corrosion of Metal Roofing
Because they are
made from active metals, aluminum and zinc roofing
panels, as well as steel roofing with aluminum and zinc
coatings (galvanized steel, Galvalume®, etc.), are vulnerable
to galvanic corrosion if allowed to come in contact
with more passive metals. [Click any image or drawing to see a larger copy]
For example, never use copper
or lead flashings with aluminum, zinc, or galvanized roofing
materials. Even water dripping from a copper pipe,
flashing, or gutter can lead to corrosion of coated-steel or
aluminum roofing materials.
How common flashing materials
react with metal roofing and other metal building
materials is shown in Table 2-12 above.
Where incompatible metals must be used in close
proximity, use the following precautions:
Separate the two dissimilar metals with building
paper, bituminous membrane, durable tapes, or
sealants so they are not in direct contact.
Coat the cathodic (less active) metal with a nonconductive
paint or bituminous coating.
Avoid runoff from a cathodic metal (e.g., copper gutters)
onto an anodic metals (such as galvanized steel).
Other Incompatible Materials Found on Metal Roofs
In addition to galvanic corrosion, a number of other common
building materials can harm the finishes on metal roofing
or lead to etching or corrosion of the material itself:
Wet Mortar Effects on Metal Roofing
Aluminum roofing materials and aluminum based
coatings can be damaged by alkali solutions such as
wet mortar. Where contact with wet mortar cannot be
avoided, one option is to spray the metal with lacquer or a
clear acrylic coating to protect it until the mortar is dry.
Pressure-Treated Wood Effects on Metal Roofing
Roof panels treated with
aluminum and zinc coatings should not come into direct
contact with pressure-treated (PT) wood, which can damage
the finish and accelerate corrosion.
Sealants & Caulks Impact on Metal Roofs
Use only sealants recommended by the manufacturer.
Never use acid-cure silicones (the most common
type, with a vinegar smell) or asphalt roofing cement with
coated-steel roofing, as these will mar the finish. Commonly
recommended products include butyl tape and
gunnable terpolymer butyl or urethane sealant.
Salt Spray Impact on Metal Roofs
Saltwater spray is very hard on metallic coated–
steel products and may lead to corrosion within
5 to 7 years. In these areas, the best choices are copper,
stainless steel, or painted aluminum. Hylar/Kynar® finishes
hold up best.
Other Examples of Corrosion Between Dissimilar Metals and the Need for Dielectric Fittings in buildings
Corrosion Protection for Electrical Panels, Wiring, & Grounding
Corroded copper grounding wires can also be unreliable as our photo shows. The copper wire was bonded to a galvanized-iron water pipe where corrosion was exacerbated by the combination of dissimilar metals and wet conditions.
at ELECTRICAL GROUND PIPE CORROSION we describe how stray voltage into the ground system can cause plumbing leaks or even damage to air conditioners and heat pumps.
Also see ALUMINUM WIRING HAZARDS where corrosion may be a factor in the reliability of some aluminum wiring connections, particularly in damp or wet locations and where aluminum is joined to copper without using the appropriate connectors and antioxidants.
Corrosion in electrical components, possibly including galvanic effects can cause more subtle hazards such as poor connections inside of electrical panels, switches, and junction boxes.
"Phase II Report, Evaluation of Residential Molded Case
Circuit Breakers", Wright-Malta Corp., (by J. Aronstein, for U.S. Consumer product Safety Commission, Project
#CPSC-C-81-1455), March 10, 1984 (Contains experimental analysis of materials,
construction, and performance of molded case circuit breakers, including FPE.
Lack of corrosion resistance of certain internal parts is considered to be a factor in the failure of the circuit
At above-left we illustrate an uninsulated aluminum grounding conductor that corroded through where it contacted a masonry block wall.
Galvanized to Copper Pipe Connections - Use a Dielectric Fitting to Avoid Corrosion
When connecting iron or galvanized iron pipes to copper in buildings, often corrosion and leaks will occur at the meeting of these two dissimilar metals.
Using a brass fitting to connect these two metals, or more commonly, using plastic or bronze fittings at the joint between these two metals will avoid future corrosion and leaks.
The photo (left) shows a galvanized iron union used to connect copper to galvanized iron. In the upper image you can just make out the black bronze ring built into this plumbing connector to avoid corrosion where the copper presses against the galvanized iron.
How do we explain that in some buildings we see direct copper-to-iron pipe connections with no corrosion? Luck? Maybe. But the corrosivity of the water is probably a factor in how rapidly copper-to-galvanized pipe connections will corrode and leak.
Spelling note that may help some searches: it's not dialectic pipe fittings, but dielectric pipe fittings.
Steel Underground Storage Tanks, Oil Piping, and Galvanic Corrosion
Dielectric material means a material
that does not conduct direct electrical
current. Dielectric coatings are used to
electrically isolate UST systems from the surrounding soils. Dielectric bushings
are used to electrically isolate
portions of the UST system (e.g., tank
at OIL TANK FAILURE CAUSES we provide details about sources of corrosion in underground oil storage tanks and in their piping & connections.
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Borenstein, S. Microbiologically influenced corrosion handbook. Elsevier, 1994.
Mansfeld, F., D. H. Hengstenberg, and J. V. Kenkel. "Galvanic Corrosion of Al Alloys I. Effect of Dissimilar Metal." Corrosion 30, no. 10 (1974): 343-353.
Abstract: Galvanic interaction between the Al alloys 1100, 2024, 2219, 6061, 7075, and Ag, Cu, Ni, Sn, Cd, Zn, the stainless steels 301, 304L, 347, A286, PH13-8Mo, Steel 4130, Inconel 718, Haynes 188 and Ti-6AI-4V has been studied in air saturated 3.5% NaCl by weight loss measurements and continuous monitoring of the galvanic current in 24 hour tests. Results show that the potential difference of uncoupled dissimilar metals, while in most cases accurately predicting the direction of current flow, is a poor indicator of the extent (rate) of galvanic corrosion of coupled dissimilar materials. The values of the average galvanic current density agree well with the increase of dissolution rates due to galvanic coupling. In general, galvanic corrosion of Al alloys coupled to dissimilar metals decreases in the order Ag > Cu > steel 4130 ≫ stainless steels ≈ Ni > Inconel 718 ≫ Ti-6AI-4V ≈ Haynes 188 >Sn > Cd. Coupling to zinc does not result in cathodic protection for all Al alloys studied, but can lead to increased attack as shown by weight loss data. Galvanic series for each Al alloy based on galvanic current data have been established. The results of all 95 couples have also been ranked according to the absolute increase of dissolution rates which is proportional to the average galvanic current density (cd). A ranking based on the relative increase of dissolution rates shows pronounced susceptibility to galvanic corrosion for the Al alloys 1100 and 6061, which have low corrosion rates when not in contact with dissimilar metals. According to the relative increase of dissolution rates which the materials studied cause to Al alloys, they have been placed in three clases (compatible, borderline, and incompatible with Al alloys).
North, R. F., and M. J. Pryor. "The influence of corrosion product structure on the corrosion rate of Cu-Ni alloys." Corrosion Science 10, no. 5 (1970): 297-311.
Jia, Jimmy X., Guangling Song, and Andrej Atrens. "Influence of geometry on galvanic corrosion of AZ91D coupled to steel." Corrosion Science 48, no. 8 (2006): 2133-2153.
Sarin, P., V. L. Snoeyink, J. Bebee, W. M. Kriven, and J. A. Clement. "Physico-chemical characteristics of corrosion scales in old iron pipes." Water Research 35, no. 12 (2001): 2961-2969.
Sarin, P., V. L. Snoeyink, J. Bebee, K. K. Jim, M. A. Beckett, W. M. Kriven, and J. A. Clement. "Iron release from corroded iron pipes in drinking water distribution systems: effect of dissolved oxygen." Water Research 38, no. 5 (2004): 1259-1269.
Waldeman, Jonathan, "Rust: The Longest War", [at Amazon.com], Simon & Schuster, (2015), ISBN-10: 1451691599,
Excerpt: Rust has knocked down bridges, killing dozens. It’s killed at least a handful of people at nuclear power plants, nearly caused reactor meltdowns, and challenged those storing nuclear waste. At the height of the Cold War, it turned our most powerful nukes into duds. Dealing with it has shut down the nation’s largest oil pipeline, bringing about negotiations with OPEC. It’s rendered military jets and ships unfit for service, caused the crash of an F-16 and a Huey, and torn apart the fuselage of a commercial plane midflight. In the 1970s, it was implicated in a number of house fires, when, as copper prices shot up, electricians resorted to aluminum wires. More recently, in the “typhoid Mary of corrosion,” furnaces in Virginia houses failed as a result of Chinese drywall that contained strontium sulfide. They rusted out in two years. One hundred fifty years after massive ten-inch cast iron guns attacked Fort Sumter, rust is counterattacking. Union forces have mobilized with marine-grade epoxy and humidity sensors. Rust slows down container ships before stopping them entirely by aiding in the untimely removal of their propellers. It causes hundreds of explosions in manholes, blows up washing machines, and launches water heaters through the roof, sky high. It clogs the nozzles of fire sprinkler heads: a double whammy for oxidation. It damages fuel tanks and then engines. It seizes up weapons, manhandles mufflers, destroys highway guardrails, and spreads like a cancer in concrete. It’s opened up crypts.
Zhang, Xiaoge Gregory. Corrosion and electrochemistry of zinc. Springer Science & Business Media, 2013.
Zhang, Zhe, Janet E. Stout, L. Yu Victor, and Radisav Vidic. "Effect of pipe corrosion scales on chlorine dioxide consumption in drinking water distribution systems." Water research 42, no. 1 (2008): 129-136.
Zhang, X. G. "Galvanic corrosion of zinc and its alloys." Journal of the Electrochemical Society 143, no. 4 (1996): 1472-1484.
Zhang, X. G., and E. M. Valeriote. "Galvanic protection of steel and galvanic corrosion of zinc under thin layer electrolytes." Corrosion science 34, no. 12 (1993): 1957-1972.
ARMA - Asphalt Roofing Manufacturer's Association - http://www.asphaltroofing.org/
750 National Press Building, 529 14th Street, NW, Washington, DC 20045, Tel: 202 / 207-0917
ASTM - ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959 USA The ASTM standards listed below can be purchased in fulltext directly from http://www.astm.org/
"The Fight Against Corrosion - A Study of the Nature of Corrosion and its Problems in Water Services and Heating Systems", Daniel Davies, Research and Development Services, Stansted Mountfichet, Essex, England, World Plumbing Conference-IV, "Plumbing and the World Environment, Compendium of Workshop Papers, October 3-6, 1996, Hyatt Regency Chicago, Chicago, IL", [personal correspondence, DJF - Author, July 2011]
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