Metal roofing rust in Key West Florida (C) Daniel Friedman Galvanic Scale & Galvanic Corrosion

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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

Badly rusted corrugated metal roofing (C) Daniel FriedmanThis article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

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.

[Click to enlarge any image]

Article Contents

Definition of Galvanic Corrosion

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

Definition of the Galvanic Scale: Tables of Noble to Less-Noble Metals

What is the Galvanic Scale

Table 2-11: the Galvanic Scale of Corrosion Between Dissimilar Metals - using Metal Roofing (C) J Wiley, S Bliss

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 corrode.

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 Comments
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)
Most Passive or Least Active Least Passive or Most Active  
Graphite, Gold, Platinum, Titanium, Nickel, Alloy 20 Stainless Steel, Silver 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

  1. Galvanized iron pipe = zinc plated mild steel pipes
  2. Mild steel pipes
  3. Cast iron pipe
  4. Lead pipe
  5. Yellow or red brass pipes
  6. Copper pipes
  7. Red bronze ( connectors)
  8. 50/50 lead-tin solder mix
  9. Stainless steel pipes & tubing grades 304, 316, 410, 416 or 430

Electrical Wiring & Connector Metals: least-noble to most-noble

  1. Aluminum or aluminum alloy wire or connectors
  2. Brass connectors
  3. Copper wire or copper connectors, also possibly copper-plated wires or connectors depending on effects of cuts, scratches, mechanical damage
  4. 90/10 copper nickel alloy
  5. 80/20 copper nickel alloy
  6. Silver (solder)
  7. Gold (or gold plated wires and connectors)

Roofing & Architectural Metals: roof surfaces or flashings: least-noble to most-noble

  1. Zinc roof flashings
  2. Aluminum sheet metal roofing or roof flashings
  3. Galvanized steel roofing (zinc-plated steel)
  4. Mild steel roofing (common corrugated metal roofing)
  5. Cast iron or wrought-iron components (railings, facades)
  6. Aluminum-bronze alloys
  7. Naval brass, yellow brass, red brass
  8. Copper roof or roof flashings
  9. Tin or tin-plated metal roofing or lead-tin solder (some metal roofs particularly antique)
  10. Stainless steel roofing (grade 410, 416)
  11. Lead roof flashings or lead-coated roofing (particularly antique)
  12. Stainless steel roofing (alloy grade 302, 304, 321, 347)
  13. Stainless steel roofing (alloy grade 316, 317)
  14. Stainless steel roofing (alloy grade 20)

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.

  1. Pure zinc
  2. Aluminum with zinc protection
  3. Galvanized steel with zinc protection
  4. Cadmium with zinc protection
  5. Mild steel & wrought iron with zinc protection
  6. Cast iron with zinc protection
  7. Lead-tin solder with zinc protection
  8. Lead with zinc protection
  9. Brass or bronze with zinc protection
  10. Copper with zinc protection
  11. Stainless Steel with zinc protection

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.

Antioxidant paste on an aluminum wiring connection - Aronstein

The following concepts can guide steps to minimize galvanic corrosion on or in buildings:

Measures to Protect Specific Building Materials from Galvanic Corrosion

  1. 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.

  2. 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.

  3. 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.

  4. Metal roof corrosion is retarded or prevented by application of special coatings or by choice of a metal such as copper or special steel alloys that form their own protective oxide coating.


Galvanic & Other Corrosion Warnings for Metal Roofs

Corrosion Standards for Metal Roofs

Steel roofing materials are tested for corrosion-resistance in a salt spray cabinet per ASTM B117 and also in a condensation chamber per ASTM D4585.

Adapted/paraphrased with permission from Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES:

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]

Table 2-12 Flashings vs other Metal Roofing (C) J Wiley, S Bliss

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:

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.

Also see our metal roofing home page,

-- Adapted with permission from Best Practices Guide to Residential Construction.

Other Examples of Corrosion Between Dissimilar Metals and the Need for Dielectric Fittings in buildings

Corrosion Protection for Electrical Panels, Wiring, & Grounding

Corroded copper ground wire (C) Daniel Friedman

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.

As we discuss at ELECTRICAL GROUND SYSTEM INSPECTION, we would be reluctant to trust this connection for the building grounding system.

You might also take a look at WATER PIPING GROUND BOND.

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.

Corroded aluminum electrical ground wire (C) Daniel Friedmanh

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 breakers.

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

Water supply piping connection: copper to galvanize (C) Daniel Friedman Diaelectric fitting

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

Quoting from 40 CFR part 280 Technical Standards and Corrective Action Requirements for Owners and Operators of Underground Storage Tanks (UST)

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 from piping).

at OIL TANK FAILURE CAUSES we provide details about sources of corrosion in underground oil storage tanks and in their piping & connections.

Oil Storage Tank Corrosion Protection Standards

The list given below suggests building components where rust and corrosion is a particular problem.


Continue reading at ANODES & DIP TUBES on WATER HEATERS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see these

Corrosion of Building Components: Articles

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