Metal roofing rust in Key West Florida (C) Daniel Friedman The Galvanic Scale
Causes & rates of corrosion between dissimilar metals
Catalog of corrosion & rust sources in building components


  • GALVANIC SCALE & METAL CORROSION - CONTENTS: Definition of the galvanic scale. Examples of types of corrosion & rust that occur on buildings & in building component including building roofing, siding, piping, HVAC equipment, electrical wiring, oil storage tanks, well casings, ductwork, boilers, furnaces & other building components.Metal Roofing - galvanic and other corrosion warnings. Electrical Components - Corrosion Protection for Electrical Panels, Wiring, & Grounding. Plumbing Components - Galvanized to Copper Pipe Connections - Use a Dieelectric Fitting to Avoid Corrosion. Underground Oil Storage Tanks - Steel Underground Storage Tanks and Corrosion
  • POST a QUESTION or READ FAQs about galvanic scale and 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.

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.

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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

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.

What is the Galvanic Scale?

Table 2-11: the Galvanic Scale of Corrosion Between Dissimilar Metals - using Metal Roofing (C) J Wiley, S BlissThe galvanic scale (see Table 2-11 at left) 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.

[Click any image or drawing to see a larger copy]

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.

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:

  • 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.

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 FriedmanCorroded 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 FriedmanhCorrosion 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 Dieelectric 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

  • U.S. Environmental Protection Agency EPA Underground Oil Storage Tank Regulations include - Complete 40 CFR part 280 Technical Standards and Corrective Action Requirements for Owners and Operators of Underground Storage Tanks (UST) (454K byte PDF). This document defines dielectric material (coatings) used to protect underground oil storage tanks from corrosion: More about the galvanic scale and corrosion between dissimilar metals is at GALVANIC SCALE & METAL CORROSION.
  • API Recommended Practice 1632, "Cathodic Protection of Underground Petroleum Storage Tanks and Piping Systems"
  • NACE RP 0169, "Standard Recommended Practice: Control of External Corrosion on Underground or Submerged Metallic Piping Systems"
  • NACE RP 0285 "Standard Recommended Practice: Corrosion Control of Underground Storage Tank Systems by Cathodic Protection"
  • NACE Test Method TM 0101, "Measurement Techniques Related to Criteria for Cathodic Protection on Underground or Submerged Metallic Tank Systems"
  • NACE Test Method TM 0497, "Measurement Techniques Related to Criteria for Cathodic Protection on Underground or Submerged Metallic Piping Systems"
  • STI R892, "Recommended Practice for Corrosion Protection of Underground Piping Networks Associated with Liquid Storage and Dispensing Systems"
  • STI-R-972, "Recommended Practice for the Installation of Supplemental Anodes for STI-P3 USTs"
  • UL 1746, Standard for Safety: "External Corrosion Protection Systems for Steel Underground Storage Tanks"

The More Reading links below list common building components where rust and corrosion is a particular problem.


Continue reading at ANODES & DIP TUBES on WATER HEATERS or select a topic from the More Reading links shown below.


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GALVANIC SCALE & METAL CORROSION at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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