The Galvanic Scale - Metal Corrosion
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Definition of the galvanic scale
Galvanic scale and corrosion on metal roofs
Galvanic scale and metal corrosion in plumbing systems
Galvanic scale explains corrosion between dissimilar metals
Corrosion Protection for Electrical Panels, Wiring, & Grounding
Steel Underground Storage Tanks, Oil Piping, and Galvanic Corrosion
Questions & answers about galvanic scale and corrosion

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.

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The Galvanic Scale and Its Role in Corrosion of Metals

This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

Also see our metal roofing home page, METAL ROOFING and see CORRUGATED ROOFING and COPPER ROOFING, our roofing home page: ROOFING INSPECTION & REPAIR. Other topics involving the galvanic scale include PLUMBING SYSTEM INSPECT DIAGNOSE REPAIR and STRUCTURAL INSPECTIONS & DEFECTS and HEATING SYSTEMS.

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 at:

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

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?

The 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]

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.

-- 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 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.

More about the galvanic scale and corrosion between dissimilar metals is at GALVANIC SCALE & METAL CORROSION.

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 breakers.

Galvanized to Copper Pipe Connections - Use a Dieelectric Fitting to Avoid Corrosion

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"

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Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education. Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
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GALVANIC SCALE & METAL CORROSION

METAL ROOFING
  ALUMINUM ROOFING
  Aluminized Steel Roofs
  Barn Roofing, Metal
  COPPER ROOFING
  Corrugated Metal Roofing
  Enameled steel roofing
  Galvalume Metal Roofing
  GALVANIC SCALE & METAL CORROSION
  Galvanized Steel Roofing
  LEAD in ROOFING, EFFECTS
  LIFE EXPECTANCY of METAL ROOFS
  METAL ROOF COATINGS & PAINTS
  METAL ROOF EXPOSED FASTENER SYSTEM
  METAL ROOF EXPOSED FASTENER FLASHING
  Metal Shingle Roofs, Embossed, Antique
  Metal Roofing Sources & Manufacturers
  METALS USED IN ROOFING
  MODULAR METAL ROOF SHINGLE SYSTEM
  NOISE TRANSMISSION in ROOFS
  SNOW GUARDS on SLATE & METAL ROOFS
  Soldered Flat Shingle and Flat Copper Metal Roofs
  Stainless Steel Roofing
  STANDING SEAM METAL ROOF INSTALLATION
  TERNE METAL ROOFING
  Zinc Metal Roofing

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/
Cedar Shake & Shingle Bureau, CSSB, U.S.: Sumas, WA 98295-1178, Tel: 604-820-7700, In Canada: Cedar Shake and Shingle Bureau #2 - 7101 Horne Street, Mission, BC V2V 7A2 Tel: (604) 820-7700, E-mail: info@cedarbureau.com , website: http://www.cedarbureau.org/
Forest Products Laboratory, US FPL, One Gifford Pinchot Drive, Madison, WI 53726, Tel: (608) 231-9200, Email: mailroom_forest_products_laboratory@fs.fed.us608-231-9200, website: http://www.fpl.fs.fed.us/
NRCA - National Roofing Contractors Association - http://www.nrca.net/, 10255 W. Higgins Road, Suite 600, Rosemont, IL 60018-5607, Tel: (847) 299-9070
"Steel [Roof] Decks: Issues for the 1990's", Thomas L. Smith, AIA, CRC., Professional Roofing, [date pending] p. 74, NRCA
"Steel [Roof] Deck Corrosion Bulletin, NRCA", Thomas L. Smith, AIA, CRC., Professional Roofing, [date pending] p. 58, NRCA
"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]
"The Many Aspects of Metal [Roof] Shingles", Thomas L. Smith, AIA, CRC., Professional Roofing, [date pending] NRCA

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

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The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop, and from the InspectAPedia bookstore. The 2010 edition of the Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Amazon.com and also Wiley.com. See our book review of this publication.
Decks and Porches, the JLC Guide to, Best Practices for Outdoor Spaces, Steve Bliss (Editor), The Journal of Light Construction, Williston VT, 2010 ISBN 10: 1-928580-42-4, ISBN 13: 978-1-928580-42-3, available from Amazon.com
The Journal of Light Construction has generously given reprint permission to InspectAPedia.com for this article. All rights and contents are ©Journal of Light Construction and may not be reproduced in any form.
Architectural elements: the technological revolution: Galvanized iron roof plates and corrugated sheets; cast iron facades, columns, door and window caps, ... (American historical catalog collection), Diana S Waite, available used out of Amazon.
Asphalt Roofing Residential Manual,
Building Pathology, Deterioration, Diagnostics, and Intervention, Samuel Y. Harris, P.E., AIA, Esq., ISBN 0-471-33172-4, John Wiley & Sons, 2001 [General building science-DF] ISBN-10: 0471331724 ISBN-13: 978-0471331728
Building Pathology: Principles and Practice, David Watt, Wiley-Blackwell; 2 edition (March 7, 2008) ISBN-10: 1405161035 ISBN-13: 978-1405161039
Built-Up Roof Systems, Manual, C.W. Griffin, Mcgraw-Hill (Tx); 2nd edition (July 1982), ISBN-10: 0070247838, ISBN-13: 978-0070247833
Concrete Folded Plate Roofs, C. Wilby PhD BSc CEng FICE FIStructE (Author), Butterworth-Heinemann, 1998, ISBN-10: 0340662662, ISBN-13: 978-0340662663
Concrete Shell Roofs, C. Wilby PhD BSc CEng FICE FIStructE (Author),
Concrete Dome Roofs (Longman Concrete Design and Construction Series),
Concrete Roofing Tile, History of the, Batsford, 1959, AISN B000HLLOUC (availble used)
Copper Roofing, by CDA
Copper Roofing, Master specifications for copper roofing and sheet metal work in building construction: Institutional, commercial, industrial, I.E. Anderson, 1961 (hard to find)
Corrugated Iron, Building on the Frontier, Simon Holloway
Green Roof Plants: A Resource and Planting Guide, Edmund C. Snodgrass, Lucie L. Snodgrass, Timber Press, Incorporated, 2006, ISBN-10: 0881927872, ISBN-13: 978-0881927870. The text covers moisture needs, heat tolerance, hardiness, bloom color, foliage characteristics, and height of 350 species and cultivars.
Green Roof Construction and Maintenance, Kelley Luckett, McGraw-Hill Professional, 2009, ISBN-10: 007160880X, ISBN-13: 978-0071608800, quoting: Key questions to ask at each stage of the green building process Tested tips and techniques for successful structural design Construction methods for new and existing buildings Information on insulation, drainage, detailing, irrigation, and plant selection Details on optimal soil formulation Illustrations featuring various stages of construction Best practices for green roof maintenance A survey of environmental benefits, including evapo-transpiration, storm-water management, habitat restoration, and improvement of air quality Tips on the LEED design and certification process Considerations for assessing return on investment Color photographs of successfully installed green roofs Useful checklists, tables, and charts
Handbook of Building Crafts in Conservation, Jack Bower, Ed., Van Nostrand Reinhold Company, NY 1981 ISBN 0-442-2135-3 Library of Congress Catalog Card Nr. 81-50643.
Historic Preservation Technology: A Primer, Robert A. Young, Wiley (March 21, 2008) ISBN-10: 0471788368 ISBN-13: 978-0471788362
Historic Slate Roofs : With How-to Info and Specifications, Tina Skinner (Ed), Schiffer Publishing, 2008, ISBN-10: 0764330012 , ISBN-13: 978-0764330018
Low Slope Roofing, Manual of, 4th Ed., C.W. Griffin, Richard Fricklas, McGraw-Hill Professional; 4 edition, 2006, ISBN-10: 007145828X, ISBN-13: 978-0071458283
- Roof failure causes in depth (and specific methods for avoiding them)
- Roof design fundamentals and flourishes, based on voluminous industry research and experience
- New technologies and materials -- using them safely and correctly
- Comprehensive coverage of all major roofing systems pecifications, inspection, and maintenance tools for roofing work
Metal Roofing, an Illustrated Guide, R.A. Knowlton , [metal shingle roofs],
Patio Roofs, how to build, Sunset Books
Problems in Roofing Design, B. Harrison McCampbell, Butterworth Heineman, 1991 ISBN 0-7506-9162-X (available used)
Roofing The Right Way, Steven Bolt, McGraw-Hill Professional; 3rd Ed (1996), ISBN-10: 0070066507, ISBN-13: 978-0070066502
Slate Roofs, National Slate Association, 1926, reprinted 1977 by Vermont Structural Slate Co., Inc., Fair Haven, VT 05743, 802-265-4933/34. (We recommend this book if you can find it. It has gone in and out of print on occasion.)
Roof Tiling & Slating, a Practical Guide, Kevin Taylor, Crowood Press (2008), ISBN 978-1847970237,
If you have never fixed a roof tile or slate before but have wondered how to go about repairing or replacing them, then this is the book for you. Many of the technical books about roof tiling and slating are rather vague and conveniently ignore some of the trickier problems and how they can be resolved. In Roof Tiling and Slating, the author rejects this cautious approach. Kevin Taylor uses both his extensive knowledge of the trade and his ability to explain the subject in easily understandable terms, to demonstrate how to carry out the work safely to a high standard, using tried and tested methods.
This clay roof tile guide considers the various types of tiles, slates, and roofing materials on the market as well as their uses, how to estimate the required quantities, and where to buy them. It also discusses how to check and assess a roof and how to identify and rectify problems; describes how to efficiently "set out" roofs from small, simple jobs to larger and more complicated projects, thus making the work quicker, simpler, and neater; examines the correct and the incorrect ways of installing background materials such as underlay, battens, and valley liners; explains how to install interlocking tiles, plain tiles, and artificial and natural slates; covers both modern and traditional methods and skills, including cutting materials by hand without the assistance of power tools; and provides invaluable guidance on repairs and maintenance issues, and highlights common mistakes and how they can be avoided. The author, Kevin Taylor, works for the National Federation of Roofing Contractors as a technical manager presenting technical advice and providing education and training for young roofers.
The Slate Roof Bible, Joseph Jenkins, www.jenkinsslate.com, 143 Forest Lane, PO Box 607, Grove City, PA 16127 - 866-641-7141 (We recommend this book).
Slate Roofing in Canada (Studi4es in archaeology, architecture, and history),
Smart Guide: Roofing: Step-by-Step Projects, Creative Homeowner (Ed), 2004, ISBN-10: 1580111491, ISBN-13: 978-1580111492
Solar heating, radiative cooling and thermal movement: Their effects on built-up roofing (United States. National Bureau of Standards. Technical note), William C Cullen, Superintendent of Documents, U.S. Govt. Print. Off (1963), ASIN: B0007FTV2Q
Tile Roofs of Alfred: A Clay Tradition in Alfred NY
"Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
Wood Shingle Roofs, Care and Maintenance of wood shingle and shake roofs (EC), Stanley S. Niemiec (out of print)
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The Galvanic Scale and Its Role in Corrosion of Metals

Galvanic & Other Corrosion Warnings for Metal Roofs

Corrosion Standards for Metal Roofs

What is the Galvanic Scale?

The Area Effect Determines the Rate of Metal Corrosion

Galvanic Corrosion of Metal Roofing

Other Incompatible Materials Found on Metal Roofs

Wet Mortar Effects on Metal Roofing

Pressure-Treated Wood Effects on Metal Roofing

Sealants & Caulks Impact on Metal Roofs

Salt Spray Impact on Metal Roofs

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

Corrosion Protection for Electrical Panels, Wiring, & Grounding

Galvanized to Copper Pipe Connections - Use a Dieelectric Fitting to Avoid Corrosion

Steel Underground Storage Tanks, Oil Piping, and Galvanic Corrosion

Oil Storage Tank Corrosion Protection Standards

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

The Galvanic Scale - Metal Corrosion
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