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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.
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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.
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.
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:
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
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.
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
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