Galvalume Technical Bulletins

Compatibility of Materials Used in Metal Building Construction

In the metal building industry, there are a number of alternative choices for materials used for roofing and siding panels, fasteners, flashings and other trim elements. How and where these various components are installed in relation to one another will have a direct impact on the effective life of a metal building or retrofit application. A decision to use one material rather than another should take into account the compatibility among components that will be in direct contact with, or in close proximity to, one another.

What to Avoid:

The table on the following page provides a quick reference guide for comparing the compatibility among the more widely used materials in the metal building industry. Use of the table and careful consideration of materials specification will help you avoid problems in three general situations: direct contact, standing water and rainwater runoff.

• Copper and copper-containing brasses, lead (for flashing or as contaminants in other materials) and graphite (from pencils used to mark components) should not be used in any construction where Galvalume® or aluminum-coated sheet is specified.
• Wherever possible, standing water conditions should be avoided.
• Drainage of rainwater from one component onto or over another building element should take into account the materials involved. The table should be consulted before materials are specified for roofing, flashing, fasteners, rain gutters and other components to avoid future problems.

Why?:

Galvanic Corrosion: Galvanic corrosion can occur when two dissimilar metals are in direct electrical contact with each other. This contact can be either the metals touching one another or connected by some other means of completing the electrical circuit. By dissimilar metals, we mean metals or alloys that have different activities when exposed to corrosive environments such as our atmosphere and rainwater.

BUILDING PANEL MATERIAL
Panel, Flashing or Accessory Material	Galvalume	Prepainted Galvalume	Galvanized	Prepainted Galvanized	Aluminum-Coated Type II
Galvalume	Compatible		Compatible, although galvanized may have a shorter life and will eventually have an adverse effect on Galvalume.		Compatible, although inferior resistance of aluminum-coated steel to standing-water and cut-edge corrosion could result in rust-staining of adjacent Galvalume materials.
Prepainted Galvalume	Compatible		Compatible, although galvanized may have a shorter life and will eventually have an adverse effect on Galvalume.		Compatible, although inferior resistance of aluminum-coated steel to standing-water and cut-edge corrosion could result in rust-staining of adjacent prepainted Galvalume materials.
Galvanized	Compatible, although galvanized may have a shorter life and will eventually have an adverse effect on Galvalume. Small areas of unpainted galvanized subject to water runoff from Galvalume panels should be avoided		Compatible	Compatible, although small areas of unpainted galvanized subject to water runoff from painted panels should be avoided.	Compatible, although galvanized may have a shorter life and may eventually have an adverse effect on aluminum-coated steel. Small areas of unpainted galvanized subject to water runoff from aluminum-coated steel should be avoided.
Prepainted Galvanized	Compatible, although galvanized may have a shorter life and will eventually have an adverse effect on Galvalume.		Compatible		Compatible, although inferior resistance of aluminum-coated steel to standing-water and cut-edge corrosion could result in rust-staining of adjacent prepainted galvanized materials.
Aluminum-coated Type II	Compatible, although inferior resistance of aluminum-coated steel to standing-water and cut-edge corrosion could result in rust-staining of adjacent bare or prepainted Galvalume panels.		Compatible, although inferior resistance of aluminum-coated steel to standing water and cut-edge corrosion could adversely affect the galvanized coating and/or result in rust-staining of adjacent bare or prepainted galvanized panels.		Compatible
Copper	Not compatible -- direct contact or exposure to water runoff may seriously affect Galvalume		Compatible		Not compatible -- direct contact or exposure to water runoff may seriously affect aluminum-coated steel.
Lead
Stainless Steel	300 series grades are compatible. 400 series grades with >1.0 mil zinc or cadmium coatings are compatible.		300 series grades are compatible. 400 series grades with >1.0 mil zinc or cadmium coatings are compatible.		300 series grades are compatible. 400 series grades with >1.0 mil zinc or cadmium coatings are compatible.
Graphite	Not compatible -- avoid contact		Compatible		Not compatible -- avoid contact
Platics	Compatible		Compatible		Compatible

Precautions were mentioned previously concerning the use of copper, lead and graphite with Galvalume and aluminum-coated sheet. The field of corrosion study has defined an "activity" scale, shown schematically at left, which shows zinc and aluminum more active than either copper, lead or graphite. The farther they are apart on the scale, the more dissimilar they are and the stronger the potential for reaction between them. When a galvanic "couple" is formed by electrical contact, the more active metal will sacrifice itself (or dissolve) to protect the less active component of the couple. A couple consisting of aluminum and graphite, for instance, will corrode the aluminum away much faster than the copper would dissolve in a copper/graphite couple.

On a metal building, when copper, lead or graphite either come in contact with, or in the case of copper used in close proximity to Galvalume or aluminum-coated sheet coatings, the coatings will corrode much faster than normal. The protective oxide film which naturally forms on aluminum surfaces is broken down by copper, lead or graphite in localized areas. When this occurs, pitting corrosion ensues which is a highly accelerated form of attack. Zinc coatings are not generally subject to pitting when in contact with the same materials. Therefore, the galvanic corrosion reaction is slower and spread over a wider area.

Rainwater Run-off:
The compatibility table also contains guidelines for selection of materials when one component will drain rainwater onto another. Pure rainwater contains very little dissolved metal. When it contacts galvanized steel, for example, it will dissolve some zinc from the coating. As the water drains onto another area, such as a lower roof elevation, it will be less corrosive relative to the pure water. Where there is a building element with a less active coating over one with an inferior coating, there will be accelerated corrosion of the lower element. Such a situation occurs where rainwater drains from a prepainted Galvalume roof into an unpainted galvanized gutter. In industrial, acid rain or marine environments, these effects will be accelerated due to the increased corrosivity of the rainwater.

Standing Water:
New applications for metal roofing have required that roof slopes be minimized to as low as 1/4:12. At times, areas of roofs can be flat depending on the particular building. These conditions, either by accident or intent, can create areas where water can collect and remain for extended periods of time with subsequent possibility of accelerated corrosion. Where an unfavorable galvanic couple exists, as discussed above, the presence of standing water for prolonged periods will allow the corrosion reaction to continue for a longer time than it normally would. Even in cases where an adverse couple does not exist, enough water can complete the necessary electrical contact and corrosion will proceed as long as the water maintains the circuit.

The appearance of roofing panels can suffer even when all materials within a water-ponding area are compatible. Aluminum-coated steel panels are not as resistant to standing water as Galvalume sheet. When the aluminum-coated panel begins to rust, the standing water can disperse and deposit rust particles on an adjacent Galvalume sheet panel, resulting in an unsightly stain.

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