Designing Metal Roofing and Siding to Avoid Premature Corrosion
In this article:
- Many metal roofing and siding products such as standing seam and corrugated metal roofing are coated with a durable metallic coating which provides long lasting protection from corrosion.
- Improper installation of metallic-coated products can lead to the creation of a phenomenon known as galvanic corrosion.
- Galvanic corrosion is a premature, rapid form of corrosion created by specific metal to metal contact in certain environments.
- With the correct product selection, environment preparation, and installation techniques, this phenomenon is entirely avoidable.
Steelcape’s products feature a metallic-coating that provides long-lasting resistance from corrosion. This durable barrier makes these products ideal for exterior applications including metal roofing and siding products. The coating is applied during the steel production process prior to the application of paint and consists of either zinc (sold as TruZinc® or galvanized) or a specialty mix of aluminum and zinc (sold as ZINCALUME®).
Special considerations must be taken during the design and installation phase of metallic-coated products to avoid the unintentional creation of galvanic corrosion. This rapid form of corrosion is created when metals of dissimilar types are installed in direct contact with one another in a corrosive environment.
Galvanic or bi-metallic corrosion is a reaction in which one metal will sacrifice itself (or dissolve) to protect the different, less active metal, leading to visible corrosion. Galvanic corrosion occurs between dissimilar metals in certain corrosive environments and typically requires three criteria in which to occur. This includes:
- Two or more dissimilar metals,
- Metal to metal contact, and,
- Both metals to reside in the same conducting solution/ corrosive environment, i.e. salt air or water.
Learn more about Galvanic Corrosion in this article from the American Galvanizers Association.
Metals can be viewed as active or noble based on their position on the galvanic scale, simplified in the picture below. The further apart these metals are on this scale, the more ‘dissimilar’ they are in nature and the greater the potential for a reaction between the metals when in direct contact with one another.
In addition to the dissimilarity of the metals, the severity of the environment will influence the potential for galvanic corrosion. Common factors influencing environmental severity can include direct proximity to salt water or chemical spray, the frequency of breaking surf (which generates airborne salt-laden particulates), and how frequently the surface is rinsed by rainfall.
Installations compromised by galvanic corrosion are commonly attributed to improper fastener and accessory selection in marine environments. This is where a fastener of a noble metal type, such as a stainless-steel rivet or copper bracket is installed in direct contact with the metal roof panel, close to breaking surf. The result is rapid corrosion of the metal product as the metallic coating ‘sacrifices’ itself.
Other common causes of galvanic corrosion can include stainless-steel light fixtures, window frames and other accessories recessed in metallic-coated panels in corrosive environments. Initially galvanic corrosion may appear as the apparent bubbling of the painted metal roof or siding surface around the stainless-steel fixture. Often falsely identified as a paint failure, this bubbling is the steel underneath corroding, resulting in the loss of paint.
Galvanic corrosion can be avoided by ensuring the correct fasteners, accessories, and other metal components are identified and selected in advance. Remember, multiple conditions are required to create galvanic corrosion and the selection of fasteners will be based on environmental conditions of the project. Reputable metal roofing and siding product manufacturers will provide guidance and supply the right fasteners based on their products and your project’s needs.
Also, important to avoiding galvanic corrosion is the selection of a skilled metal installer who is familiar with the effects of galvanic corrosion. A skilled installer will ensure the right fasteners, accessories, trim, and overall project detailing is correct based on the installed environment of the project.
Galvanic corrosion and its effects, such as red rust, paint bubbling, peeling, and cracking, is an installation issue, and not covered by any Steelscape finish or substrate warranty or panel manufacturer warranty. If galvanic corrosion is identified, this issue must be rectified as promptly as possible due to the rapid rate at which it can spread. Remedies may include the replacement of the panels impacted by corrosion and the substitution or relocation of the associated dissimilar metals. For more information on the identification or rectification of galvanic corrosion, please contact Steelscape.
Galvanic corrosion on metal roofing and siding can be an unsightly condition and costly to remedy. However, by following manufacturer guidance, selecting the right accessories, and installing metal roofing and siding components correctly, this issue is entirely avoidable.
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