In general construction, particularly in the building cladding industry, contact between two different types of metal should be avoided. But why is this so important? The answer is galvanic corrosion, which refers to corrosion induced when two dissimilar materials are coupled in a corrosive electrolyte. More simply put, when two non-similar metals are connected by an electrolyte, such as water, the materials in contact may oxidize or corrode. In order for this to occur, there are three conditions that must be met:
1 – There must be two electrochemically different metals present (i.e. Aluminum and Galvanized Steel).
2 – The metals must be in contact with one another or there must be an electrically conductive path between them.
3 – The two metals must be subjected to an electrolyte, such as water.
In roofing, the most common occurrence of galvanic corrosion occurs predominantly at a point where a panel has been cut or punctured. For example, if an anodic (less noble) metal fastener (galvanized steel) penetrates through a more cathodic (more noble) metal (Copper) the risk of severe corrosion on the galvanized steel fastener is very high.
The second major factor that determines the severity of galvanic corrosion is the voltage difference between the two metals on the Galvanic Series. In reference to the galvanic series chart, each alloy is given a range of voltage starting at the top with the least noble (anodic) and increasing voltage to the bottom where the most noble (cathodic) alloys are present. Basically, the farther apart the alloys are from each other on the chart, the greater the severity of galvanic corrosion, i.e. galvanized steel vs. copper.
Another way that galvanic corrosion can occur in the roofing industry is by the transfer of ions from water. For example, never use copper or lead flashings with aluminum, zinc or galvanized roofing materials. Even water dripping from a copper flashing “upstream” of a steel or aluminum flashing or gutter can lead to corrosion of the coated-steel or aluminum roofing materials.
Even though galvanic corrosion can be severely damaging to metal roofing components and accessories, if the proper precautions are taken, the risk of its occurrence can be greatly reduced. Be sure to use the same types of metal throughout the roof system. If copper flashings are present on the roof system, either replace them with the same metal being used on the rest of the project, or assure that any new metal being installed is also copper, if it will be in contact with, or “downstream,” of existing flashings. Another way to reduce the risk of galvanic corrosion is to use nonmetallic, nonabsorbent insulators between the dissimilar metals to prevent current flow.
Although galvanic corrosion is a small consideration when compared to the overall scope of the project, it is important nonetheless. After all, some of the most critical components of the roof can be subject to this type of corrosion. Assuring that the proper precautions are taken when designing a roof system and its components can help maintain the performance and beauty of the finished product for years to come.