COR-TEN®, corten, CorTen, and so on are names for steel alloys that contain copper. COR-TEN® is actually a trademark owned by U.S. Steel, who began marketing these materials more than 50 years ago, and the name has since been genericized by tradespeople to include products produced by other mills and having slightly varied alloy ratios. Originally, their intended use was for street and traffic light poles, railroad cars and structural beams and columns used outdoors without need of further coatings or treatments. They perform well from a corrosion standpoint provided however that their surface is always well drained. Tendencies to early corrosion are exacerbated when moisture is retained on their surface from capillary entrapment. The designator and standard for thin sheet typically used for roofing is ASTM A606.
More appropriately, these alloys should be generically referred to as “weathering” steels. A handful of U.S. domestic mills produce weathering steels. Typically, carbon steel is alloyed with small amounts of copper (usually less than .5%), chrome and nickel in various ratios by producing mills. The oxides formed by copper, chrome and nickel are much denser and tighter that is iron oxide, hence these alloyed metals tend to “seal” the surface and protect the steel from corrosion that would occur much more rapidly on ordinary carbon steel (“black steel” or “cold-rolled”). Weathering steels are sometimes called “bleeding” steel because of the red colored oxides that leach off the material during the patination process—potentially staining adjacent construction.
Copper and aluminum are dissimilar metals and electrolytic contact of these metals may induce and accelerate corrosion of the aluminum. The severity of electrolytic corrosion depends upon many factors including the severity and frequency of electrolytic contact (accelerating factors); the relative sizes of the interfacing materials and the behaviors of the parent metals and their various oxides as the interfacing materials weather (which may help to retard the corrosion process). Aluminum is more anodic in behavior than copper, chrome or nickel, so given adequate electrolytic severity, the aluminum would corrode, not the weathering steel.
Typically, weathering steel has been used on corrugated roofs, but in recent years is also becoming popular on standing seam profiles. In a practical sense, aluminum forms an oxidizes quite rapidly and the oxide layer is much more inert and stable than the parent metal that created it (passive in behavior rather than anodic). Hence in practice, aluminum brackets or clamps should pose little (if any) threat of adverse corrosive effects with weathering steels, except perhaps in a highly corrosive environment, such as sea coast applications where breaking surf (seawater) is the electrolyte, or in a severe acid-rain environment. In such cases, the aluminum is threatened, not the weathering steel roofing.
Possible Safeguard Options
The variables mentioned earlier may change from project to project as each application will have its own environmental conditions and issues. With all that said the climate could be such, or the application such that corrosion is of minimal concern for the given application. The aluminum clamps in electrolytic contact with a dissimilar metal in an arid climate will perform differently than in a marine environment. Ultimately, the decision must be made by the user based upon the real risk and actual expense. So, when considering additional safeguards, the user must ask, “is the juice worth the squeeze?”
Anodizing is an electro-chemical passivation process used to increase the thickness of the natural oxide layer on the surface of aluminum, increasing corrosion resistance and wear resistance of clamps or brackets. Anodized aluminum is often postulated as an appropriate method to separate aluminum from a dissimilar metal. There are anodizers in virtually any “Major City, USA”. This precaution adds considerable cost and inconvenience, however. It should be noted that when the anodized coating is broken (via surface scratching during packing, shipping or during and post installation), the scratch location is more subject to accelerated corrosion. And the aluminum oxides will form naturally on their own with outdoor exposure.
Mill finish with a separator:
Depending on seam/joint geometry, a stainless-steel separator, butyl tape or EPDM pad can be used to separate the metals. (S-5! Brackets come with butyl or EPDM factory-installed.)
Stainless steel clamps:
Stainless is compatible with all weathering steels however it will depend on the seam geometry and stainless is expensive. In some cases, a stainless clamp is available from S-5! Stainless brackets are not, except on a very large, custom order basis.
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