Terne in Commercial Construction
Today’s Terne Metals
Terne metals are produced by coating carbon steel, stainless and other select metals with a specially formulated alloy consisting of zinc, tin and trace amounts of other elements in order to dramatically increase a metal’s corrosion resistance as much as ten times.
When terne metal was first used, during the colonial era, it contained roughly 80 percent lead and 20 percent tin. However, in the latter half of the 20th century, as lead was found to have potentially detrimental health effects, the lead/tin alloy had to be replaced. Seven years of metallurgic research and development produced a new and superior zinc/tin alloy in the mid-1990s. This new alloy for terne metals, proven through ASTM corrosion resistance testing, provides improved performance and aesthetics over the original, minus potential risk to health.
Besides stainless and carbon steel, the zinc/tin alloy may also be applied to other metals such as copper, bronze, tin and titanium.
Available in a variety of gauges and widths, today terne metals are used on industrial, commercial, institutional, and residential structures for roofing, gutters and downspouts, siding, soffits, facias and numerous other architectural applications.
Manufacturing Process
The terne metal manufacturing process begins with careful cleaning and inspection of the chosen substrate. Then, the substrate is plated with a specific thickness of the zinc/tin terne alloy through a hot-dip coating process.
Terne-coated stainless steel may then be micro-embossed under high pressure rollers to achieve a soft grey, satin-like, low sheen appearance. Terne-coated stainless and terne-coated copper need never be painted and are recommended for even the most corrosive applications, such as roofs, in the most corrosive environments, such as coastal areas.
After the zinc/tin plating, terne-coated carbon steel is either factory coated with a high-performance solar reflective color coating, or it is delivered to the fabricator or installer uncoated, where it will be painted after installation using a fast drying, specially formulated acrylic water-based primer and finish coat.
Performance
A terne roof using a carbon steel substrate can easily last more than 100 years with very little maintenance required. A terne-coated stainless steel roof leaves little to no chance of corrosion in any environment. Even the patina layer that forms over unpainted terne metals works as an inert corrosion barrier, shielding the alloy and further enhancing the overall protection of the substrate.
Aesthetics
Over time, unpainted terne-coated stainless steel roofs react with oxygen in the air to weather naturally to a soft grey patina. However, this can take months. To obtain a similar patina immediately upon installation, terne-coated stainless may be micro-embossed, a process that reduces the reflectivity of the metal without affecting its performance. This creates a uniform satin-like appearance remarkably similar to the natural patina of solid zinc.
Painted or factory-coated terne metals are most commonly identified by a low-sheen color coating, much unlike the shiny, high-gloss appearance of common color coated architectural metals.
Advantages
Modern zinc/tin terne metal maintains the time-tested qualities and historic aesthetics of the original lead/tin terne, with improved capability for resisting corrosion:
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Corrosion Resistance
The principal advantage of zinc/tin alloy roofing is corrosion resistance, even in harsh marine and industrial environments.
The zinc/tin alloy prevents corrosion from the presence of carbon dioxide, chlorides, or other reducing agents in the environment, significantly extending the life of the material. Also, the alloy inhibits corrosion of exposed metal anywhere in the vicinity of the coating. This assures that minor discontinuities in the coating – possibly caused by the coating process, fabrication or installation – will not fail to protect the substrate from rust.
- Strength and Formability
Zinc/tin alloy’s dull surface has excellent low temperature properties, exceptional solderability and formability, and is easy to install in any weather condition.
Zinc/tin coated stainless steel has about the same tensile and yield strength as mild steel, but a much higher ductility. The higher strength of zinc/tin-coated stainless steel means that higher loads of force are required to bend the material. However, stainless is generally used at a much thinner gage, enabling it to be formed easily with conventional roofing tools. This flexibility and formability is critical to installation.
Testing
Metals coated with modern zinc/tin terne alloy are safer and perform better than historic terne metals. Steel roofing materials are tested for corrosion-resistance in a salt spray cabinet per ASTM B117; a condensation/atmospheric chamber per ASTM D4585; and also for electrochemical testing. Non-terne roofing materials may typically exceed 2,000 hours before the material begins to develop red rust, whereas micro-embossed terne-coated steel has surpassed 8,000 hours, and counting, of continuous exposure to salt spray with no sign of red rust.
