Terne
Terne is an alloy coating that was historically made of lead and tin used to cover steel, in the ratio of 20% tin and 80% lead. Until the year 2012 lead had been replaced with the metal zinc and was used in the ratio of 50% tin and 50% zinc. This alloy had a low melting point of approximately 360 degrees Fahrenheit but is no longer available. Today terne coated metal is coated with 99.9% tin, instead of hot-dipping, a more consistent galvanic deposition process is applied. Additionally the substrate has been changed from steel to stainless steel, benefitting from the corrosion resistance of stainless steel alloys. The pure tin alloy makes soldering easier, as the melting point is homogenised and doesn't need to match the melting point of two metals with different melting temperatures.
Use
Terne was historically used to coat sheet steel to inhibit corrosion. It was one of the cheapest alloys suitable for this, and the tin content was kept at a minimum while still adhering to a hot-dipped iron sheet, to minimize the cost.
Historical terne metal must be painted. If the paint is maintained, terne metal can last 90 years or more. Terne coated stainless steel (TCS II or Roofinox) or copper is commonly used to replace terne metal roofs as either material will outlast terne metal. Terne coated stainless steel roofing can last 100 years or more unpainted, copper roofing can last 50 years or more unpainted.
Terne II used zinc in place of lead for environmental reasons. Today's main reason and benefit of the tin coating is its aesthetic behavior. Tin develops a natural matte grey patina when exposed to the elements. The result of the weathering process depends on the environment and climate conditions present. Depending on the environment and atmospheric conditions different substrate alloys are used. The stainless steel alloy 439 is ferritic and is used for normal environments; for coastal applications, the austenitic stainless steel alloy 316L is used providing an increased corrosion resistance to chlorides.
Terminology
Terne-coated steel is also known as terne-metal and often referred to as "tin" (see the Slate Roof Bible). Traditional terne-coated steel cannot be purchased anymore, it had been replaced by a terne coated stainless steel product (Roofinox tin-plated (Terne)), available at most roofing supply stores.