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The Alloy

 

What is ZT Alloy?

ZT® alloy is a proprietary coating of Follansbee Steel that is formulated specifically as a corrosion resistant coating for sheet steel. ZT alloy is comprised of zinc, tin, and traces of other elements.

Compared to aluminum/zinc alloys, ZT alloy has been proven, both in laboratory and real-world installations, to be far more resistant to atmospheric corrosives. This quality makes it ideal for locations like coastal environments with salt-laden air, heavily industrialized areas, etc. ZT alloy coated steel sheet is the most sensible material choice for exterior architectural applications such as walls, roofs and other non-architectural exterior applications because of its remarkable corrosion-resistance.

Over the past decade, the US architectural and building owner communities have embraced the principles of sustainability. As a result, use of ZT alloy coated steel has risen sharply during this time. Because ZT alloy coated steels so dramatically outlast their aluminum/zinc alloy counterparts, an increasing number of architects and owners are willing to invest a premium for ZT alloy materials in order to achieve a minimally quadrupling of material lifecycle.

 

Why Should You Invest in Premium ZT Alloy Coated Steel?

The answer is simple: corrosion resistance. No other steel sheet material outperforms ZT alloy coated steel. It lasts nearly 18 times as long as aluminum/zinc coated steel, and 94 times longer than G90 galvanized.

ASTM B117 salt spray test remains a standard method to assess a galvanized coating for corrosion-resistance. In this test a sample is exposed to a fine mist containing 5% sodium chloride (NaCl) for a period of time at room temperature. When 5% of the surface area shows signs of red rust, the test is terminated.

In ASTM B117 testing, Follansbee ZT alloy coated carbon steel outperforms Aluminum/Zinc coated carbon steel by a factor of 17.9 and galvanized steel by a factor of almost 94.

Material Hours exposed before 5% or more corrosion product had been produced on the surface
Aluminum/Zinc coated carbon steel 1,075*
G90 galvanized steel 205*
Terne II ZT Alloy coated carbon steel 19,250**
 

* Data from GalInfo Center, GalvInfoNote 3.4, Rev 1.1 Jan 2011
** Data from Matco Services, Inc., Report No. 905-50686 May 25, 2009

 

How Does ZT Alloy Coated Stainless Improve Type 304 stainless?

After all, isn’t stainless already impervious to corrosion? In fact, no it isn’t. True, stainless is far more corrosion resistant than carbon steel – but stainless is anything but impervious to atmospheric corrosives. Type 304 stainless steel found in a coastal areas, for example, will pit, corrode, and fail over time.

Protecting stainless with ZT alloy can greatly extend the life of the material. In ASTM B117 testing, Follansbee ZT alloy coated stainless steel surpassed 30,750 hours without reaching the required 5% of visible surface corrosion.

It should be noted that most metallurgists find no definitive correlation between ASTM B117 and real-world applications. Some research suggests that type 304 stainless tested for 100 hours in ASTM B117 is the equivalent of one month of exposure in the most extreme coastal environments. So, according to this formula, ZT alloy coated stainless should last 25 years in this worst case scenario.

Perhaps the better predictor of the corrosion resistance of any Follansbee Steel material are the hundreds of real world applications of exterior roofs and walls, many of which are located in the harshest environments on the planet. The first terne coated stainless roofs installed in the late 1960‘s still continue to look and perform perfectly, more than 40 years later.

 

What are the Advantages to Using ZT Alloy?

 
The principal advantage of ZT alloy coated metals – stainless (TCS II and TCS II Satin) and low carbon steel (Terne II and KlassicKolors) – is its excellent corrosion-resistance in marine and industrial locations where other metals corrode readily. The alloy has a dull surface, excellent low temperature properties, and is not degraded by solar energy. It is easily worked, and can be installed quickly in any weather, especially in wet weather when built-up roofing materials cannot be installed.
 
The Power of Electroprotection
 
ZT alloy is a two-phase metallic coating that, when applied to stainless steel or low carbon steel materials, forms a highly corrosion-resistant coating, which reduces the corrosion rate of the materials when exposed to the atmosphere. By adding zinc to tin, the two phase metallic coating exhibits corrosive resistant properties that exceed that of tin coatings and original lead-based terne materials, especially in marine environments.
 
ZT alloy is electroprotective under corrosive conditions. This quality inhibits corrosion of exposed metal near the zinc/tin coating. As a result, minor discontinuities in the ZT alloy coating do not result in oxidation of the exposed metal; the opposite results if only a tin coating is used. The zinc/tin coating may contain small amounts of other metals to modify the physical properties of the zinc/tin, two-phase metallic coating; however, these secondary metal components contribute primarily to the strength of the coating and not to the corrosion-resistant properties of the coating. 
 
Flexibility of Application
 
ZT alloy is applied to both stainless steel and carbon steel sheet materials through a hot-dip coating process. However, the coating may be applied by other means, such as electroplating and airknife processing, etc. ZT alloy coating is not limited to only protecting stainless steel and carbon steel, but may also be applied to other metals such as copper, bronze, tin, titanium, etc.
 
ZT Alloy Even Protects Itself
 
ZT alloy coating greatly extends the life of a metal by acting as a barrier to the harsh atmosphere, thereby preventing the metallic coating from corroding in the presence of oxygen, carbon dioxide, chlorides, or other reducing agents in the environment. Although ZT alloy coating corrodes in the presence of various corrosive agents in the atmosphere, the rate of corrosion is significantly slower than that of other materials. For example, ZT alloy coated stainless steel materials (TCS II) has surpassed 30,000 hours of direct exposure to the sea mist in a marine environment. 
 
Furthermore, the outermost zinc/tin protective layer, which forms on the coating surface, provides corrosion-resistance to the tin-zinc coating itself. This further enhances the overall corrosion protection afforded by this coating. The zinc/tin protective layer also reduces the reflectivity of the zinc/tin coating.
 
 
Desired Aesthetic
 
Follansbee steel discovered that the novel ZT alloy formulation forms a colored coating which closely matches the popular gray, earth tone color of weathered traditional lead-based terne materials.
 
The aesthetic qualities of ZT alloy vary in different environments. In outdoor exposure with excessive pollution, or in the presence of chlorides, zinc/tin coatings on stainless steel develops a dull gray earth-tone patina, which gives the overall look of a rich gray finish.
 
  • In rural environments, nature will provide sufficient cleaning, and the surface will probably retain its appearance for many years.
  • In an industrial environment, some airborne particles can adhere to the surface, which, after many years, may require cleaning. Industrial environments may cause the formation of light gray corrosion products. The production of protective corrosion residues in the pores of the coating may give additional corrosion resistance in industrial areas. Under normal atmospheric conditions both H2S and SO2, reports show little effect on the tarnishing of the exposed ZT alloy coating.
  • In the very corrosive marine environment, because of the constant presence of chlorides, the material develops a compact gray protective corrosion product, depending on the distance from the ocean.