The glass ceiling
A metal providing good performance in one application can corrode rapidly under different circumstances. Strength, weight, desired appearance, corrosion resistance, expected service life, and costs associated with installation and long-term maintenance must be considered.
This article provides some guidelines for determining the service environments where architectural metals are likely to perform well and where problems may occur. Physical and mechanical properties will be summarized and comparative appropriate thicknesses in sample applications will be shown.
Stainless steelsStainless steels contain at least 12% chromium. They form a thin (invisible), protective, corrosion-resistant passive film on their surface. The film self-repairs quickly if it is damaged or removed during fabrication or polishing. If an appropriate stainless steel is selected, it will retain its original appearance with no corrosion product discoloration of surrounding materials.
The stainless steels most commonly used in architecture and construction are types 430, 304, and 316. Type 430 and other 400-series stainless steels are magnetic. Types 304 and 316 and other 300-series stainless steels are not magnetic and can be strengthened by cold work.
In most architectural applications, stainless steel is used in the annealed or "dead soft" condition. In structural and sheet applications subjected to higher loads or impact, cold working can be an economical means of reducing weight and increasing strength.
Type 430 is used in indoor, rural, or urban locations with low corrosion potential. Type 304 is used in indoor, rural, moderately polluted urban applications, and coastal applications with low humidity and temperatures. Type 316 is used for marine, coastal, or deicing salt exposures and in industrial and aggressive urban locations. In some very aggressive sites, a more corrosion-resistant stainless steel may be needed.
If an appropriate stainless steel is used, there is no reason to apply a protective coating. A variety of mechanical and colored finishes are available. Stainless steel is available in sheet, strip, plate, bar, tube, pipe, castings, and extrusions.
Stainless steel is sometimes mistakenly ordered by gauge number. There is no standard definition of stainless-steel gage numbers, and specifiers should always specify the desired thickness.
Weathering steelsHigh-strength, low-alloy weathering steels are carbon steels with combined additions of up to two per-cent copper, chromium, nickel, and phosphorus. They are used in exterior applications and have mechanical properties similar to those of structural carbon steels.
In climates with wet and dry cycles, they form a protective, dense, adherent, stable, corrosion-resistant oxide (rust) layer. Corrosive substances can still pass through the oxide film, but corrosion rates decrease as the layer thickens.
This uniform protective layer will not form indoors, or in arid or continuously wet environments. If this layer is allowed to form and is not disturbed, weathering steels provide significantly better corrosion resistance than carbon steels.
Weathering steels should not be used where mechanical or chemical disruption of the film is likely. Mechanical damage could result from windborne abrasive particles, rubbing, or scratching. Chlorides and industrial pollutants disrupt the film. Weathering steels are not suggested for coastal, marine or deicing salts, or industrial environments.
Water coming into contact with weathering steel can stain other materials. Designs must drain water away from other materials and permit the surface to dry. Elements not boldly exposed to the atmosphere should be painted.
Carbon steelCarbon steels are at least 95% iron with two percent or less carbon and are classified by grade based on their carbon content. Strength and hardness increase with carbon content but ductility decreases.
The grades typically used in architecture are very mild (0.05% to 0.15% carbon), mild (0.15% to 0.25% carbon), and medium (0.25% to 0.35% carbon) carbon steel.
Carbon steels are available in the product forms needed for architecture including structural shapes, pipe, tube, sheet, and strip. Because bare carbon steels will rust in air, they are normally protected with metal coatings, porcelain enamel, or paint.
Common metal coating processes include aluminized (aluminum), chromium, copper, brass, galvanized (zinc), Galvalume? (aluminum-zinc), terne (lead-tin or tin-zinc), and tin. (Galvalume? is a registered trademark of Bethlehem Steel Corp.)
The precautions discussed in the zinc alloys section also influence the life of zinc coatings. The corrosion rate is dependent on the environment and protective coating integrity or the effectiveness of the cathodic protection system (e.g., zinc).
Regular exposure to moisture (rain, fog, high humidity levels), industrial pollution, sulfur oxides, and chlorides can significantly increase corrosion rates. If protection is not maintained, rust can discolor surrounding materials, and rust may undermine more of the protective surface layer. Surface rust buildup should be removed or it will retain moisture, concentrate corrosive chemicals, and accelerate corrosion.
Copper and copper alloysThe natural color of copper alloys varies considerably with alloy content and includes shades of red, gold, and silver. Copper and its alloys form a thin adherent oxide layer or patina, which gradually changes the metal's color. The color will vary with the alloy and may not be uniform.
If moisture is present, copper can stain porous materials. Detailing should prevent water run-off onto these surfaces. A wide range of product forms are available. Intricate castings are possible.
Electrolytic tough pitch (99.9% pure) copper, used for roofing, provides good corrosion resistance to chlorides and deareated nonoxidizing acids. Alkalis (which may leach from concrete), ammonia, and sulfur compounds will cause corrosion. Fire-retardant treated wood may be treated with chemicals that corrode copper.
Brass has good resistance to atmospheric corrosion, alkalis, and organic acids. In some potable waters and in seawater, brasses with 20% or more zinc may suffer corrosive attack. Moist ammonia, mercury, and moist chlorine gas can cause stress corrosion cracking. Brasses used in architecture include commercial bronze, red brass, cartridge brass, yellow brass, and Muntz metal.
Nickel silvers are copper-nickel-zinc alloys with a golden-silver luster. They were particularly popular during the 1920s and 30s when art deco architecture was in style. Other names used for this alloy family are German silver, white copper, white bronze, Paris metal, and dairy bronze.
MonelMonel is a trade name for a highly corrosion-resistant family of silver-white colored nickel-copper alloys. (Monel? is a registered trademark of Special Metals Corp.)
Alloy 400 is the generic name for the alloy used in architecture. It was very popular from 1909 to the mid-1950s and is sometimes mistaken for stainless steel in restoration work. It continues to be used in roofing.
In exterior applications, it will develop a patina ranging from light gray-green to medium brown. It provides excellent resistance to atmospheric corrosion, including relatively severe chloride levels, alkalis (which may leach from concrete), or industrial pollution
Aluminum alloysAluminum alloys are lightweight, silver-colored, and provide corrosion resistance in a variety of environments. Some alloys used in architectural applications are 3003, 3105, 5005, 6061, and 6063. The 3000 and 5000 series alloys can be strengthened by heat-treating ("H" tempers). The 6000 series alloys can be strengthened by cold work ("T" tempers).
Aluminum forms a protective oxide film with a high level of corrosion resistance when clean metal is exposed to air. Anodizing strengthens and thickens the film, providing greater corrosion protection. Either anodized or resinous organic coated aluminum is normally specified for exterior applications. Ceramic coatings can also be applied. Aluminum is available in sheet, strip, bar, plate, tube, pipe, forgings, castings, and extrusions.
When aluminum corrodes, the surface becomes rough and chalky with a white or gray film. The film color makes corrosion less visible but there can be some staining of adjacent surfaces. Atmospheric sulfates and coastal and deicing salts can cause corrosion. A heavy protective coating is needed if aluminum will be in contact with concrete, masonry, plaster, or soil.
Wood in contact with aluminum should be sealed with a primer and paint. However, fastenings into oak, chestnut, or western cedar should be avoided unless the wood is well seasoned. Under damp conditions, insulating products can release byproducts corrosive to aluminum unless a sealant is used.
ZincZinc's low creep strength at room temperature and high thermal expansion coefficient should be considered during design of roof and wall panels. Zinc alloys with at least 0.05% titanium and 0.10% copper have improved creep resistance and mechanical strength and weather to a grayer patina than pure zinc. A phosphate pretreatment can be used to produce a darker gray patina on these alloys.
Zinc is resistant to chloride corrosion, but industrial atmospheres containing sulfur dioxide can accelerate corrosion. The only common woods that can be used in contact with zinc are pine, poplar, and spruce.
If zinc is in contact with another metal and moisture is present, the metals must be electrically insulated from each other because zinc corrodes in preference to all other architectural and engineering metals. This effect is so strong zinc is regularly used as a sacrificial anode to protect other metals.
Zinc and zinc coatings must be protected from condensation and standing water. There should be a free flow of air behind zinc panels to prevent condensation. If thermal insulation is used, the space between the zinc and roof sheeting should be vented. Zinc should not be placed in direct contact with cement, mortar, or other moisture retaining materials.
Extending service lifeLike all materials used in exterior environments, metals can accumulate dirt and debris. These deposits can be corrosive. The corrosion resistance of metals is improved by regular removal of corrosive deposits from the surface.
In environments with plentiful rain, designs encouraging natural rain cleaning can be effective in removing corrosive deposits, particularly if the surface finish is smooth. Building exteriors often have elements not easily washed by rain. Regular cleaning to remove deposits can enhance the corrosion resistance of metals, extend their service life, and keep them attractive.
Mechanical and physical propertiesTypical mechanical and physical properties of some common architectural metals are shown in Table 1 and Table 2. The ASTM specifications should always be used to specify the desired grade or alloy and its mechanical properties. Additional information on physical properties can be obtained from metal suppliers, industry associations, and metals reference books such as the ASM International Metals Handbook series.
Specifying metal thicknessMetal strength, strength-to-weight ratios, corrosion resistance, and wear resistance determine the appropriate thickness or weight for a project. Assuming the same thickness is appropriate for two different metals (e.g., steel versus stainless steel) can lead to costly over or under specification.
Industry associations and manufacturers can be a good source of guidelines. The Architectural Sheet Metal Manual published by the Sheet Metal and Air Conditioning Contractors' National Association is an excellent resource for determining appropriate metal thicknesses, gauges, or weights. Examples are shown in Table 3 and Table 4.
Fire resistanceResistance to fire and thermal radiation damage can be an important safety consideration. The metals used for architecture exhibit significant differences in strength retention at elevated temperatures even after brief exposure to high temperatures.
Copper and aluminum start to experience strength reductions at fairly low temperatures. Aluminum alloys begin to show a reduction in strength at temperatures above 100?C (212?F). At 204?C (400?F), the tensile strength of copper has decreased about 25% and 6061-T6 aluminum has decreased by about 60%.
Under continuous loading, carbon steel is usually limited to a maximum temperature of 370?C (700?F). By the time steel reaches 500?C (930?F), it has lost about 30% of its tensile strength. Unprotected weathering steel loses about half of its tensile strength above 538?C (1000?F).
Both carbon and weathering steel are normally encased in insulating materials to limit temperature increase during fires. Types 316, 304, and 430 stainless steel can withstand short exposure times up to 870?C (1600?F).
Five industry associations and metals producers sponsored fire and radiation resistance testing of structural materials: galvanized carbon steel, fiberglass-reinforced plastic (FRP), aluminum, and Type 316 stainless steel.10 The tests used commercially available 10-m (33-ft) long cable ladders uniformly loaded to simulate the weight of electrical cables. The FRP and aluminum ladders failed both tests.
In the five-minute test for fire-resistance, the metals were exposed to direct flames at 1,000?C to 1,050?C (1,832?F to 1,922?F). The galvanized mild steel passed the five-minute exposure requirement and reached 642?C (1,188?F). Type 316 ladder passed the five-minute test.
The test time was extended to 45 minutes, and it reached 705?C (1,300?F). The deflection of the galvanized steel after five minutes was 166.5 mm (6.6 in.). The deflection of the stainless steel after 45 minutes was 80.5 mm (3.2 in.).
Radiation testing simulates heating by radiation rather than with direct flame. The test was continued until the ladder temperature stabilized or structural failure occurred. The galvanized steel ladder reached a stable temperature of 552?C (1,026?F) in two hours. The stainless steel reached temperature stability at 556?C (1,033?F) in three hours. The deflection of the stainless steel after three hours was about one-third of the galvanized steel ladder after two hours.
When specifying a metal, it is important to identify the specific grade or alloy and the appropriate ASTM standards for the desired product form. Properties and corrosion resistance can vary significantly within a metal family. Specifying just stainless steel, aluminum, or copper is not adequate. The wrong grade or alloy within the metal family could be supplied, and it may not meet application requirements.
Metal performance can vary significantly with the environment. Environments should be carefully reviewed to avoid metal corrosion that could be detrimental to aesthetic, building envelope, or structural requirements. The manufacturer or industry association should be consulted to determine if the metal, coating, thickness, or weights are appropriate for the application.