CapabilitiesCapabilities andand LimitationsLimitations ofof ArchitecturalArchitectural Metals:Metals: Part 1 by Catherine Houska etal offers many attractive design possibilities. But to Muse metals safely and eco- nomically, architects, contractors, and building owners must under- stand both their capabilities and limitations. A metal providing good performance in one application can corrode rapidly under different cir- cumstances. Strength, weight, desired appearance, corrosion resistance, expected service life, and costs associ- ated with installation and longterm maintenance must be considered. PHOTOS COURTESY NICKEL DEVELOPMENT INSTITUTE October 2000 The Construction Specifier 1 ADDITIONAL INFORMATION 304 and 316 and other 300-series stainless oxide (rust) layer. Corrosive substances can steels are not magnetic and can be still pass through the oxide film, but corro- Author strengthened by cold work. In most archi- sion rates decrease as the layer thickens.1 CATHERINE HOUSKA, CSI, is a tectural applications, stainless steel is used metallurgical engineer with an MBA in in the annealed or "dead soft" condition. In This uniform protective layer will not form industrial marketing. She specializes in architectural applications for metal and structural and sheet applications subjected indoors, or in arid or continuously wet is market development manager at to higher loads or impact, cold working can environments. If this layer is allowed to Technical Marketing Resources, Inc., a be an economical means of reducing form and is not disturbed, weathering consulting firm in Pittsburgh, Pennsylvania. She can be reached via weight and increasing strength. steels provide significantly better corrosion e-mail at [email protected]. resistance than carbon steels. Type 430 is used in indoor, rural, or urban MasterFormat No. locations with low corrosion potential. Weathering steels should not be used 05050—Basic Metal Materials Type 304 is used in indoor, rural, moder- where mechanical or chemical disruption and Methods ately polluted urban applications, and of the film is likely. Mechanical damage coastal applications with low humidity and could result from windborne abrasive par- Key Words temperatures. Type 316 is used for marine, ticles, rubbing, or scratching. Chlorides alloys coastal, or deicing salt exposures and in and industrial pollutants disrupt the film. oxide industrial and aggressive urban locations. Weathering steels are not suggested for corrosion In some very aggressive sites, a more corro- coastal, marine or deicing salts, or industri- 3 Abstract sion-resistant stainless steel may be needed. al environments. Metal has become increasingly popular because it provides unique design If an appropriate stainless steel is Water coming into contact with weathering opportunities, strength, and corrosion used, there is no reason to apply steel can stain other materials. Designs must resistance. Each metal family has a protective coating. A variety of drain water away from other materials and unique characteristics. Performance over time depends on the designer mechanical and colored finishes are permit the surface to dry. Elements not understanding the advantages and available. Stainless steel is available in boldly exposed to the atmosphere should limitations of each metal and taking sheet, strip, plate, bar, tube, pipe, castings, be painted. (See photo at end of article.) these into consideration during the design process. and extrusions. Stainless steel is some- This article provides some guidelines for times mistakenly ordered determining the service environments by gage number. There where architectural metals are likely to is no standard definition perform well and where problems may of stainless-steel gage occur. Physical and mechanical properties numbers, and specifiers will be summarized and comparative should always specify the appropriate thicknesses in sample applica- desired thickness. tions will be shown. Selection Criteria: Selection Criteria: Stainless Steels Weathering Steels Stainless steels contain at least 12 percent High-strength, low-alloy chromium. They form a thin (invisible), weathering steels are car- protective, corrosion-resistant passive film bon steels with combined on their surface. The film self-repairs additions of up to two per- quickly if it is damaged or removed dur- cent copper, chromium, ing fabrication or polishing. If an appro- nickel, and phosphorus. priate stainless steel is selected, it will They are used in exterior retain its original appearance with no cor- applications and have rosion product discoloration of sur- mechanical properties rounding materials. similar to those of struc- tural carbon steels. In cli- The stainless steels most commonly used in mates with wet and dry architecture and construction are Types cycles, they form a protec- The United States Steel Worker’s Building (formerly the IBM Building) was completed in 1963. It has Type 304 stainless steel 430, 304, and 316. Type 430 and other 400- tive, dense, adherent, cladding on the steel lattice trusses and anodized aluminum panels series stainless steels are magnetic. Types stable, corrosion-resistant under the windows. Architect: Curtis & Davis. 2 The Construction Specifier October 2000 The corrosion rate is nonoxidizing acids. Alkalis (which may dependent on the envi- leach from concrete), ammonia, and sul- ronment and protective fur compounds will cause corrosion. Fire- coating integrity or the retardant treated wood may be treated effectiveness of the with chemicals that corrode copper. cathodic protection sys- tem (e.g., zinc). Regular Brass has good resistance to atmospheric exposure to moisture corrosion, alkalis, and organic acids. In some (rain, fog, high humidity potable waters and in seawater, brasses with levels), industrial pollu- 20 percent or more zinc may suffer corrosive tion, sulfur oxides, and attack. Moist ammonia, mercury, and moist chlorides can significantly chlorine gas can cause stress corrosion increase corrosion rates. If cracking. Brasses used in architecture protection is not main- include commercial bronze, red brass, car- tained, rust can discolor tridge brass, yellow brass, and Muntz metal. surrounding materials, and rust may undermine Nickel silvers are copper-nickel-zinc alloys more of the protective with a golden-silver luster. They were par- surface layer. Surface rust ticularly popular during the early 1900s buildup should be when Art Deco architecture was in style. removed or it will retain Other names used for this alloy family are moisture, concentrate German silver, white copper, white bronze, corrosive chemicals, and Paris metal, and dairy bronze. accelerate corrosion.3 Selection Criteria: MONEL® Selection Criteria: MONEL® is a trade name for a highly Copper and corrosion-resistant family of silver-white One Oxford Center has a painted carbon steel curtain wall and was Copper Alloys colored nickel-copper alloys. Alloy 400 is completed in 1983. Architect: Hellmuth, Obata, & Kassabaum. The natural color of cop- the generic name for the alloy used in per alloys Selection Criteria: Carbon Steel varies considerably with Carbon steels are at least 95 percent iron alloy content and includes with two percent or less carbon and are shades of red, gold, and classified by grade based on their carbon silver. Copper and its content. Strength and hardness increase alloys form a thin adher- with carbon content but ductility decreas- ent oxide layer or patina es. The grades typically used in architecture which gradually changes are very mild (0.05 percent to 0.15 percent the metal’s color. The carbon), mild (0.15 percent to 0.25 percent color will vary with the carbon), and medium (0.25 percent to alloy and may not be uni- 0.35 percent carbon) carbon steel.2 form. If moisture is pre- sent, copper can stain Carbon steels are available in the product porous materials. Detail- forms needed for architecture including ing should prevent water structural shapes, pipe, tube, sheet, and run-off onto these sur- strip. Because bare carbon steels will rust in faces. A wide range of air, they are normally protected with metal product forms are avail- coatings, porcelain enamel, or paint. able. Intricate castings Common metal coating processes include are possible. aluminized (aluminum), chromium, cop- per, brass, galvanized (zinc), Galvalume® Electrolytic tough pitch (aluminum-zinc), terne (lead-tin or tin- (99.9 percent pure) copper, zinc), and tin. The precautions discussed in used for roofing, provides the zinc alloys section also influence the life good corrosion resistance The copper details on the dome of Midtown Towers were installed in of zinc coatings. to chlorides and deareated 1908. Architect: Thomas Hannah. October 2000 The Construction Specifier 3 Aluminum forms a pro- darker gray patina on these alloys. tective oxide film with a high level of corrosion Zinc is resistant to chloride corrosion, but resistance when clean industrial atmospheres containing sulfur metal is exposed to air. dioxide can accelerate corrosion. The only Anodizing strengthens common woods that can be used in contact and thickens the film, with zinc are pine, poplar, and spruce. If zinc providing greater corro- is in contact with another metal and mois- sion protection. Either ture is present, the metals must be electrical- anodized or resinous ly insulated from each other because zinc organic coated alu- corrodes in preference to all other architec- minum is normally spec- tural and engineering metals. This effect is ified for exterior