To forestall corrosion, carbon steel fasteners should be plated or coated with zinc, cadmium, nickel, or chromium in accordance with the specifications in TIR-A9 Section 4, “Protection against Corrosion and Hydrogen Embrittlement.”
The specifier and purchaser must be aware of these matters and make the best compromise, all factors considered, in the selection of the fasteners. Designs for curtain wall anchoring systems must account for the stresses for which fasteners must be selected and the coatings to be employed to eliminate problems due to galvanic action and stress corrosion.
Various ASTM standards govern coatings or platings in terms of class, service condition, and type, which dictate acceptance testing such as duration of salt-spray application. Class designation relates to the coating thickness, while service condition indicates the severity of the exposure for which the coating is intended (ranging in four steps from mild or indoor to very severe). The type indicates the composition of the coating, such as whether chromate or phosphate layers are added.
Stainless steel offers better corrosion protection than plated carbon steel, but can present difficulties in terms of matching finishes to the framing.
Hydrogen embrittlement is a condition of low ductility in metals resulting from the absorption of hydrogen, which typically occurs during the manufacturing process (particularly electroplating), although it also can occur through in-service corrosion. Bolts and screws—with a hardness of C35 or greater on the Rockwell C scale—are particularly subject to embrittlement. Hydrogen embrittlement can cause unpredictable and potentially disastrous failure, especially of a fastener under tensile load. Tests are available to assess whether hydrogen embrittlement is present
in a batch of fasteners.
Beyond developing a thorough specification, it is essential the fastener purchaser has a means for determining the fasteners that are received meet the specifications.
ASTM standards provide chemical and mechanical requirements for the steels used in fasteners, as well as quality control procedures for shipment lot testing, source inspection, alloy control, heat control, permeability, manufacturer’s identification, and material identification. Additionally, the end user should consider adding a requirement that fasteners be produced under a recognized quality assurance (QA) program, such as that governed by the quality management system standard International Organization for Standardization (ISO) 9001, Quality Management Systems–Requirements.
|AAMA curtain wall design references|
|Additional American Architectural Manufacturers Association (AAMA) curtain wall design references for design professionals and specifiers include:
Dean Lewis currently serves as American Architectural Manufacturers Association’s (AAMA’s) educational and technical information manager, bringing his knowledge of technical training to advance the Fenestration Masters professional certification program. Lewis began his career in the fenestration industry at PPG Industries with positions in project engineering, product design, and sales and customer technical support. He has served on committees of ANSI, ASTM, and ASHRAE. Further experience includes teaching in the industrial and military sectors, and 35 years of managing technical training, publishing, and certification. Lewis can be contacted by e-mail at email@example.com.