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Know Your Equivalent Coatings

H ow durable do construction materials need to be? Anyone who has ever walked onto a project would probably say, “pretty tough.” Construction materials must survive transport, storage, assembly, installation, modification by other trades and ultimately the environment for the life of the building. Steel is considered one of the toughest materials in construction with a prolonged life span when manufactured, handled and installed to industry standards. For steel framing, the base material specification is ASTM A1003/A1003M, Standard Specification for Steel Sheet, Carbon, Metallic- and Nonmetallic-Coated for Cold-Formed Framing Members. ASTM A1003 defines the mechanical properties, chemistry and coating required for steels that are formed into studs and track.




The durability of steel framing is inherently linked to its ability to resist corrosion. By nature, bare steel will oxidize (rust) quickly when exposed to oxygen, typically in the form of moisture. The coatings on the surface of steel protect the base metal by creating a barrier that prevents moisture from reaching the substrate. Generally, coatings can be classified in three ways: sacrificial coatings that react in the presence of moisture but corrode over a longer period of time than steel; barrier coatings that also react to oxygen to form protective oxide films but do not corrode further; and paints that provide a barrier but are not impervious to moisture.




ASTM A1003 specifies the acceptable metallic coatings for studs and track and classifies them for use in structural and nonstructural products. These coatings include Hot-dipped Galvanized (G40 and G60), Galvannealed (A40 and A60), Galvalume (AZ50), Galfan (GF30), Electro-galvanized (60G/60G and 90G/90G), and Type 1 and Type 2 Aluminized (T1-25 and T2-100). Minimum weights of each of these coatings are specified in ASTM A1003 Table 1 for structural (Type H and Type L) and nonstructural (Type NS) applications. Because structural framing is often installed in the building envelope, and therefore closer to the exterior environment, a higher level of performance and coating weight is required than for nonstructural members installed in the interior of the building. Table 1 of ASTM A1003 is reproduced below for reference. Each coating type is further defined in other ASTM standards indicated in the footnotes.




Most industry members are familiar with popular, standard coatings like galvanized and galvanealed, however, new “EQ” or equivalent coatings are hitting distribution yards and job sites. These new coatings are produced by a variety of manufacturers and vary in both appearance and performance. Commonly, EQ coatings are the combination of a base metallic coating and an outer coating that chemically bonds to the base. The base metallic coatings are generally standard coatings that do not meet the minimum weight requirements of ASTM A1003. To improve the performance of the base coating, a supplemental coating is added to improve the coating corrosion resistance. Luckily, ASTM A1003 provides performance-based requirements for alternative coatings.




Alternative Coatings


ASTM A1003 defines performance levels for three types of alternative coatings for use on structural and non-structural members: metallic-coated; painted metallic-coated; and painted nonmetallic-coated. The coatings are evaluated using the methods specified in ASTM A1004/A1004M, Standard Practice for Establishing Conformance to the Minimum Expected Corrosion Characteristics of Metallic, Painted-Metallic, and Nonmetallic-Coated Steel Sheet Intended for Use as Cold Formed Framing Members. For each type of coating, ASTM A1004 outlines a specific procedure that includes testing samples in a salt spray (salt fog) apparatus as defined by ASTM B117. Because each coating type acts differently, the test durations and evaluation methods are different.




In accordance with ASTM A1004, metallic-coated sheet steels not listed in Table 1 must exhibit “the minimum corrosion characteristics of G60 for the Type H and Type L steels [structural] and G40 for the Type NS steels [nonstructural].” The minimum corrosion performance test requires that samples be exposed to a salt spray for 75 hours for nonstructural steels and 100 hours for structural steels. The samples are then evaluated for coating loss. For a metallic coating to be compliant, it must exhibit no more than 10 percent red rust on the surface of the sample.




Although less common in the market, painted products are permitted by ASTM A1003. Painted metallic-coated and painted nonmetallic-coated sheet steels are also evaluated for corrosion protection in a salt spray test. The samples are scribed (scraped) through the non-metallic (paint) layer down to the base coating. Painted metallic-coated specimens are tested for 500 hours, and painted nonmetallic-coated specimens are tested for 250 hours. The specimens are then evaluated for creepage and blistering of the paint in accordance with other ASTM test methods.




Get Used to Change


While EQ coatings have become more prevalent in recent years, innovation is not new to the steel framing industry. Over the last seven years the industry has seen several generations of EQ drywall studs and track, in which material properties such as thickness, shape and yield strength have been optimized to create cost saving innovations. The industry adapted to these products and association certification programs inspect, test and certify these products for compliance. The results of EQ stud efforts were reduced end-user costs on materials and improved industry oversight and compliance. The EQ coating movement is on a similar path as alternative coating provisions are already integrated into industry associations’ code compliance programs.




Innovation on coatings is not new either. The automotive and appliance industries regularly change the steel coatings to improve cost, performance and aesthetics. Many of the coating producers and applicators that service the steel framing industry also supply the automotive and appliance manufacturers. The synergies between industries produces economies of scale and that allows stud manufacturers to access state-of the art coating facilities, chemists and other technologies with limited investment. These benefits ultimately drive competitive advantage and cost savings to the contractor. While some of these coating technologies are new, the concept of coating or recoating steel is not. The quality and performance of these coating is clearly defined in ASTM standards that have been in place for over a decade. The standards are written to accommodate innovation in coating technologies and test methods insure adequate levels of performance for the applications.




It is imperative for the steel framing industry innovate to stay competitive with other construction materials. EQ coatings allow a manufacturer to purchase and process a wider range of raw materials, effectively reducing input costs and ultimately their selling price. The ASTM standards provide guidelines for a minimum level of performance and the code compliance programs certify products for quality and adherence to the standards. Through continuous improvement and innovation, steel framing will remain a cost-effective, durable building product.




Additional resources for coatings can be found at the Steel Framing Industry Association’s website at www.steelframingassociation.org and at www.galvinfo.com.




J. Ryan Smith, P.E., LEED AP, is a principal with StructureSmith LLC, a design firm in the steel framing industry located in Norfolk, Va.

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