ADVANCED MATERIALS & PROCESSES | JULY/AUGUST 2025 22 staff or vendors to sort and de-coat the old signs. The Texas DOT estimates that 51 million highway road signs are currently in use in the United States[9]. Assuming an average life of 10 years, more than 5 million new signs would be required each year just to replace existing stock. ALLOY 5052 DEVELOPMENT AND APPLICATION In a February 1936 issue of Metals and Alloys magazine, Dix and Bowman describe the new “52S” alloy created by the (Alcoa) Aluminum Research Laboratories: “containing 2.5% magnesium and 0.25% chromium…selected to give a non-heat treatable alloy of moderate strength that could be produced at a reasonable price.” They note the relatively high purity of the alloy and its position as an alternative to foreign alloys with higher levels of magnesium[10]. The first use of 52S took place in 1934, when few other alloys were available, DC casting barely existed, and rolling facilities were rudimentary. It could be produced as a soft (“O”) temper or a hard-rolled (“H”) temper to give a wide range of properties. In 1954, 52S was registered by the Aluminum Association as alloy 5052. As new product applications grew with the aluminum industry after World War II, 5052 was a popular replacement for steel due to its moderate strength, ease of fabrication, and excellent corrosion resistance. In the 1960s and early 1970s, as material standards became more common, application and design engineers consulted a materials handbook to choose an aluminum alloy. Alloy 5052 was documented as the best mid-strength range alloy applicable to a variety of general applications. As time passed, it was easier to use an existing alloy than to spend development time and resources to create a new specification. This preference for Alloy 5052-H38 sheet has been the most widely used sheet material for road signage in the United States for decades. The sheet product used for road signs requires a strong temper (hence H38), with a flat clean surface and no forming or joining requirements. While the alloy has excellent corrosion resistance in all tempers, there is no actual test requirement. The minimum yield strength of the H38 product is 32 ksi (220 MPa) and typical properties are closer to 37 ksi (255 MPa)[5]. SIGN FABRICATION States and municipalities can produce their own signs or buy them from private companies. The highly reflective plastic films necessary for high night visibility are first laminated to the aluminum substrate. The lettering is then silk screened on the substrate. Relatively small signs can be produced on simple equipment from sheet as seen in Fig. 2. Large signs produced from sheet can require the backing of extruded panels or thick sheet strips to provide the required rigidity to the sign structure. Multi-lane directional signs used along the U.S. Interstate highway system are produced from horizontal extruded panels that are bolted together to give a rigid final structure as shown in Fig. 3. The Interstate Signways company produces signs up to 20 meters in width in their facility[7]. The lifetime of signs is governed more by the deterioration of the reflective coating than by corrosion, mechanical, vandalism, or storm damage. Highway signs are typically in service for 6 to 12 years. Signs can be bar-coded to specify an installation date and location to facilitate replacement. End-of-life road signs are collected by the various transportation authorities. They may be simply sold for scrap and recycled in combination with other 3xxx or 5xxx alloy materials. Some authorities actually re- process (surface treat, laminate, and paint) undamaged signs, Fig. 4) after the coatings are stripped off with high-pressure water jets. This reduces material cost but incurs the expense of Fig. 2 — (a) Laminating of reflective white surface to aluminum sheet blank. (b) Silk screen printing to produce a stop sign at the New York City Department of Transportation sign shop[6]. (a) (b) Fig. 3 — Production of large signs from extruded aluminum sections in the Interstate Signways’ Little Rock, Ark., factory[7]. Fig. 4 — Recycled sign blanks that have been de-coated and prepared for resurfacing at the SC Department of Transportation sign shop, Columbia, S.C.[8].
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