By Joseph C. Benedyk, Editor.
In the October 2016 issue of Light Metal Age, J. Gilbert Kaufman’s book on the fire resistance of aluminum and its alloys was reviewed. The book offered substantial documentation and evaluation of the properties of aluminum and its alloys in various structural product forms upon fire exposure in applications such as the marine and building and construction industries.1 With his decades of experience in the aluminum industry and as a noted chronicler of physical and mechanical properties of aluminum and its alloys, Kaufman analyzed and reported on the fire resistance of aluminum and its alloys by concluding: “The natural physical characteristics of aluminum and aluminum alloys are such that they do not burn under normal atmospheric conditions nor do they contribute to flame spread or act as a fire accelerant.”
Case closed — well, not quite.
New White Paper on Aluminum Fire Safety
Recently, the issue of establishing the fire resistance of aluminum and its alloys has come back under discussion. This has prompted the Aluminum Association to publish a white paper on the flammability of aluminum and its alloys, presenting the results of their recent study and analysis of the non-combustibility and fire safety of aluminum.2 The Association published this white paper in response to recent concerns regarding potential combustibility of aluminum and its alloys in building construction, based on erroneous information within professional circles that has been spreading to the general public. This white paper follows others, published by European aluminum groups, also denouncing claims about the combustibility of aluminum in marine and building construction applications.3-5
Perhaps, the most famous of these misconceptions revolved around the sinking of the British guided-missile destroyer HMS Sheffield during the Falklands War in May 1982. The ship was hit by an Argentine Exocet missile, which detonated inside the ship causing extensive damage and starting a fire. The crew eventually abandoned the ship, which sank days later while being towed. Media reports were quick to blame aluminum as a possible contributor to the fire, but these reports were incorrect as the Sheffield and her sister ships of the Type 42 class were built entirely from steel.
The media confusion stemmed from the presence of aluminum on several earlier U.S. Navy and Royal Navy ships, which suffered from fires in the 1970s, despite the fact that both navies determined that the use of aluminum in the ships’ designs had nothing to do with their loss. In those cases, certain aluminum components, such as ladders and part of the superstructure had melted in the extreme temperatures, but did not combust or otherwise contribute to the fires.
The Aluminum Association’s new white paper provides valid arguments about the fire safety of aluminum and its alloys (as has been demonstrated time and again in previous white papers). It also provides test data on the non-combustibility of aluminum and various aluminum alloys used in building construction, based on fire performance tests conducted according to ASTM E 136-11 (Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750°C). The International Building Code (IBC) Fire Safety Committee, adopting the National Fire Protection Association (NFPA) guidelines, defines a noncombustible material as “a material that, under the conditions anticipated, will not ignite or burn when subjected to fire or heat; materials that pass ASTM E 136 are considered noncombustible.”6
Aluminum Properties in a Fire
As the white paper notes, any discussion of aluminum’s combustibility must begin with consideration of the physical and mechanical properties of aluminum and its alloys that relate to behavior in a fire. Unalloyed aluminum will melt at around 660°C (1,220°F) and, although most of its alloys begin to lose strength at temperatures above 150°C (300°F), the high specific heat, thermal conductivity, and reflectivity of aluminum help to give the metal some fire resistant characteristics, making it viable for construction applications. Each of these properties alone increase the reliability of aluminum in a fire, but together they greatly mitigate the drawbacks of its relatively low melting point by increasing the time it takes the material to reach that melting point, all while allowing designers to take advantage of its utilitarian benefits to better manage heat, intentional or otherwise.
The effect of increased temperature on mechanical properties of aluminum and its alloys exposed in a fire have been extensively studied, especially for columns and beams used in building construction.1,7-9 To be sure, however, never is it the case that combustibility of aluminum is an issue in ambient atmospheres. It should be mentioned also that, in manufacturing aluminum products, very often arc welding and laser welding are used to join aluminum components together at temperatures well above the melting point, without combustion taking place in the weld zone.
Testing Combustibility of Aluminum
The Aluminum Association commissioned a series of ASTM E 136 tests, first in 2011 and then again in 2020, to provide additional scientific data proving that solid bulk aluminum does not burn. The alloys chosen for testing in 2011 were 3003, 5052, 5083, and 6061. These four alloys were selected as they represent alloys in three alloy groups (3xxx, 5xxx, and 6xxx) and are some of the most commonly produced alloys in the industry. The 2020 tests featured five additional aluminum alloys (5005, 6006/6105, 6005A, 6063, and 6351) along with commercially pure aluminum (P1020A). These alloys were selected for their widespread use in construction. The tests were commissioned at the Fire Technology Department of the Southwest Research Institute (www.fire.swri.org).
The ASTM E 136 test specimen is a dry 38 x 38 x 51 mm specimen heated in a small vertical tube furnace at 750°C (1,382°F). This temperature is maintained either until the sample has failed, or until 30 minutes have passed, whichever occurs first. Thermocouples are used to monitor the temperature inside the furnace to ensure that there is no spike in temperature resulting from the material burning, and the sample is observed to ensure that no flames are present.
To ensure the statistical validity of the test, a minimum of four samples are tested, of which at least three must pass the criteria. If the weight loss of the test specimen is 50% or less, the criteria are: the specimen surface and interior temperatures during test do not increase more than 30°C above the temperature measured on the surface of the specimen prior to the test and no flaming from the specimen after the first 30 seconds. If the weight loss exceeds 50%, the criteria are no temperature rise above the stabilized temperature measured prior to the test, and no flaming is observed at any time.
All the alloys supplied by the Aluminum Association and tested this year per ASTM E 136 met the required criteria for passing the test.10 As noted in the conclusion of the Association white paper on Fire Safety of Aluminum and Its Alloys, “The various scientific tests commissioned by the Aluminum Association used to conclusively determine aluminum’s non-combustibility meet the requirements laid out by the applicable government and non-governmental entities tasked with setting and enforcing public safety standards. As such, aluminum is a perfectly safe material to use from a fire safety perspective for all applications for which the use of aluminum was properly considered in the design.”
References
- Kaufman, J. Gilbert, Fire Resistance of Aluminum and Its Alloys and Measuring the Effects of Fire Exposure on the Properties of Aluminum Alloys, ASM International, 2016.
- Lemmon, A. and J. Weritz, “Fire Safety of Aluminum and Its Alloys,” Aluminum Association, September 8, 2020.
- “Fire Resistance of Aluminium,” Alcan Marine.
- “Aluminum and Fire: UK Aluminum Industry Fact Sheet 11,” Alfed, December 14, 1982.
- “Aluminum and Fire Safety,” European Aluminium, May, 7, 2020.
- Hirschler, Marcelo M., International Building Code – Fire Safety, FS5-07/08.
- Mazzolani, F.M., “Chapter 10: Fire Resistance,” Aluminum Alloy Structures, 2nd Edition, E&FN Spon, 1995, pp. 657-683.
- Maljaars, J., “Literature Study on Aluminum Structures Exposed to Fire,” Netherlands Institute for Metals Research, Technical University of Eindhoven, June 2005, p. 177.
- Czerwinski, F., “Review: Thermal Stability of Aluminum Alloys,” Materials, Vol. 13, No. 15, August 2020, 13, p. 50.
- Alloy Combustibility Test Reports, Aluminum Association.
Editor’s Note: This article first appeared in the October 2020 issue of Light Metal Age. To receive the current issue, please subscribe.