Chapter 9 FIRE RESISTANCE OF ALUMINIUM

1. The Falklands conflict ...... 137

2. Reaction to fire of aluminium ...... 137 2.1 Non-flammability ...... 138 2.2 No release of smoke ...... 138 2.3 No sparking ...... 138

3. Classification of aluminium ...... 139

4. Passive protection of aluminium ...... 139

5. Thermal properties of aluminium ...... 140

6. Change in the physical properties of aluminium as a function of temperature ...... 140 6.1 Change in mechanical properties ...... 140 6.2 Change in physical properties ...... 142 Alcan Alcan Marine

135 9. FIRE RESISTANCE

HETHER for a ship or a within a structure, any means of However despite a number of W building on land, the protection provided for it, its per- reservations relating in particular resistance to fire of the materials formance in a fire (retention of to its low melting point and the that are used in its construction is rigidity, extent to which it heats up controversy that arose following one of the factors – without doubt etc.), generally imposed by regula- the Falklands conflict in the the most important factor – tions such as SOLAS and the HSC Eighties, the marine applications affecting the safety of the persons collection for high speed ships. of aluminium have advanced who use it and of the survival of steadily since 1960 in naval con- the vessel in the event of fire. In shipbuilding, aluminium was struction, in passenger ships, liv- first used during the period 1920 – ing quarters, link bridges and in A material’s behaviour in a fire is 1930 to make furniture for passen- offshore structures [3, 4] . determined by its intrinsic physical ger cabins on liners to replace properties (melting point, specific wooden furniture that was consid- heat etc.), its tendency to ignite ered to be dangerous in the event and to give off smoke and fumes of a fire as it represented a fire in the event of combustion, and load and its combustion gave off changes in its mechanical charac- smoke and fumes whose cata- teristics. Any decrease in the latter strophic effects were already well as temperature rises will reduce known [1, 2] . the material’s ability to support a load etc. The use of aluminium has long been challenged in a number of However fire resistance also industries, e.g. petrochemical and depends on the conditions of use offshore, that are highly exposed of the material, e.g. its position to fire hazards.

VESUVIO JET Alcan Alcan Marine

136 OF ALUMINIUM

1. The enquiry that followed showed 2. that out of the nine ships damaged THE FALKLANDS REACTION and sunk, only three had alu- CONFLICT TO FIRE minium alloy superstructures, and OF ALUMINIUM the Sheffield’s superstructure was The resistance to fire of aluminium made of steel !!! was the subject of much contro- It is now a generally accepted and versy during the Falklands conflict There is nothing that establishes a established fact that “solid” alu- in May 1982, in which British naval cause and effect relationship minium (1) does not burn, does forces were attacked by Argentine between aluminium and the loss not give off smoke when exposed aircraft and nine British ships were of these ships. to fire and does not emit sparks on sunk [5] . impact. In a “White Paper” published by Among these nine were two Type the Ministry of Defence on the 14 42 destroyers, and one of these – December 1982, the British HMS Sheffield –was struck by an authorities concluded that “there Exocet missile. is no evidence that aluminium has contributed to the loss of any ves- It was subsequently reported in sel” [6] . (1) Like most metallic powders, certain sections of the media that aluminium powder is very flammable and so hazardous to handle. Its these vessels had sunk because behaviour in fire is wholly different from their aluminium alloy superstruc- the behaviour of the “solid” metal tures had caught fire upon impact covered with its natural oxide film. The solid fuel motors of rockets usually from missiles or bombs dropped consist of a pyrophoric charge based on from aircraft. specially conditioned aluminium powder.

HSV2 Alcan Alcan Marine

137 In recycling foundries, aluminium 2.1 scrap of all alloys is fed into open Non-flammability furnaces as it is. The temperature There is no evidence that solid or of the melt is in the region of 750 liquid aluminium catches fire by - 800 °C !!! itself in a fire [7] . and this has been confirmed by experience and by The molten aluminium can be numerous laboratory tests. sprinkled with jets of water. The amount of water broken down by Tests have shown that in pure oxy- the molten aluminium is minimal gen , at a pressure of 1013 bar, the because the metal’s reactivity is ignition temperature of aluminium inhibited by the formation of the is above 1000 °C, higher than that oxide film. Very little hydrogen is of other common metals such as given off and so there is no risk of 930 °C for steel and 900 °C for zinc. explosion. The level of ignition temperatures is not dependent on melting point [8] . There are metals whose ignition 2.2 temperature is below their melting No release of smoke point, and vice versa (table 62). As with all common metals and alloys, aluminium does not give off RADAR MAST The extreme difficulty in getting any smoke or toxic gases when aluminium to burn is because the heated up or melted (4). natural oxide layer inhibits the reaction of the metal with air or oxygen by “locking” the liquid 2.3 metal in an envelope that provides No sparking a seal from the surrounding envi- It is worth noting that neither alu- ronment tight enough to prevent minium (nor its alloys) produce ignition (2). Put another way, we sparks upon low impact [4] , which (2) Experience shows that it is can say that there are two com- is why aluminium alloy equipment impossible to set fire to aluminium cooking foil 6 to 7 microns thick with any peting processes: the formation of has been used in coal mines for a type of flame. the oxide layer (oxidation) and the long time (5). (3) The oxide film can form under very combustion of aluminium (3). low pressures of oxygen (approx. 1 millibar and less) and at high speeds of the order of 1 millisecond. This explains Moreover, in mixtures of oxygen why the machining and sawing of and argon, aluminium will only aluminium present no risks. burn if the temperature exceeds (4) Except those from the combustion or the melting point of alumina (2250 hot decomposition of any coatings. °C) and aluminium only burns by (5) In Europe, most road tankers that carry petroleum products have itself if the temperature attains its aluminium alloy tanks (made from 5083, boiling point (3073 °C) [9] . 5186 and XTral728).

IGNITION TEMPERATURES IN OXYGEN

Metal Ignition Temperature in Pure Oxygen (°C) Melting Point (°C)

Magnesium 623 650

Molybdenum 750 2 620

Lead 870 327

Zinc 900 419

Iron 930 1 540

Alcan Alcan Marine Aluminium 1 000 666 138 Table 62 9. FIRE RESISTANCE OF ALUMINIUM

that surface, including joints, does 3. 4. not rise by more than 225 °C CLASSIFICATION PASSIVE above the initial temperature, at OF ALUMINIUM PROTECTION the end of the following times: OF ALUMINIUM Class B – 15 15 minutes Aluminium is classed as non-flam- Class B – 0 0 minute mable: The rules for the protection of alu- More effective thermal insulation ■ according to British standard BS minium alloy structures are there- products are now used on board 476 [10] , fore the same as for steel struc- aluminium alloy ships [13] . These ■ according to ASTM test E136 [11] . tures (7), and can obviously be are “synthetic vitreous fibres (of applied without any difficulty: silicates) of random orientation, The International Convention for whose percentage by weight of the Safety of Life at Sea (SOLAS) ■ to type A ship divisions: alkaline oxides and alkaline-earth [12] classes aluminium among the - they must be constructed to pre- oxides (Na O + K O + MgO + 2 2 non-combustible materials, and vent the passage of smoke and BaO) exceeds 18%”. explicitly permits the use of alu- flames until the end of a standard They are non toxic and conform to minium alloys in naval construction one-hour fire test, according to European directives [14] . (6): resolution A.754 (8), Steel or other equivalent material: - they must be insulated with They have an improved insulation Whenever the words “steel or approved non-combustible materi- capacity but are much thinner and other equivalent material” occur, als such that the mean tempera- so easier to apply. the term “equivalent material” ture on the unexposed surface shall be taken to mean any non- does not rise by more than 139 °C They therefore represent a signifi- combustible material which, by above the initial temperature, and cant weight saving of some 40 % itself or with insulation, possesses the temperature at any point on compared with traditional “rock- properties equivalent to those of that surface, including joints, does wool”, as shown in table 63. steel in regard to mechanical not rise by more than 180 °C strength and integrity at the end of above the initial temperature, at Experience shows that for a 40 the standard fire test (e.g. a suit- the end of the following times: metre passenger ship, 700 to 800 ably insulated aluminium alloy) . Class A – 60 60 minutes kg of INSULFRAX blanket is Class A – 30 30 minutes enough to insulate those parts of Class A – 15 15 minutes the ship that are classed type A. Class A – 0 0 minutes

■ to type B ship divisions: - these must be constructed to prevent the passage of flames until the end of the first half-hour of a standard one-hour fire test, - they must have a level of insula- tion such that the mean tempera- ture on the unexposed surface does not rise by more than 139 °C above the initial temperature, and the temperature at any point on

EXAMPLE OF INSULATION WITH UNIFRAX FIBRES

Structure Type Thickness (mm) Weight (*) (Kg m -2 ) (6) Solas, Chapter II-2, Rule 3, Section 7. Deck A30 38 3,65 (7) Solas, Chapter II-2, Rule 3, Section 3. A60 50 4,80 (8) Resolution A.754: Recommendations Bulkhead A30 38 3,65 on fire resistance tests for “A”, “B” and “F” class divisions. International code A60 50 4,80 for the application of fire test methods, Alcan Marine Code FTP IMO publications. 139 (*) on a flat surface. Table 63 Aluminium’s reflective power is 5. very high, 80 to 90 % of incident 6. THERMAL radiation compared with 5 % for CHANGE IN THE PROPERTIES painted steel and 25 % for stain- PHYSICAL PROPER- OF ALUMINIUM less steel. The effect of this high TIES emissivity is that aluminium alloy OF ALUMINIUM The melting point of aluminium is structures that are exposed to AS A FUNCTION much lower than that of steel, 666 heat radiation emitted by a fire OF TEMPERATURE °C as against 1530 °C (table 64). take longer to heat up, so limiting the spread of the fire. Like most common metals, a rise The thermal properties of alu- in temperature modifies the physi- minium are distinctly superior to Aluminium’s reflective power falls cal properties of aluminium to a those of steel: very little when its surface is greater or lesser degree. ■ its thermal conductivity is 3 exposed to high temperatures. For times higher for the 5000 series a surface temperature of between alloys and 4 times higher for the 500 and 600 °C, the reflective 6.1 6000 series, power remains at a high level – in Change in mechanical ■ its specific heat is twice that of the region of 70 % – even on sur- properties steel. faces that are very old and oxi- This is a very important parameter dized. and one that must be taken into As a result, aluminium’s thermal consideration to maintain the behaviour is quite different from When the surface is painted and integrity of structures and their that of steel (9). For an equal mass covered in soot, the reflective ability to take the initial loads. of metal, far more calorific energy power falls very significantly to is needed to heat aluminium than less than 20 to 30 % of a clean The longitudinal modulus of elastic- steel; the energy is dissipated surface [15] . ity of aluminium alloys decreases more readily because of alu- with temperature (table 65) [xvi] as minium’s very good thermal con- shown in figure 116. ductivity. Its superior ability to conduct heat away eliminates hot The mechanical properties of alu- spots and increases the period of minium alloys, including the elastic serviceability. limit, start to fall as the tempera- ture rises above 150 °C, dropping However, in facilitating the diffu- to 50 % between 200 and 250 °C sion of heat energy, aluminium will (see table 64 and figure 118). The help to heat up contiguous ele- mechanical properties of steel also ments (volumes or structures), decrease with temperature, with and aluminium alloy structures the 50 % threshold coming at must be insulated to avoid this. (9) Cf. Chapter 6. around 600 °C (figure 119).

THERMAL PROPERTIES OF ALUMINIUM Property Aluminium 1050A O 5083 O 6005A T5 Steel E24

Melting range (°C) 645/658 574/638 605/655 1 400/1 530

Boiling point (°C) 2 425 2 425 2 425 2 860

Melting heat (kJ.Kg -1 ) 390 390 390 250

Specific heat (J.Kg -1 .K -1 ) 900 900 940 420

Thermal conductivity (W.m -1 .K -1 ) 229 117 188 54

Coefficient of linear expansion Alcan Alcan Marine (10 -6 .K -1 , 20/100 °C) 23,5 24,2 23,6 13,5 140 Table 64 9. FIRE RESISTANCE OF ALUMINIUM

CHANGE IN MECHANICAL CHANGE IN YOUNG’S CHARACTERISTICS OF 6082 MODULUS

100

80 Rm (MPa) Rp (MPa) 0,2 60 130 400 400 130 40 300 200 160 300 160 20 250 200 200 300 250 200 0 100 100 % Modulus of elasticity 0 100 200 300 400 500 300 Temperature of metal ° Celsius 1/ 1/ 1 6 12 1 2 30 100 720 1/ 1/ 1 6 12 1 2 30 100 720 4 2 4 2 From TALAT de l’EAA Hours Days Hours Days Figure 116 Holding time Holding time The period of exposure to temper- ature (10) has little effect on the Temperature °C mechanical characteristics of From TALAT de l’EAA strain hardened alloys when they Figure 117 are in the annealed condition, 5083 O and H111, 5086 O and H111, etc. However it will have an CHANGE IN THE ELASTIC LIMIT OF 5083 O AND 6061 T6 annealing effect on age hardened alloys such as 6005A, 6061, 6082

σe(θ) σe(θ) in the T6 temper when the tem- σe σe 1 1 perature exceeds 150 °C. 0,8 0,8 0,6 0,6 In practice, if a load-bearing struc- 0,4 0,4 0,2 0,2 ture made from age hardened alu- 0 0 minium alloys is exposed to tem- 0 100 200 300 400 0 100 200 300 400 Temperature °C Temperature °C peratures above 150 °C for sev- 5083 O 6061 T6 eral hours, then the residual mechanical characteristics of Figure 118 components made from alloys belonging to the 6000 series will have to be tested after the fire. The higher the temperature to CHANGE IN THE MODULUS OF ELASTICITY which the alloy has been heated, OF ALUMINIUM ALLOYS the faster its loss of mechanical characteristics (figure 117). Temperature (°C) Modulus of elasticity (MPa)

20 70 000

50 69 300 (10) Cf. table 23, Chapter 3.

100 67 900 CHANGE IN THE ELASTIC 150 65 100 LIMIT OF STEEL [17]

200 60 200 σe(θ) σ e 250 54 600 1,0

300 47 600 0,5 350 37 800

400 28 000 0 0 200 400 600 800 1000 550 0 Temperature °C Alcan Marine 141 Table 65 Figure 119 CHANGE 6.2 IN THERMAL CONDUCTIVITY Change in physical properties 250

Aluminium’s thermal conductiv- -1 .K

-1 200 ity increases with temperature between 0 and 400 °C accord- 150 ing to the empirical formulae (figure 120): 100

50 ■ = 0.07 + 190 for alloys λal θ belonging to the 1000, 3000 and 0 0 200 400 600

6000 series, Thermal conductivity in W.m Temperature °C ■ = 0.10 + 140 for alloys λal θ belonging to the 2000, 5000 and From TALAT de l’EAA 7000 series. Figure 120

Its specific heat also increases with temperature, between 0 and CHANGE IN MASS 400 °C, (Figure 121) according to THERMAL CAPACITY the empirical formulae: Mass thermal Mass thermal ■ Cp = 0,418 + 900 for the capacity (J.kg -1 .K -1 ) capacity (J.kg -1 .K -1 ) al θ 5083 alloy ■ Cp = 0,710 + 880 for the 1100 1200 al θ 1100 6061 alloy. 1000 1000 Its coefficient of linear expansion 900 900

increases with temperature 0 100 200 300 400 0 100 200 300 400 between 0 and 500 °C (figure 122) Temperature °C Temperature °C according to the formula: 5083 6061 l/l = 0,1.10 -7 2 + 22,5.10 -6 – ∆ θ al θal 4,5.10 -4 Figure 121 with : ■ l = length at 20 °C, ■ ∆l = expansion due to tempera- CHANGE IN THE COEFFICIENT ture, OF LINEAR EXPANSION ■ = temperature. θal

40 -1

.K 30 -6

20

10

0 0 200 400 600

Temperature °C Coefficient of expansion in 10 From TALAT de l’EAA

Figure 122 Alcan Alcan Marine

142 9. FIRE RESISTANCE OF ALUMINIUM

Bibliography [7]“Resistance à l’incendie des [12] SOLAS, Consolidated Edition of constructions en aluminium”, V. J. H , 1997, Consolidated Text of the [1]“Evolution des applications de ILL Rapport comité International de International Convention of 1974 for the l’aluminium au cours des cinquante développement de l’aluminium. CIDA Safety of Life at Sea, and of the Protocol dernières années [1886–1936]”, Revue de Report 7132, 1971. of 1978: Articles, attachments and l’Aluminium , No. 84, 1936, pp. 381-398. [8] “Combustion of metals in oxygen”, A. certificates, International Maritime [2]“Les constructions navales”, P. DE V. G ROSSE , J. B. C ONWAY , Industrial and Organisation , London 1997. LAPEYRIÈRE , Revue de l’Aluminium , No. Engineering Chemistry, Vol. 50, 1958, pp. [13]“Fire insulation meets demanding 117, 1945, pp. 183-192. 663-672. standards”, P. H , Speed at Sea , 1999, [3] Fire , ALFED Notice, 1999 - Aluminium YNDS [9] “Température de l’aluminium pendant p. 31. Federation Limited, Broadway House, sa combustion dans les mélanges [14] European Directives 97/69, 80/1107, Calthorpe Road, Five Ways, Birmingham oxygène/argon dans l’azote et dans l’air”, 89/391, 98/24. B15 1TN, UK. R. B , Compte rendu de [15] “Material Aspects of Fire Design”, [4] “Application of aluminium to offshore OURRIANES l’Académie des Sciences, Paris, Vol. 275, STEINAR LUNDBERG , Hydro Aluminium topside structures”, M. J. B AILEY , First 1972, pp. 717-720. Structures , Karmoy, TALAT Lecture International Offshore and Polar [10] BS 476, Classification of Materials F2502. Engineering Conference, Edinburgh, for Fire Resistance. [16] Eurocode 9: Design of Aluminium 1991, pp. 265-272. [11] ASTM, Designation E136: Standard Structures – Part 1–2 General : Structural [5] “Fire Resistance and Flame Spread Test Method for behavior of materials in fire design, CEN TC 250, ENV 1999-1 Performance of Aluminum and Aluminum a vertical tube furnace at 750 °C. 2:2000. Alloys”, The Aluminium Association , First [17]“Stabilité au feu des charpentes Edition, December 1997. métalliques, matériaux de protection”, C. [6] “The Falklands Campaign: The A CAT , J. K RUPPA , G. L AMBOLEY , lessons”, presented to Parliament by the IMONE CTCIM BP1, F78470 Saint-Rémy-lès- Secretary of Defence by command of Chevreuses, 1987. Her Majesty, December 1982.

INSULATION IN THE ENGINE ROOM Alcan Alcan Marine

143 MOTOR YACHT Alcan Alcan Marine

144