upon as the principal method of fixing; the aluminium should be bolted or nailed to the substructure. Aluminium bolts should be made from NG6-M or HE9-WP. drive screws from NC6; they should comply with the requirements of B.S.2-165. Aluminium fixing acces- sories for building purposes.' Nails may be of NG4, NG6, HG9 or HG20 alloy.
Flashings should be made from the purer grades of aluminium, and several producers can supply 20 or 22 s.w.g. aluminium in widths to meet most requirements. Where con- siderable working of the metal is involved, the super-purity grade S1- H is to be preferred. Site welding is readily accomplished with $1 or SIA material, using off-cuts from the metal or a filler rod (GI of B.S.1475) and the appropriate flux. Brazing may also be done using an alum- inium 10% silicon alloy filler rod NG2 of B.S.1475), an appropriate brazing flux, and a neutral or slightly reducing oxy-acetylene flame. All flux residues must be removed from the surface. Soldering cannot be recommended for permanent installa- tions because of the danger of chemical
from entrapped attack
fluxes and corrosion due to the con- tact of dissimilar metals. Double or single lock welts are the more usual methods of jointing.
Window and curtain walls
Aluminium alloys, being easily extruded and rolled, lend themselves readily to the manufacture of all the usual types of windows including casement. double-hung sashes and horizontal sliding windows. Many of the windows produced are of pro- prietary design planned to make the most of the properties of the metal. The section shapes shown in B.S.990 for steel casement windows are not entirely suitable for aluminium he- cause of its lower elastic modulus. The 'Z' range of metal windows in- cluded in this Standard are generally preferred although not all designs may be available.
Twenty or more proprietary systems of curtain walling for large or small installations are now available in aluminium. The alloy
The alloy most fre- quently used for these and for win- dows is H9. Precautions against
101
condensation and corrosion are essential.
DURABILITY
The surface of aluminium always carries a continuous but
very thin protective film of aluminium oxide; if removed this film immediately re- forms. In clean, dry air the ap- pearance of the metal will remain unaltered for a long time but under normal outdoor conditions in Great Britain the surface, originally bright and light grey in colour, tends, un- less specially finished, to become duller and darker, and eventually roughened, owing to the formation of corrosion products and the de- position of dirt. Washing, either artificially or by rain, removes dirt and other debris that accelerates corrosion, and so is beneficial.
Since the natural oxide film is self- healing, greying of the surface al. though indicative of corrosion does not imply progressive thinning of the metal; some pitting of the surface may nevertheless occur, the tendency being greater on sheltered areas than on those washed by rain. The ten- dency to corrosion varies according to the composition of the metal and also the heat-treatment and the na- ture of the surface; it is least on pure aluminium and greatest on alloys containing high percentages of cop- per. The latter, also, will suffer severe loss of strength with age un- less coated with pure aluminium by cladding (alclad) or spraying. They can be made less susceptible to cor- rosion by extended heat-treatment. but this causes some loss of strength. In general, however, the results of long-term experiments in America and in Britain have shown that most of the alloys likely to be used as building elements are durable materials capable of withstanding normal atmospheric conditions with- out serious deterioration.
Although the structurally impor- tant alloys are slightly more suscep- tible to corrosion than the purer aluminium used in sheet form, the greater thickness of the former com- pensates for this; H20 and H30 used structurally are usually not less than 0.080 in. thick and extrusions of H9 are usually limited to a minimum thickness of 0.048 in., by the manu- facturing process. In heavily pollut ed industrial and marine atmospheres
corrosion is accelerated and the metal needs to be protected, usually by painting. The B.S. Code of Prac tice C.P.231 deals with the painting of aluminium, which will also be discussed in the next Digest.
In poorly ventilated industrial buildings, particularly those with uninsulated single-skin roofs, acid vapours condensing on the underside of aluminium roof sheets will attack the protective film of natural oxide and hasten corrosion of the surface. Thus, in certain instances, the under- surface of aluminium roofing has suffered severe corrosion through the use of open stoves for space heating, without adequate ventilation. The remedy is to reduce condensation by improving the ventilation, but pro- tective painting may also be needed.
Contact with water
or
Water itself does not attack alu- minium, but it may become corrosive if it is contaminated with substances from neighbouring materials, those present in industrial or marine atmosspheres. Also, static pools of water remaining for long periods on an aluminium surface can produce corrosion by limiting the access of oxygen to parts of the surface. designing aluminium structures it is important, therefore, to eliminate as far as possible any crevices where water might lodge, or else to arrange for them to be sealed with a water- proof corrosion-inhibitive composi tion such as a chromated jointing compound.
In
Domestic water supplies contain small amounts of metallic salts which would be liable to 'plate out' on the surface of aluminium supply lines, and lead to corrosion. For this reason aluminium and its alloys can. not be recommended at present for general use in water supply systems. Industrially, however, in specially treated waters, aluminium tubing, etc. is of much importance.
Washing by rainwater is generally beneficial and aluminium rainwater goods have given excellent service for many years, apart from some made from high-strength copper- bearing alloys which are no longer used for this purpose. Cast and wrought alloys have different surface appearance and corrosion charac- teristics; hence unless a deliberate distinctive effect is required, alloys
THE HONG KONG & FAR EAST BUILDER — VOLUME 17, NUMBER 6
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