of the same type should be used throughout a rainwater installation, apart from small items such brackets. stays, etc.
as
Aluminium is not recommended for waste pipes in kitchens and bath rooms since the waste waters are liable to harm the metal. Hopper heads receiving domestic waste water should be protected internally by painting. Aluminium should not be used in any plumbing system which contains copper or copperbased ma- terials. or in any situation where water passes over a copper surface before reaching the aluminium.
Contact with building materials Cement products
cause
can
Under dry conditions the metal is not attacked by contact with cernent. In wet situations, corrosion occur to a degree depending on the alloy composition and the nature of the building material. Mortars and concrete will
corrosion. especially those rich in cement. With alloys 1C, H9 and H30 the corrosion is light. but anodising coatings are rapidly destroyed. Corrosion of embedded metal can lead to cracking of the embedding medium and un- sightly salt efflorescence above the level of embedment. Under condi- tions which are constantly damp it is advisable to protect the metal with at least two coats of bitumen paint. Flashings should preferably be given the same protection, particularly where workability aids containing chlorides are used in the mortar mix. Lime mortars affect aluminium in the same way as cement, but less severe- ly.
Plasters
Gypsum plasters of any type are liable to cause some corrosion in the earlier stages while they are still damp, but under normal conditions the corrosion should not continue to give rise to trouble once the plaster has dried out. Prolonged dampness. however, may lead to further cor rosion, and cracking of the plaster.
Magnesium oxychloride
Unprotected aluminium should not be embedded in magnesium oxych- loride flooring since the high mag- nesium chloride content and the tendency of this salt to absorb mois-
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ture from the air both promote cor- rosion of the metal. Aluminium framed partitions on such flooring should be separated from it by moisture barrier, such as a bitumen felt strip or a plastics separator. Wood
Aluminium is resistant to the or- ganic acids present in most woods but. under wet condition, western red cedar, Douglas fir, oak and sweet chestnut can be used for cedar wood shingles and sidings provided the heads are covered by the overlapping shingle but they may be severely at- tacked if directly exposed to rain- water. If aluminium nails are used for fixing replacement shingles, the exposed heads should be protected with a paint or mastic coating. The metal is not harmed by direct con- tact with wood preservatives such as zinc naphthenate, creosote or bitu men. but those based on copper com- pounds may cause corrosion. Al- uminium adjacent to wood that has been treated with a copper preserva. tive should therefore be separated from it by a layer of felt or other impervious separator. Aluminium paint can safely be used on wood and is, in fact, widely employed as a priming paint.
Soil
Direct contact with soil should be avoided; the metal should be pro- tected with a bitumen coating or a paint system consisting preferably of two coats of zinc chromate or barium chromate primer and two further coats of paint.
Painted surfaces
Aluminium should be kept out of direct contact with paints containing copper or mercury fungicides. gra. phite or lead.
Other metals
When any two dissimilar metals are in direct contact under damp conditions. the more electronegative of the two is liable to become corrod. ed as the result of galvanic action. In relation to other metals common- ly used in building, aluminium is often the more electronegative. Be- cause of this, it must be kept out of direct contact with copper, brass or bronze unless the junction can be protected with a waterproof coating.
Similar precautions are desirable at contacts between aluminium and iron or mild steel. For the same reason, rainwater flowing over cop- per or its alloys should not be al- lowed to come into contact with the metai.
Contact with lead or stainless steel is not usually objectionable except in marine or heavily polluted atmos pheres, but where the aluminium is in the form of thin sheet it is better to avoid any direct contact with lead since. where the metal is already thin. no acceleration of corrosion can be tolerated.
Aluminium is less electronegative than zinc or cadmium and so is not itself affected by contact with these mttals but may, on the other hand. tend to accelerate their rate or cor- 10sion; however, galvanized fixing accessories for aluminium roofing are permitted by C.P.143. 'Sheet Roof Coverings' Part 1.
HANDLING AND STORAGE
To preserve the finished appear- ance of aluminium components, pre- cautions should be taken in storing and handling them on site. Alumin- ium roofing sheets should preferably be stored under cover and away from copper, rusty steel, lime, cement and other harmful materials. Where no cover is &vailable. loose sheets should be stacked vertically so that no water is retained between in- dividual sheets. Rolled material, de- livered in coil, should be protected by waterproof wrappings. Windows, doors, handrails and the like should be handled and stored with care, par- ticularly when colour-anodised or bright finished. Preferably, such units, unless protected with a coating of strippable lacquer or wax before delivery to the site, should not be in- stalled until most of the wet trades have finished.
It is not possible to restore on site the original appearance of unprotect- ed anodised units which have been splashed with cement mortar or with plaster. Untreated aluminium show- ing super ficial corrosion may be cleaned by abraiding with wire wool (preferably stainless steel) or by treatment with a phosphoric acid solution, but the original sheen will be destroyed. (Crown copyright)
THE HONG KONG & FAR EAST BUILDER — VOLUME 17, NUMBER 6