recommended. with thicker films in localities where air pollution is high. Thinner films. on the other hand, can be used on work that is cleaned regularly and often, such as shop fronts or entrance halls. Lacquers or waxes applied to the surface help to reduce frequency of maintenance.
Anodised aluminium should be washed with water, a weak soap solution or a mild detergent. For removing ingrained dirt, nylon or stiff bristle brushes should be used. Organically dyed surfaces should preferably be washed with clean water or special detergents only, since certain cleaning compositions are known to have an adverse effect on the dye.
Physical properties
to
Anodic films in general are hard and brittle and so are liable crack if the metal is permanently deformed. The hardness can vary from 100 to 400 V.P.N. according to the method of processing; at low temperatures, using special clectroly- tes, very hard films, up to 500 V.P.N., can be produced. Thin films produced in chromic acid electrolytes are softer and more flexible than those formed in sulphuric acid. The harder coatings are appropriate for items subject to abrasive wear such as doors, furni- ture or handrails.
The coefficient of thermal expan- sion of the film is only about one- fifth that of the aluminium and. because of this, crazing of thick. hard films is liable to occur at tem-
peratures above 100°C. Exposure to heat or direct sunlight may thus lead to surface crazing, particularly with dark finishes; the risk can be minimized by using light colours and by careful orientation. The softer films are more tolerant of heat but their thinness makes them less resistant to atmospheric attack.
Chemical sealing impairs the resistance to abrasion. and hard films are often used unsealed or sealed with wax, lacquers, etc. Coatings sealed in this way, while suitable for many purposes, offer less resistance to corrosion than chemically sealed films a factor to be taken into account in outdoor use.
The
reflectivity of a bright anodised surface is usually 80 to 90% and the specular reflection is from 50 to 75% of the incident light. Reflectivity falls off with anodic film thickness and greatest with superpurity aluminium of anodising quality.
is
The cost of anodising will depend upon the pre-treatment of the metal. the film thickness, and the colour. Without quoting average figures for cost, which might be misleading, it may be said that good colour anodising is roughly 25% dearer. and black anodising 50% dearer than a plain anodised finish.
MISCELLANEOUS FINISHES
For technological or decorative purposes, aluminium can be elec- troplated with copper, brass, silver. nickel. gold or chromium. The cleaning, surface preparation and deposition of the coating are costly and in the case of chromium plate. the methods for producing anodised bright trim are cheaper and pro- Pro- duce a
similar effect. very prietary solutions are available which permit the direct deposition of chromium or nickel without preplating with brass, copper, zinc or cadmium.
Laminated coatings with insulat ing or decorative materials are available; one of the more familiar finishes is the aluminium/p.v.c. laminates combining the decorative properties of the textured plastic with the light weight and corrosion resistance of the backing material.
For extreme decorative effect. aluminium has been covered with gold leaf. but it is particularly im- portant here, as in the case of plated deposits. that porosity of the sur- face is eliminated. Porous coatings or gaps between the edges of foil set up corrosion cells resulting in rapid pitting of the basis metal. For a gold finish, an alternative procedure is to use gold anodising on a well polished surface.
(Crown copyright by the Building Research Station, Garston, Herts.)
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122
THE HONG KONG & FAR EAST BUILDER VOLUME 18, NUMBER 1