Aluminium in Building- -2: Finishes
FOR many purposes, the appearance of aluminium 'as-cast" or "as- manufactured' is satisfactory and no special finishing treatment is required. Where necessary, however, the appearance can be preserved, enhanced or varied, and the metal itself protected, by adopting one or other of the the methods of finishing described in this Digest.
The preceding article described the variety of materials included under the general name of alumin- ium and showed how they can he chosen to suit different building purposes. according to the proper- ties required; however, the ques tions of appearance and special finishes were not discussed.
For many purposes, the appear- ance of aluminium 'as-cast' or 'as- manufactured is satisfactory and no special finish is needed. Some dulling of the surface will occur in most environments, but this may not be considered objectionable provided the surface does not become serious. ly roughened or pitted.
It is also possible, where con- venient. to preserve the original condition of the surface by regular washing or. if necessary, by abrasive cleaning. The frequency of such maintenance may vary from every few months to once a year; it de- pends upon the composition of the alloy but also. more particularly, on the local atmospheric conditions and the surface condition regarded as acceptable. Surfaces sheltered from rain need more frequent cleaning than rain-washed areas, to maintain the same appearance.
for
In other circumstances example, where appearance is all- important and a particular decora- tive effect is sought, or where the metal needs protection against ag- gressive conditions the various finishes described below offer wide choice. Each has its particular uses; the order in which they are discussed is broadly that of increasing durability, but there is
no
a
Own
hard and fast distinction be- tween them in this respect.
CONVERSION COATINGS
The so called 'conversion coatings are chemical treatments of the alu- minium surface which have been
116
developed for instance, as anti-glare coatings, for decorative effect. as pretreatment for painting and for corrosion protection. All are design- ed either to increase the thickness of the naturally formed oxide film or to build up a complex oxide film containing chromates or phosphates. The corrosion protection obtained is intermediate between that of the natural oxide film and an anodically formed film. but the costs involved are very much lower than in anodis- ing. All the following processes are patented and are available under proprietary names.
Chromate-oxide coatings are pro- duced by immersion in alkaline metal chromate solution at 90°
The Bohmite process produces a transparent monohydrated alumina film which finds limited use as a protective finish.
Etch primers are chemical agents usually formulated from an etching solution. a resin medium and a cor- rosion-inhibitive pigment such as zinc chromate. Two-pack or one pack primers are available for ap plication by brush or spray, and for the best results the treatment should be followed immediately by a chromate-based priming paint.
PAINTING AND LACQUERING
Successful painting depends on care in pre-treatment. The material as delivered from the manufactur- er may be covered with grease or wax and this should be first remov- ed by a solvent such as a mixture of white spirit and light solvent naptha. A pre-treatment primer, a chemical conversion coating or a phosphoric acid type etchant should then be applied to provide a key for the priming paint;
small areas. sand or grit-blasting or wire- brushing may serve the purpose. Weathered aluminium should be vigorously abraded. preferably with stainless-steel wire; on are
100°C. On the pure grades of alu- minium this gives a grey coating which can be adopted for decorative or anti-glare purposes; it is. how- ever, essentially a factory-applied finish and not suitable for spray application on site.
Chromate-fluoride coatings produced by immersion in or spray- ing with an acidic chromate-fluoride solution at 15°-55°C; they are yellow to golden brown according to their thickness. This process is used as a pre-treatment prior to painting or to vitreous enamelling.
Chromate phosphate coatings are green non-crystalline films some 10-6 to 10-7 in. thick and are widely used for decorative and anti-glare pur- poses and as a base for paint. The uniform opaque green colour pro- duced by some processes is known as 'architectural green."
are
Zinc phosphate processes produce a light grey crystalline coating of zinc phosphate. Such films thicker than the chromate-phosphate coatings and are less flexible. The process, however, is of importance commercially since different metals can be treated in the same bath.
on
no account
should copper or brass wire pads be used, Flame-cleaning, also. should never be used on aluminium. be- cause of the risk of melting the metal. For priming the surface after cleaning, a lead-free primer is recommended, preferably one taining a chromated pigment. Fin- ishing coats must be compatible with the primer employed. For fur ther details reference may be made to B.S. C.P.231.
con-
some-
Mill-finished products are times finished with a methacrylate or cellulose acetate butyrate lacquer. Formulations are available which are reported to be non-yellowing and non-peeling and have the use- ful property, on re-application, of dissolving the original lacquer film. They are resistant to alkalis. Thin coatings of the order of 0.0006 in. thick prevent peeling, and such coatings are said to make cleaning
THE HONG KONG & FAR EAST BUILDER
—
VOLUME 18, NUMBER 1
No comments yet.
Private notes are available after approval.