easier. The lacquering of mill finished products has not, however, received universal approval and in- stances have occurred where super- ficial corrosion of a filiform nature has developed below the lacquer film, disfiguring the appearance of the metal. Lacquered anodic coat- ings do not show this effect.
STOVE ENAMELLING
Stove-enamelling is a factory-ap- plied finish in which special types of paint are hardened, after applica- tion, by heating. The finish is more uniform and durable than that of ordinary site-applied painting. Care is needed. however, in handling stove-enamelled members since there is no satisfactory way of making good any damage to the finish on site.
Certain formulations of stove- enamel produce films that are suf- ficiently elastic to tolerate perman- ent deformation of the metal after coating. without flaking of the paint film.
Table 1
ANODIC COATINGS Minimum average thickness (B. S. 1615)
Thickness
Grade
Microns
in.
AA35
35
0.0014
AA25
25
0.0010
AA15
15
0.0006
AA10
10
0.0004
AA5
5
0.0002
AA3
3
AAI
1
0.00012 0.00004
VITREOUS ENAMELLING
Vitreous enamel
on aluminium,
as on steel. coats the surface with a continuous inorganic glaze. Special frits are required because of the much lower melting point of al- uminium. This type of finish is used extensively in the United States. Its use has increased ap- preciably in the last few years and several firms are now able offer a wide range of colours in gloss, matt or semi-matt finishes. Suitable alloys. correctly cleaned. give coatings with good adhesion, abrasion resistance and an acid re- sistance equivalent to Class B of B.S.1344. Frits are normally bond-
118
19
ed to the surface at temperatures around 560 ̊C, usually with lead- free formulations.
Tentative specifications for the re- quirements of this material for architectural use have been issued by the Vitreous Enamel Develop- ment Council. These call for a coating thickness of 0.003 to 0.007 in. after firing. Recommended alloys include 1C, N3, H20 and H30 in the wrought alloys, and LM6. and LM18 in the cast alloys. The American counterpart of alloy H20 has been particularly successful in the United States.
When successfully prepared. vitreous enamelled aluminium may be sawn, drilled, cut and punched without chipping, thus offering a distinct advantage over vitreous enamelled steel; the low tempera. ture frits do, however, produce a softer coating. Care must be taken to seal cut edges with a corrosion- inhibitive paste so as to prevent unsightly staining by products.
corrosion
The heating necessary to fuse the glass causes a reduction in strength of the metal and because of this. vitreous enamelling should be con- fined to non-loadbearing com- ponents.
ANODISING
Anodising is a process in which a protective film of oxide. very much thicker than that
that formed naturally on
exposure to air. is produced on the surface of the metal by electrolytic oxidation. The film is chemically stable and relatively hard and, being also porous, it forms an excellent base for paint or for dyestuffs. On the pure metal the film as formed is transparent but it is often coloured for decora- tive effect. After formation the film is sealed. The combined anodising and sealing, besides enhancing the appearance, increases the resistance to corrosion and abrasion.
The electrolyte most widely used for anodising is sulphuric acid. Other solutions, e.g. oxalic acid
or
phosphoric acid. are used for special purposes. Chromic acid is sometimes employed where a grey opaque finish is required. Al- though it gives films having fewer pores per square inch and greater
corrosion resistance for equal film thickness than those prepared in sulphuric acid, this advantage is offset by the limit of thickness available. which is about 10 microns (0.0004 in.); generally not more than 5 microns is possible.
With sulphuric acid the film thickness normally used architecturally varies from 10 to 35 microns. The thick- nesses included in B.S.1615 are given in Table 1.
Bright surfaces with the highest reflectivity can be obtained only with alloys based on super-purity aluminium and by special processing techniques, but for most purposes the brightness obtainable with alu minium of 99.8% purity is adequate. The anodic film thickness has to be low since thick films obscure the brightness of the surface. Thin films. however, have lower corrosion resistance and such finishes if used externally should be confined to work that is regularly cleaned.
·
or
The suitability of different alloys for anodising varies, as is shown in Table 2 which relates particularly to sulphuric acid anodising. Most of the supply companies provide a special *anodising quality' 'polishing quality grade of metal which has been specially processed and may contain minor alloying constituents to facilitate anodising or brightening. At the present time. S1. SIB. and Al-5% silicon sheet are available in this special grade. as well as the 'bright trim' materials and also extrusions and castings of the lastnamed alloy.
par.
may
For decorative purposes, ticularly where large panels are placed side by side, colour matching is important. This can only be re- liably achieved on selected material since different forms of the same alloy composition, or different bat- ches of the same material. show some variation in colour and appearance after anodising. It is advisable, therefore, that architects selecting aluminium from normal grades for purposes where colour matching will be required should consult the anodiser and supplier as soon as the other requirements of the design have been settled. This is particularly important where the metal requires fabrication prior to anodising since the geometry of the surface and the effects on the metal
THE HONG KONG & FAR EAST BUILDER
VOLUME 18, NUMBER 1