No_1_June_1963 — Page 126

of some methods of jointing in- fluence the quality and appearance of the finished work.

The texture of the finish can be varied as required by suitable pre- paration of the surface before anodising lustrous finishes by chemical, electrolytic or mechanical polishing, satin and matt finishes by brushing with stainless steel wire and by shot or grit-blasting respec- tively, patterned finishes by rolling. It may be noted that slight dif ferences in colour are less noticeable on textured surfaces than on smooth or polished ones.

The transparent or grey film pro- duced by anodising can be coloured immediately after formation by in- troducing organic dyes or inorganic pigments. These must be light-fast; B.S.1006 describes a method of test for the organic dyes. For indoor use on aluminium. the rating in this test should not be less than 5; for outdoor use not less than 8. Ex- perience over many years has pro- duced a satisfactory but limited range of colours for outdoor use. It includes silver, produced by an un- coloured film; gold and bronze pro- duced by inorganic pigments; and yellow, green, blue, black and red produced by certain organic dyestuffs. In choosing a colour for a particular purpose it is advisable to consult the anodising company before making a final decision. in order to reach agreement not only on the required shade but also on the limits of variation of shade that can be accepted.

The composition of the alloy used affects the appearance of the anodis- ed film. Thus, on alloys 1B and H9 the uncoloured film appears silver; on H30 the manganese constituent imparts a brown tint, N3 gives a still darker tint; H20 and the cop- per-bearing alloys produce yellow films while that on N21 is grey.

Newer anodic processes designed to produce self-colour films are now available in this country. Very thick films up to 75 microns (0.003 in.), produced anodically at low temperatures in sulphuric acid, derive their variation in colour from the composition of the alloy used. Other processes give thinner films (up to 0.0014 in.) produced in an electrolyte based on sulphosalicylic

120

acid. The colour obtained is again. dependent on alloy composition. The thickness and compactness of the films would be expected to result in increased corrosion resistance but experience with such coatings in this country is very limited.

The facilities available for anodis- ing in this country can accept sheet material up to 12 ft X 8 ft and sections up to 22 ft long.

Durability and maintenance

The process of anodising pro- duces a thick oxide film on the sur face; in effect the surface is 'pre- corroded.' When adequately sealed, this oxide film is highly resistant to atmospheric attack but, like the natural protective film. it can be destroyed locally

by corrosion resulting from the deposition of dust. dirt and soot. The film then becomes perforated and the alu- minium pitted. Washing by rain artificially. or

minimises

water. these effects.

The greatest single factors in obtaining a lasting finish are the

thickness of the film and the degree of sealing after anodising or dyeing. In the sealing process the porosity of the oxide film is reduced either physically, by means of lacquers, oils, or waxes, or chemically, by the action of pure water, steam or in- hibitor solutions. Tests to ensure adequate sealing and corrosion resistance of the films are included in B.S.1615.

The effect of film thickness on the corrosion resistance of anodised aluminium has been studied in field tests but the relationship between the two has not been definitely established, since the rate of corro- sion of the metal is influenced by various other factors. such as the alloy composition, the local condi- tions of exposure and the amount of washing the surface receives. Under identical conditions, however, the results of such experiments show that the relationship is not linear; doubling the thickness of the anodic. film more than doubles the durabili. ty of the coating. For general ex- ternal use on buildings a film thick- ness of 25 microns (0.001 in.) is

TABLE 2

ANODISING PROPERTIES OF CAST AND WROUGHT ALUMINIUM

Group

Aluminium—

1% Mg-1% Si 1% Mg-1% Sit 1% Mg-1% Si

Suitability for *

Protective Anodising Bright anodising and dyeing anodising

0004 0430<< _@

BODY

OPPUMPE DOOD

E

V-E

DAAG

M

G-V

G

M

Z JOZZOZ ÚZZZ

M

G-V

M

Specification

B.S. 1470-77

Wrought

B.S.1490

Cast

Nominal composition

Aluminium

99.99% A1

E

1A

99.8% Al

1B

99.5% Al

1C

99% Al

Aluminium-

manganese

N3

14% Mn

N4

21% Mg

magnesium

N5

31% Mg

N6

5% Mg

N'

7% Mg

LM5

5% Mg

LM10

10% Mg

Aluminium-

H9

magnesium --silicon

H19

H20

H30

4% Mg-1% Sit

Aluminium- copper-

H14

41% Cu-

3% Mg-3% Si

M

magnesium

-silicon

H15

41% Cu-

1% Mg-1% Si

M

M (D)

U

N21

5% Si

+(D)

LM18

5% Si

LM6

12%Si

LM20

12Si

LM2

10% Si-11% Cu

M

U

LM21

5% Si-3% Cu

M (D)

LM8

LM11

4% Si-1% Mg 41% Cu

V

U

M

V=Very good

G=Good

M÷Moderate

(D)=Oniy suitable for dark colours

U-Unsuitable

† Manganese 0.2% max.

* Manganese 0.4-1.0%

Aluminium-

silicon- copper

Aluminium-

silicon- copper

Others

*E Excellent

M

M

EU PU

M(D)

MOOD

00 3C

M

M

G

לס כס כככככ

U

THE HONG KONG & FAR EAST BUILDER — VOLUME 18, NUMBER 1