Aluminium Anodising Process

Sulphuric Acid. Is the predominant Anodising process today. Coatings 0.1-1.0 mil thick formed in a 15 pct. solution, 12asf, 18-24 volts, 70F for 10-60 min. This coating is usually organically dyed or coloured with deposited metals (two-step). Offers a wide array of applications. Most anodisers use this method, although other acids are available for specialty purposes.

Organic acids. These integral one-step processes employ 90-100 g/litre solutions of organic acids, containing a small amount of sulphuric acid (for increased conductivity). Operating conditions: 70-80F, 24asf, voltage up to 75; produces amber, bronze and black coatings. The resulting oxide coating is light-fast and weather resistant.

Here is a detailed description of the processes used - and the results obtained - for six Anodising colour systems:

 

Surface Coating

Working method
Pre treatment
Grounding
(Chromatising)
Colour Coating
Curing of lacquer coat

Structure

Grip coat ca. 1 pm of Al oxide hydrates. Al-Cr phosphates. Lacquer coat 30-80 pm organ. polymers. Colour pigments distributed in lacquer coat

Shades
Unlimited, including white; various degrees of gloss

Appearance of surface
Metallic character of the Aluminium lost, pigments cover underlying surface

Overall resistance
Good, depending on binder type and pigment quality; chalking observable in most cases

Chemical stress
Very good resistance to alkaline construction materials and corrosion influences. May show chalking after a few years.

Mechanical stress
Less resistant than Anodised aluminium, flaking of lacquer coat possible

Colour repairs
Possible without dismantling, but with limitations; Shade/gloss is a problem. Adhesion of repair lacquer may vary

Main applications
Buildings with a normal to high representative character

Integral Colouring

Working method
Pre-treatment
Anodising(special alloy + organic acid)
Sealing*

Structure

Colouring matter distributed in hard Al oxide, inseparably bound to the aluminium; anodic film thickness 15-40 pm

Shades
Brown, bronze, grey, slate; no special colours; no white

Appearance of surface
Metallic character of the Aluminium retained

Overall resistance
Outstanding, virtually unlimited durability, no chalking

Chemical stress
Sensitive to alkaline construction materials

Mechanical stress
Extremely resistant; no flaking
These coats are considerably harder than surface coatings

Main applications
Buildings with a highly representative character

 

Organic Dyeing

Working method
Pre-treatment
Anodising (no special alloy)
Dyeing (with highly-fast organic dyes, without current)
Sealing*

Structure

Colouring matter (org. dye) in most cases incorporated throughout the coating, which is inseparably bound to the aluminium; minimum anodic film thickness 20pm

Shades
Spectral colours; deep yellow, d. red, d. blue, d. turquoise, d. black; no brown, bronze or grey tones

Appearance of surface
Metallic character of the Aluminium retained; high brilliance

Overall resistance
Very Good

Chemical stress
Sensitive to alkaline construction materials

Mechanical stress
Like normal electrolytic dyeing; no flaking
These coats are considerably harder than surface coatings

Colour repairs
Impossible without dismantling (but alien retouching possible on the building)

Main applications
Buildings with a highly representative character and a distinctive colourful appearance

 

Inorganic Dyeing

Working method
Pre-treatment
Anodising (no special alloy)
Dyeing (with highly-fast, inorganic dye salts, without current 1- or 2- bath)
Sealing*

Structure

Colouring matter (heavy metal oxide hydrates; Fe, Co, Mn) incorporated in outer region of coating; minimum anodic film thickness 20 pm

Shades
Pale to dark bronze and gold tones, muted

Appearance of surface
Metallic character retained; anodic film somewhat duller, especially with gold tones

Overall resistance
Very Good

Chemical stress
Sensitive to alkaline construction materials

Mechanical stress
Practically as with absorptive dyeing; no flaking
These coats are considerably harder than surface coatings

Colour repairs
Impossible without dismantling (but alien retouching possible on the building)

Main applications
Buildings with high requirements as to appearance (metallic character)

 

Electrolytic Colouring

Working method
Pre-treatment
Anodising (no special alloy)
Dyeing with metal salts and A.C.
Sealing* or Coating with colourless lacquer, then curing

Structure

Colouring matter at base of pores consists of metals in highly dispersed form inseparably bound to the aluminium; minimum anodic coat thickness 20 pm

Shades
Brown, bronze, grey, slate, black, pink, burgundy, no spectral colours, no white

Appearance of surface
Metallic character of the Aluminium retained

Overall resistance
Outstanding, no chalking

Chemical stress
Sensitive to alkaline construction materials

Mechanical stress
Slightly less hard than colour-Anodised coats; no flaking
These coats are considerably harder than surface coatings

Colour repairs
Impossible without dismantling (but alien retouching possible on the building)

Main applications
Buildings with high requirements as to appearance (metallic character)

 

Electrolytic Colouring Combined With Organic Dyeing

Working method
Pre-treatment
Anodising (no special alloy)
Dyeing with metal salts and A.C.
Sealing* or Coating with colourless lacquer, then curing

Structure

One colouring matter at base of pores, the other incorporated in the coating; min. anodic coat thickness 20 pm

Shades
Virtually unlimited, muted shades, no white

Appearance of surface
Metallic character and surface appearance of the Aluminium retained

Overall resistance
Outstanding, no chalking

Chemical stress
Sensitive to alkaline construction materials

Mechanical stress
Slightly less hard than colour-Anodised coats; no flaking
These coats are considerably harder than surface coatings

Colour repairs
Impossible without dismantling (but alien retouching possible on the building)

Main applications
Buildings with high requirements as to appearance (metallic character)

*Sealing = Closing the pores