Current Anodizing Processes

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

Organic Acids.
These intergal one-step processes employ 90-100 g/liter solutions of organic acids, containing a small amount of sulfuric acid (for increased conductivity). Operating conditions: 70-80F, 24 asf, 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 anodizing color systems:

System Coloursystem and distribution (diagrammed) Working method Structure Shades Appearance of surface Overall resistance Atmospheric erosion(loss of coating thickness) Chemical stress Mechanical stress Colour repairs Main applications
1.Surface coating 1 -Pretreatment-Grounding(chromatizing)

-Colour coating

-Curing of lacquer coat

Grip coat ca 1 pm of Al oxide hydrates. Al-Cr phosphates lacquer coat 30-80 pm organ. polymers Color pigments distributed in lacquer coat. Unlimited, including white, various degrees of gloss Metallic character of the aluminum is lost, pigments cover underlying surface. Good, depending on binder type and pigment quality; chalking observable in most cases. ca. 1 pm/year Very good resistance to alkaline construction materials and corrosion influences. May show chalking after a few years. Less resistant than anodized aluminum, flaking of lacquer coat possible Possible without dismantling, but with limitations-Shade/gloss is a problem-Adhesion of repair lacquer may vary Buildings with normal to high representative character
2.Integral colouring -Pretreatment-Anodizing (special alloy + organic acid)-Sealing* Colouring matter distributed in hard Al oxide, inseparable bound to the aluminum; anodic film thickness 15-40 pm Brown, bronze,grey, slate; no special colours; no white Metallic character of the aluminum retained Outstanding, virtually unlimited durability, no chalking None detectableA special protective treatment after sealing the dyeing prevents any loss of the coat thickness due to erosion Sensitive to alkaline construction materials Extremely resistant; no flakingThese coats are considerably harder than surface coatings Impossible without dismantling (but alien retouching possible on the building) Buildings with a highly respective character
3.Organic dyeing -Pretreatment-Anodizing (no special alloy)-Dyeing (with highly-fast organic dyes, without current)

-Sealing*

Colouring matter (org. dye) in most cases incorporated throughout the coating, which is inseparably bound to the aluminum; minimum anodic film thickness 20 pm Spectral colours; deep yellow, d. red, d. blue; no black; no brown, bronze or grey tones Metallic character of the aluminum retained; high brilliance Very good Hardly detectableA special protective treatment after sealing the dyeing prevents any loss of the coat thickness due to erosion As above Like normal electrolytic dyeings; no flakingThese coats are considerably harder than surface coatings Impossible without dismantling (but alien retouching possible on the building) Buildings with a highly representative character and a distinctive colourful appearance
4.Inorganic dyeing -Pretreatment-Anodizing (no special alloy)-Dyeing (with highly-fast, inorganic dye salts, without current, 1- or 2-bath)

-Sealing*

Colouring matter (heavy metal oxide hydrates; Fe, Co, Mn) incorporated in outer region of coating; minimum anodic film thickness 20 pm Pale to dark bronze and gold tones, muted Metallic character retained; anodic film somewhat duller, especially with gold tones Very good Loss of colour possible due to deposits in outer layer of colour coatA special protective treatment after sealing the dyeing prevents any loss of the coat thickness due to erosion As Above Practically as with adsorptive dyeings; no flakingThese coats are considerably harder than surface coatings Impossible without dismantling (but alien retouching possible on the building) Buildings with high requirements as to appearance (metallic character)
5.Electrolytic colouring -Pretreatment-Anodizing (no special alloy)-Dyeing with metal salts and A.C.

-Sealing* or

-Coating with colourless lacquer, then curing

Colouring matter at base of pores consists of metals in highly dispersed form inseparably bound to the aluminum; minimum anodic coat thickness 20 pm Brown, bronze, grey, slate, black, pink, burgundy, no spectral colour, no white Metallic character of the aluminum retained Outstanding, no chalking Hardly detectableA special protective treatment after sealing the dyeing prevents any loss of the coat thickness due to erosion As above Slightly less hard than colour-anodized coats; no flakingThese coats are considerably harder than surface coatings Impossible without dismantling (but alien retouching possible on the building) As above
6.Electrolytic colouring combined with organic dyeing 6 -Pretreatment-Anodizing (no special alloy)-Dyeing with higly-fast organic or inorganic dyes

-Sealing* (Nt)

One colouring of matter at base of pores, the other incorporated in the coating; min. anodic coat thickness 20 pm Virtually unlimited, muted shades, no white Metallic character and surface appearance of the aluminum retained Outstanding, no chalking Hardly detectableA special protective treatment after sealing the dyeing prevents any loss of the coat thickness due to erosion As above Like normal electrolytic dyeings; no flakingThese coats are considerably harder than surface coatings Impossible without dismantling (but alien retouching possible on the building) As above

Legend:

  1. Aluminum Metal
  2. Chromate coating (as grip for lacquers)
  3. Aluminum oxide coating (as colour anchor in anodizing process)
  4. Lacquer coating
  5.  Colouring Matter

*Sealing=Closing up pores.