CURRENT ANODIZING PROCESSES

Sulfuric 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		-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		-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.