CURRENT ANODIZING PROCESSES
Sulfuric Acid. The predominant anodizing 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 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 integral 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, 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 anodizing color systems:
Surface Coating
Working method
- Pretreatment
- Grounding
(Chromatizing) - Color 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
- Color pigments distributed in lacquer coat
Shades
- Unlimited, including white; various degrees of gloss
Appearance of surface
- Metallic character of the aluminum 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 anodized aluminum, flaking of lacquer coat possible
Color 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 Coloring
Working method
- Pretreatment
- Anodizing (special alloy + organic acid)
- Sealing*
Structure
- Coloring matter distributed in hard Al oxide, inseparably bound to the aluminum; anodic film thickness 15-40 pm
Shades
- Brown, bronze, gray, slate; no special colors; no white
Appearance of surface
- Metallic character of the aluminum 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
Color repairs
- Impossible without dismantling (but alien retouching possible on the building)
Main applications
- Buildings with a highly representative character
Organic Dying
Working method
- Pretreatment
- Anodizing (no special alloy)
- Dyeing (with highly-fast organic dyes, without current)
- Sealing*
Structure
- Coloring matter (org. dye) in most cases incorporated throughout the coating, which is inseparably bound to the aluminum; minimum anodic film thickness 20pm
Shades
- Spectral colors; deep yellow, d. red, d. blue, d. turquoise, d. black; no brown, bronze or gray tones
Appearance of surface
- Metallic character of the aluminum retained; high brilliance
Overall resistance
- Very good
Chemical stress
- Sensitive to alkaline construction materials
Mechanical stress
- Like normal electrolytic dyeings; no flaking
- These coats are considerably harder than surface coatings
Color repairs
- Impossible without dismantling (but alien retouching possible on the building)
Main applications
- Buildings with a highly representative character and a distinctive colorful appearance
Inorganic Dyeing
Working method
- Pretreatment
- Anodizing (no special alloy)
- Dyeing (with highly-fast, inorganic dye salts, without current 1- or 2- bath)
- Sealing*
Structure
- Coloring 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 dyeings; no flaking
- These coats are considerably harder than surface coatings
Color repairs
- Impossible without dismantling (but alien retouching possible on the building)
Main applications
- Buildings with high requirements as to appearance (metallic character)
Electrolytic Coloring
Working method
- Pretreatment
- Anodizing (no special alloy)
- Dyeing with metal salts and A.C.
- Sealing* or
- Coating with colorless lacquer, then curing
Structure
- Coloring matter at base of pores consists of metals in highly dispersed form inseparably bound to the aluminum; minimum anodic coat thickness 20 pm
Shades
- Brown, bronze, gray, slate, black, pink, burgundy, no spectral colors, no white
Appearance of surface
- Metallic character of the aluminum retained
Overall resistance
- Outstanding, no chalking
Chemical stress
- Sensitive to alkaline construction materials
Mechanical stress
- Slightly less hard than color-anodized coats; no flaking
- These coats are considerably harder than surface coatings
Color repairs
- Impossible without dismantling (but alien retouching possible on the building)
Main applications
- Building with high requirements as to appearance (metallic character)
Electrolytic Coloring Combined With Organic Dyeing
Working method
- Pretreatment
- Anodizing (no special alloy)
- Dyeing with highly-fast organic or inorganic dyes
- Sealing* (Nt)
Structure
- One coloring 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 aluminum retained
Overall resistance
- Outstanding, no chalking
Chemical stress
- Sensitive to alkaline construction materials
Mechanical stress
- Like normal electrolytic dyeings; no flaking
- These coats are considerably harder than surface coatings
Color repairs
- Impossible without dismantling (but alien retouching possible on the building)
Main applications
- Buildings with high requirements as to appearance (metallic character)
1- Aluminum metal, 2- Chromate coating (as grip for lacquers), 3- Aluminum oxide coating (as color anchor in anodizing process), 4- Lacquer coating, 5- Coloring matter
*Sealing = Closing the pores
