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Frequently Asked Questions - Page 2

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Frequently Asked Questions


We have seen a die-cast zinc part with a zinc chromate seal. Given the environmental concerns associated with chrome, we were wondering whether an anodic finish could be used as an alternative. Is there any such thing as zinc anodizing?

To our knowledge, there is no commercial process for zinc anodizing. While it is theoretically possible that a thin anodic oxide layer could be produced on zinc, it likely would be too thin to impart any of the beneficial surface properties we see with an aluminum oxide. No other metal has the anodizing characteristics of aluminum.

Please define Class I and Class II Anodic Coatings

Class I and Class II anodic coatings are designations created by the Aluminum Association for the purpose of codifying the specification of anodized aluminum. The American Architectural Manufacturers Association (AAMA) publication, AAMA 911-92, Voluntary Standards for Anodized Architectural Aluminum, includes the following definitions from The Aluminum Association publication #45, Designation System for Aluminum Finishes:

Class I. High performance anodic finishes used in exterior applications receiving periodic maintenance such as curtain walls. Minimum coating thickness of 0.7 mil (18 microns).

Class II. Commercial anodic coatings used in interior applications or exterior applications receiving regularly scheduled cleaning and maintenance such as store fronts. Minimum coating thickness of 0.4 mil (10 microns).

The above publication goes on to call out the designations of other more specific coatings and methods of pretreatment of the aluminum prior to anodizing.

Coating thickness can be measured by an "eddy current," nondestructive test instrument as per ASTM B 244-79, or by cutting a cross-section of the anodized aluminum, mounting it in a slide, polishing the edge, and reading the coating thickness directly with a microscope as per ASTM B 487-85 (1990).

Class I and Class II coatings should not be confused with Type I, Type II and Type III anodic coatings as described in the authoritative anodizing standard, MIL-A-8625. Type I anodize refers to chromic acid anodizing. Type II is nominal "clear" sulfuric acid anodizing. Type III is "hardcoat" using sulfuric acid or mixed chemistry electrolytes.

Various publications on both aluminum and anodizing from the above organizations may be obtained from:

The Aluminum Association, Inc.
900 19th St., N.W.
Suite 300
Washington, D.C. 20006
Fax 202-862-5164

1827 Walden Office Square
Schaumburg, IL 60173-4628
Fax 847-303-5774

What is the best dye to use to obtain a very dark black coating on aluminum?

There are several organic dyes that can be used to achieve a black color; these dyes are available to anodizers from several suppliers. Chemical suppliers can offer advice regarding the specific application. Dye selection depends upon alloy and application. What works well for exterior may not yield good results for interior applications.

Another means is an electrolytic coloring, a post-anodic coating procedure where AC current is applied to the anodized part in an electrolyte consisting primarily of tin sulfate, sulfuric acid, and various stabilizing agents. This may not produce the "deepest" blacks, however. Choice of alloy, coloring media, and coating thickness may affect the depth or intensity of color.

The surface pretreatment is also important ... the eye will perceive a "darker" black on a smooth (buffed) surface than on a heavily etched surface.

Are there different kinds of anodizing?

There are many different types of anodizing. When we talk about anodizing, we generally are talking about sulfuric acid anodizing that produces a porous oxide layer. There are other processes where other electrolytes are used such as chromic acid, phosphoric acid and sulfuric-boric acid. Using these different electrolytes produces oxide layers with different properties than those produced with sulfuric acid. In addition, process conditions such as temperatures can be changed to produce very hard oxide coatings.

How do you color aluminum?

There are four ways to color aluminum

1. Dye: The freshly anodized part is immersed in a liquid solution that contains dissolved dye. The porous anodic coating absorbs the dye. The intensity of color is related to the thickness of the anodic film, the dye concentration, immersion time and temperature, among other things.

2. Electrolytic Coloring (a.k.a. "two-step"): After anodizing, the metal is immersed in a bath containing an inorganic metal salt. Current is applied which deposits the metal salt in the base of the pores. The resulting color is dependent on the metal used and the processing conditions (the range of colors can be expanded by overdyeing the organic dyes). Commonly used metals include tin, cobalt, nickel, and copper.

3. Integral Coloring: This so-called one-step process combines anodizing and coloring to simultaneously form and color the oxide cell wall in bronze and black shades while more abrasion resistant than conventional anodizing.

4. Interference Coloring: An additional coloring procedure, recently introduced, involves modification of the pore structure produced in sulfuric acid. Pore enlargement occurs at the base of the pore. Metal deposition at this location produces light-fast colors ranging from blue, green and yellow to red. The colors are caused by optical interference effects, rather than by light scattering as with the basic electrolytic coloring process.

Should I request color range samples from my anodizer?

Generally yes. It is important to remember that color is not defined by simply "light" and "dark." Like the controls on a conventional TV set, the hue and chromaticity will have an impact on appearance as well as light-to-dark. Color is at least a three dimensional thing, and cannot be adequately represented on a one dimensional light-to-dark axis. It may be impossible for the color of a flat sheet to perfectly match an extrusion anodized at the same time because the color of the metal may be fundamentally different. Range samples are good for adjusting expectations and for representing the amount of color variation that one can expect. However, if representing the exact color is important, then one must make samples from the exact lot of material you expect to use. When striving for uniformity, it is similar to buying fabric or carpet because you want all the material to come from a single lot.

Can I expect uniform color?

The answer is relative. Anodizers have made incredible improvements in controlling the color of anodized aluminum. The color consistency is comparable to metallic paints.

What is bright dip?

Bright dipping is a process for increasing the specularity or brightness of aluminum by leveling the microscopic roughness or "peaks and valleys" on the surface of the aluminum. The process can not do macropolishing or smoothing of scratches or pits. This limitation underscores the need for careful handling prior to bright dipping.

Most commercially available bright dip baths consist mainly of phosphoric and nitric acid. Additives are introduced to reduce nitrogen oxide fumes and to enhance the brightening ability of the bath.

What is a hardcoat anodizing?

This can be a vague term, but usually hardcoat refers to a very thick and hard anodic coating. This kind of anodizing is accomplished with a bath similar to the standard sulfuric process, but with the temperature reduced to about 0°C to slow the dissolution rate. A higher voltage is applied to enable the coating to continue to build after the insulation value of the coating starts slowing down the coating formation.

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