Tag: Hard Coat

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Common Dyed Anodizing

An anodizing layer may be dyed or undyed. In the 8625 Mil spec, the term “Class” is used to designate this: Class 1 is clear or undyed, and Class 2 is dyed.

Anodizing layers are commonly dyed with organic dyes by dipping the part into a hot dye tank directly after anodizing. A wide variety of colors can be obtained through this process. By far the most commonly specified color is black, followed by blues, reds and golds. Note that only Type II anodizing can be dyed a variety of colors. Type III “hard” anodizing, which is quite dark even when undyed, is typically left clear or dyed black.
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“Clear” Anodizing

Anodize Thickness
In the image above, the coating of these parts ranges from .0003″ on the left to greater than .002″ on the right. All are undyed.

“Clear” anodizing is really a misnomer, as undyed anodizing is not really clear.

“Clear” anodizing varies from a very light gray for very thin coatings (.0001-.0003″) to a dark olive color for thick Type III coatings (.002 or greater). Very thin clear anodizing (sometimes called “cosmetic” clear) coatings mimic the look of bare aluminum (though the coating is actually slightly gray), hence the term “clear” anodizing. But on thick hardcoated parts, the coloring is anything but clear.

All of these parts above are examples of undyed, “clear” anodizing, the only difference is the thickness of the anodizing!
Clear Anodize Caps

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Care and Cleaning of Anodizing

Interested in learning more about the care and cleaning of anodized parts?

Anodizing is surprisingly durable and resilient to corrosion. However, like its aluminum base, it is susceptible to strong alkalines, such as Lye or Sodium Hydroxide. It also can be damaged by strong acids, such as nitric or sulfuric. Beware of strong alkaline cleaners, which often contain ammonia or lye, as well as masonry or cement cleaners, which can contain strong acids.

Surprisingly, anodizing is quite resistant to organic solvents. While alcohol or acetone will quickly remove ink from a permanent marker, for example, they will not damage or fade colored anodizing, and can be excellent for cleaning purposes.
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Defects in Metal and their Effects on Anodizing

Because anodizing is a conversion coating, and relies on the aluminum substrate being converted to aluminum oxide, any defects in the aluminum will have marked effects on the quality of the anodizing. The basic rule to remember is that the aluminum finish must be consistent, clean and fully exposed for the anodic layer to form properly.

Common problems occur with welds, extrusions, cold or hot-worked areas, or poorly machined surfaces.

Aluminum Weld

It is common for problems in the material or mechanical finish to have the appearance of a problem with the anodizing. Often material defects will look like chemical spotting or streaking, when the issue is material based.

GIGO (garbage in, garbage out) is truer with anodizing than nearly any other finish. For the anodic layer to be perfect, the aluminum substrate has to be perfect as well.

Sheet Metal Blemish

This blemish looks very much like a chemical spot, but it isn’t. It’s a location where a locating pin inadvertently pushed up the sheet metal. The sheet was peened back into place by the machinist, but the cold work altered the material enough to change the color of the anodizing.
This blemish looks very much like a chemical spot, but it isn’t. It’s a location where a locating pin inadvertently pushed up the sheet metal. The sheet was peened back into place by the machinist, but the cold work altered the material enough to change the color of the anodizing.

This dark spotting looks like something amiss in the anodizing tank, but it was actually caused by overly dirty water in a vibratory tumbler. The dirty media was embedded into the aluminum finish, causing blotchy areas.

Uncharacteristically, the welds on this bike frame caused little change in the anodizing color. However, poor sanding and left over spots of bondo create a surface finish that shows unanodized blotches and mechanical scratches.