How to Judge Hard Coat Color on Anodized Aluminum

Posted August 31, 2016
By Jack Tetrault

Is a Lighter Color Aluminum Hard Coat Thinner than a Darker One?

I have been asked this question often: “Is it always the case that a lighter color hard coat is less thick than a darker one?” While the answer is not simple, I usually respond by saying, “Maybe, but not necessarily.” To explain, let’s start with some basic information.

Hard-Coat-Aluminum-Anodizing_DCHN-blog.jpgSome Basics of Hard Coat Aluminum Anodizing

Normally, natural hard coat appears as a dark grey-green (some say, brown) color when .002” is created on a 6061 alloy with a low-temperature sulfuric acid bath cooled to approximately 32 degrees Fahrenheit. This is what is known by Sanford Process Corporation—known worldwide for its hard coat expertise—as “classic” hard coat. The color is not a mystery and there are scientific reasons why the aluminum takes on color during the hard coat anodizing process. The depth of color is not always the best way to judge thickness and never is the best way to judge the performance characteristics the coating will have. I’ll get more into performance in our follow up to this blog, but for now let’s stick with thickness and color.

Classic Hard Coat Color and Alloys

With classic hard coat, color is created by entrapment of alloying materials within the aluminum as the anodic coating forms and grows. In general terms, the thicker the coating the darker it becomes, because more of these alloying constituents will be trapped in the oxide. But, since each alloy is different from a metallurgical perspective, they will appear much different. Therefore, it is not a good idea to correlate color with thickness.

Even within an alloy, there are allowable amounts of other metals or silicon that are acceptable for that alloy. This variance alone can show different color, even at like thicknesses. Plus, there are other outside influences on color. Mechanical surface preparation methods, manufacturing method, and temper also play a role. This is in addition to a number of operating parameters that the anodize house controls. Bath temperature, current density, voltage, acid concentration, etc., will also effect the final color. 

Quick Answer

The quick answer is that hard coat color on anodized aluminum—while a good litmus test—should not be used to make final judgement on thickness or performance.  

There are also methods of hard coating that can reduce the entrapped alloying constituents and create an anodic coating that is much more of a clear coating. Ask us about this unique technology.

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