Once in awhile, I’ll run across a requirement to use a specification that isn’t physically possible. Something I see from time to time is the request to apply the specification of a clear hard anodize finish to the drawing of an aluminum part. This makes me chuckle (unless the requester is thoroughly convinced that this mythical beast really exists).
A hard anodize (Type III) finish is intended to provide wear and abrasion resistant surfaces with improved corrosion protection due to greater thickness and weight than common anodizing (Types I and II). The goal when using hard anodizing is to have a wear index of 1.5mg/1000 cycles, according to MIL-A-8625F.
An anodize finish on an aluminum part is achieved by growing an aluminum oxide layer on its surface using direct current through an electrolytic solution, with the aluminum object serving as the anode. The current releases hydrogen at the cathode and oxygen at the surface of the aluminum anode, creating a build-up of the aluminum oxide. The voltage required by various solutions may range from 1 to 300 V DC, although most fall in the range of 15 to 21 V.
Common methods apply an aluminum oxide layer that is .00002 – .001″ thick. A clear appearance remains as the thickness approaches .0006″ thick. Thicker than that, the layer darkens to a bronze, gray, or black color (depending on the purity of the aluminum substrate). At .0017″, the color is very pronounced. Hard anodize specification calls for .0020″ (+/-20%) thickness. This is far above the point where the anodizing process produces a colored finish. Another factor is that of temperature during the process. Hard anodizing requires the process to occur as a much lower temperature for the harder surface (higher process temperature = softer surface). Additional coloration occurs due to the lower temperatures required by the hard anodize process.
Although a balance may be struck between hardness and clearness, the specification of clear hard anodize is not an achievable specification in a strictly technical sense. Any compromise to get close to this specification is going to have some color and reduction in hardness or durability.
Reference: MIL-A-8625F (.pdf)
- Mythical creations
- Clear hard anodize finishing (mythical)
- Black Chem-film (mythical)
- Mythical Specifications: Non-accumulative tolerance