I agree with your assessment of 2024 hardcoat hardness. We know from our experience that it is significantly softer than 6061 but we do not have any test numbers. The general principle is that the higher purity alloys will always yield a denser and harder hardcoat finish. The reason for this is that the alloying elements plate out during the process leaving a porous anodized surface (in anodizing the part is the anode which is sacrificial in electroplating terms).

Aluminum, magnesium, and titanium do not plate out but oxidize under the right conditions. Also, if you check MIL-A- 8625 they will note that unsealed hardcoat gives better wear on the Tabor Abrasion test. We have tested sealed and unsealed hardcoat and can confirm that recommendation.

A principle to keep in mind is that whatever you have on the machined finish will remain after anodizing or chromate conversion. Think of the anodizing process as similar to film development. Whatever is in the substrate of the aluminum will be more visible after the anodizing process is completed.

Anodizing or chromate conversion cannot hide machine marks, scratches, extrusion lines or dents. Our etch process (depending on alloy used of course) can remove tiny scratches or fine lines but the more pronounced the mark or scratch the more likely it will show after finishing. We do offer acid etch (for an extra cost) that will help hide some of the machine marks and lines by creating a matte finish.

Question:

"Can hardcoat anodizing be reprocessed?"

Answer:

The short answer is yes, however Type III anodizing can present a number of potential issues to consider before reprocessing parts that have been hardcoated:

Dimensional Change: Reprocessing hardcoat first requires stripping off the old hardcoat anodizing. Hardcoat anodizing causes dimensional growth on parts, typically .002 inches thickness. And, hardcoat creates a 50% outward growth and 50% inward substrate penetration. Removing hardcoat anodizing will take .002 inches or more off of each surface, or over .004 inches off a dimension. Reprocessing will add back .002 inches to the dimension but it will end up being .002 smaller than it was prior to stripping. For many parts and applications this is unacceptable. This means the hardcoat process has to run longer in order to build up to the previous required dimension and it is a more complicated process to control.

Surface Finish: In addition to dimensional changes, stripping off the old hardcoat will always degrade the surface finish to some extent. It will become noticeably more matte or dull from the etching process. The amount of surface degradation varies with the aluminum alloy. 6061 alloy shows the least amount of degradation. 7075 and 2024 alloys are typically the worst, sometimes even producing noticeable pits in the surface.

Hardness: The etching process used to strip the hardcoat anodizing on a part removes some of the magnesium, copper, or silicone alloying elements leaving the surface porous. This porous surface will be softer and less wear and scratch resistant.

Main Takeaway: Our general advice is to not strip and re-anodize hardcoat finishes if at all possible. However, there are some types of parts that will do well with reprocessing. We recommend consulting with our technical staff.

See also "Can .031" 3003-H14 sheet aluminum be hardcoat anodized?"