Finish It Right From The Beginning

Expert Answers

Expert Answers (9)


"My understanding is that the substrate material of aluminum would influence the hardness of the hardcoat on it. Would this imply that the hardcoat on 2024 alloy is softer than the hardcoat on 6061 alloy? Have you ever tested the HK hardness of hardcoat on 2024 alloy?"


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.

There are two possible issues with hardcoating aluminum that thin. First, Type III hardcoat process uses a higher current density than Type II. The electrical current is transmitted to the part through some contact points (typically four). Being so thin the part would have limited ability to transfer and disseminate the heat generated at the contact. This would most likely result in burning at the contact points. The edges, and particularity the corners, would also have a propensity to burn due to the electrical field effect (similar to a lightning rod attracting more current at the point).

Secondly, if we were able to get past the first problem, the .002 hardcoat could craze if the part were bent. If you are looking for corrosion protection probably the best anodized solution would be standard Type II sealed. You will have better quality results with that process.

Higher purity alloys are always preferred for anodizing. Alloying elements such as copper and silicon do not anodize and leave microscopic voids in the aluminum oxide film. Since the anodizing process converts only the aluminum to aluminum oxide to form the anodized finish, higher purity aluminum will yield a denser and harder layer of aluminum oxide. High concentrations of some alloying elements will also affect the surface finish and color of the anodized finish and will reduce the effectiveness of the sealing process causing reduced corrosion and wear resistance and decreasing fade resistance in dyed parts.

The most popular alloys used for anodizing are 5000, 6000 and 7000 series alloys. These alloys will provide consistently excellent quality finishes for hardcoat and conventional anodizing. High purity alloys like 1100 and 3000 series will also form very good finishes. Although 2000 series alloys are popular alloys because of their strength and machining characteristics ithey are not the best choice for good anodized finishes because of their high copper content.  It has all of the disadvantages noted in the above paragraph. Alpha Metal Finishing regularly provides good anodized finishes for parts made from 2000 series alloys, however a superior anodized finish will be obtained with higher purity alloys.

The colors produced by the anodizing process are subtractive not additive as in paint systems. The anodized film transmits light to the substrate and is reflected back through the film. In short, it acts as a filter rather than a reflector as in a paint system.

In addition to the dye, anodized color appearance is influenced by alloy, surface finish (specularity), anodizing bath temperature, time, and constituents. Thus, getting colors to match from order to order is extremely difficult. We recommend establishing a color range that is acceptable and processing orders together as much as possible to minimize variation. 

The short answer is no. The only metals that can be anodized are aluminum, magnesium, and titanium. The aluminum oxide is created by electrochemically converting the base aluminum to aluminum oxide. All other metals will dissolve and plate out onto the cathodes.

If they are very fine surface scratches you can try automotive fine polishing compound and wax. Be careful. Regular clear and color anodizing is only .0005 inches thick in the coating (one half thousandths). If they are deep scratches you are out of luck. Hardcoat anodizing on the other hand is typically .002 inches thick and can be buffed and polished with stainless steel rouge to a mirror finish.


Birchwood Casey sells a Aluminum Black Touch-Up for small scratches. Here is a link to their product:

The hardcoat surface is very scratch resistant. It can be measured by a Tabor abrasion test called out in MIL Spec 9625. Sealed hardcoat will provide the best dielectric strength. The breakdown voltage of sealed 2 mil hardcoat is about 1200 volts. There is some degradation of the scratch resistance with sealing but it is still very good. Although the thermal conductivity of aluminum oxide is not as good as aluminum (because the coating is so thin) it will have very little impact on the overall thermal conductivity of the part. I would also recommend the book The Surface Treatment and Finishing of Aluminum and Its Alloys by Wernick and Pinner which addresses the electrical and thermal characteristics of anodized aluminum.

Anodizing grows perpendicular to the surface of the aluminum.  That being said, whatever surface roughness/texture is there before anodizing will be there after anodizing process.  Anodizing is only .0003 to .002 inches thick and has little effect on the mechanical properties. Optically, there will be a change in the emissivity of the anodized surface.