"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.
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.
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.
During the anodizing process, the aluminum oxide film grows perpendicular to the surface of the part. Because of this there will be less coverage on the sharp corners of parts leaving a thin spot in the anodic coating. That thin spot will then be a site of low dielectric strength and reduced corrosion resistance. In the case of parts being hard anodized, because it is a site of low electrical resistance, an increased amount of current will flow at that spot, frequently causing the part to overheat and produce a discoloration or “burn”. In some cases the temperature becomes so hot that the anodic coating and the underlying aluminum are actually dissolved in the sulfuric acid electrolyte and large amounts of material are removed from the part.
This phenomenon very rarely causes a problem in conventional anodizing. However, in the hardcoat anodizing process where power densities can approach 3000 watts per square foot, sharp corners or points can initiate a “burn” which can destroy portions of the part. Being aware of this potential problem, Alpha Metal Finishing has developed procedures and computer control technology that will minimize this possibility. However it is much better practice to design parts without sharp points or edges.
A different but equally undesirable phenomenon occurs with sharp inside corners. The two surfaces growing perpendicular to each other impinge upon each other at the corner creating a seam. The seam is not contiguous like normal anodizing and can expand and contract with temperature allowing the environment access to the base aluminum at its root. MIL-A-8625 recommends a minimum radius of .030 in. on both inside and outside corners.
Anodizing is the process of electrochemically converting the surface of an aluminum part to aluminum oxide. Aluminum oxide occupies about two times the volume as that of raw aluminum. Therefore, anodizing will cause parts to grow dimensionally. This factor should be taken into consideration when designing parts that will be anodized.
Typical standard clear and color anodizing creates an aluminum oxide film in the range of .0002 to .0008 inches, (.005 to .020 mm), on each surface. Hard anodizing is typically in the range of .0005 to .003 inches, (.013 to .076 mm), the most common being .002 inches. (.051 mm).
The process of hard anodizing a part to .002 in. film thickness will therefore grow .001 in. on each surface or .002 in. in overall dimension.
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.
Anodizing is an electrolytic process for producing controlled aluminum oxide films on aluminum. Oxide forms naturally on untreated aluminum, but the anodizing process produces a coating which is uniform, much harder, and more dense than natural oxidation. Aluminium oxide also possesses excellent thermal and electrical insulating qualities.
The anodic film is formed by converting the surface of the part into aluminum oxide. Unlike paint, which can flake off if not applied properly, anodized aluminum finishes are actually formed from the original material and cannot flake off. The aluminum oxide finish is very hard and exceptionally wear resistant. Anodizing is good for many consumer products and sporting goods due to its aesthetic and corrosion protection properties. Anodizing is the primary finish for aluminum aircraft parts including major components before assembly and painting. We also offer Teflon impregnation to further enhance your parts resistance to wear.
Parts can be dyed during the anodizing process to produce luxurious finishes with a deep color that can only be imitated by paint. Alpha Metal Finishing currently offers aluminum anodizing in clear, black, gold, red, blue, and green. A variety of other colors are available for your higher volume products.
At Alpha Metal, our standard anodizing process conforms to MIL-A-8625 Type II.
Benefits of Aluminum Anodizing: