Metal finishing Q&As since 1989

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Conductivity of "sticky" Type III Class1 hardcoat anodize

July 15, 2009

Hi

We manufacture scuba diving wrist-top computers, with a 6061-T6 aluminium hard anodized casing. Our product is regularly immersed in salt water. It also has steel contact pins glued in the aluminium which allow the charging of the battery - it is important that the anodize coating is not conductive. The electronics inside the unit are potted in a epoxy gel. I have had endless problems with the anodizing and desperately need advice. In the post below, I outline the history of the situation in a few bullets, the questions follow at the end.

- for a long time, we successfully got our 6061-T6 casings hard anodized. Then our supplier "X" fired his main technician, and the quality went down the drain (we got casings with uneven anodizing buildup)
- we then got our machine shop to anodize the casings by outsourcing to a neighboring shop "Y". "Y" started adding black pigment to the anodize and we eventually got some instances where the black anodize started peeling off. We then asked them to do a hardcoat with no pigment added, and the casings came back with a clear anodize (they were silver) rather than a hardcoat (which would be dark grey)
- I then sent casing samples to various alternate anodizing suppliers and got yellowish/copper color of the anodize (instead of matte dark grey). The anodizers said it was the quality of the metal. The machine shop said it had all the test certificates that it was 6061-T6, but perhaps the surface finish was less good than before. So the casings were sent to the machine shop to be stripped of anodizing and re-sanded.
- The re-finished casing samples were sent to various anodizing suppliers, including "X" and they came back fine from "X" - they had the even dark grey hardcoat finish, so I decided to run the rest of casings with them.
- When the rest of the casings were sent to "X", they came back "sticky" when touched with moist hands. I have read on this forum that hardcoat parts occasionally can come back "sticky", as they are not typically sealed.

QUESTIONS
- We have never had "sticky" hard anodize from "X" before, why now?
- Why were the samples not "sticky" and the rest of the casings were "sticky"? All of the units were from the same batch, same metal, similarly stripped and refinished, with the one difference that the samples were only refinished (sanded) on the front face, while the rest of the casings were refinished (sanded) on all sides. They were (supposedly) anodized in the same fashion (Type III, Class 1, 2 mil finish)
- We have a concern that our customers may not accept "sticky" computers, so we need to have a way of ensuring future parts are not "sticky" in the future, but our anodizer says they have no control over that, and says all hard anodize parts are "sticky". He claims that in the past we would get parts after a few days, rather than immediately, and the "stickiness" would lessen in time, however, he claims all hard anodize parts we have gotten are "sticky" to some degree. Is this true?
- Would "sticky" parts have different conductivity levels? I assume the stickiness comes from the anodize somehow reacting with the moisture in my hands, which doesn't sound good if you consider the parts will be used underwater.
- Finally, the anodizers all claim to be using the same process (hardcoat, type III, Class 1, 2 mil) and the machine shop claims that they always supply the same metal (6061-T6) so how is it that I get everything from transparent (silver) anodize, to yellow/copperish shiny, to yellow/copperish matte, to grey matte, to grey shiny, to grey sticky?

Margaret Malewski
Customer - Burnaby, BC, Canada

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First of two simultaneous responses -- July 20, 2009

MIL-A-8625 Type III specifies a minimum hardcoating performance, not a detailed process. Therefore, it is up to the hardcoater to choose the combination of process parameters to meet those requirements. All hardcoat processes are not the same. In fact, they can be quite different. Variables include ramp time, current density, electric waveform, acid concentration, bath additives, agitation, bath temperature, and racking method. In addition, since the aluminum oxide anodic coating is formed from the base metal, this aluminum composition has a significant effect on the final coating properties.

All fresh anodize is hydroscopic. That is, it wants to absorb water. That is what makes dye absorption in color anodizing possible. This property makes the coating feel "sticky" as it tries to draw moisture to reach equilibrium. Normally 6XXX series alloys are not as bad as 2XXX or 7XXX series. We have noticed that extruded bar stock is worse than cold rolled. Some hardcoating processes, particularly those with warmer baths, tend to be "stickier" and lighter in color. This probably can be solved by sealing the parts in a hot aqueous solution. This accelerates the water absorption process and has the added benefit of improving corrosion resistance. Sealing is not permitted on hardcoat, unless specified, as it reduces wear resistance. Without forced sealing, over time the coating naturally seals by absorbing atmospheric moisture.

Some questions to ask:

1) Is the material really 6061-T6? If so, was it extruded or cold rolled?

2) Was the uneven anodize build-up within a single part? If so, it was probably the material. If it was part to part, it still could be the material or the hardcoating process.

3) Is the hardcoat 2 mils thick? Thin coatings appear lighter or could even be a standard commercial anodize.

4) Were the parts stripped and re-hardcoated? This tends to result in a matte finish.

For your application I would recommend a full 2 mil coating applied in a classic low temperature (32F) hardcoat bath and sealed. Make sure the material is cold rolled 6061-T6. The resulting part should be dark grey to black and very corrosion resistant.

Chris Jurey, Past-President IHAA
Luke Engineering & Mfg. Co. Inc.

Wadsworth, Ohio

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Second of two simultaneous responses -- July 20, 2009

Hard anodize (per Mil-Spec) will be unsealed unless requested. Unsealed will be absorbent and "sticky" to the touch.

A seal would be recommended for salt water environments. You should be requesting seal on applicable drawings and purchase orders.

Sealed parts are more corrosion resistant and yes, sealing increases voltage breakdown, at least in the reports I have read.

Different lots of 6061 alloy may have minute variations in composition and yet be within tolerance for cert. Add variation in tempering and the anodizing process and color variation doesn't seem all that surprising.

Bill Grayson
Metal Finishing - San Jose, CA. USA

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July 21, 2009

Thanks to both of you for your posts. I pulled out one of the casings from its newspaper wrapping and handled it for a while when searching for more information on this "stickiness". Guess what - the stickiness went away!

Based on what you are saying, it strikes me that the individual newspaper wrapping of the parts was preventing the parts from absorbing moisture. When I unwrapped a few of them their stickiness was reduced (although less than the part I handled - I guess the moisture in the air is less than in my hands?). So this problem is solved.

Also useful to know is that refinished parts are more likely to be matte than first-run parts.

I appreciate the input!

Margaret Malewski
- Burnaby