Anodizing for Electrical Resistance / Dielectric

A. Hard anodize is an excellent electrical resistor. More often than not, this is considered a limitation. A lot of masking on hardcoated parts is to provide for a grounding path. However, due to the edge defect, I would not count on threaded fasteners to provide isolation. (Edge Defect: Hardcoat does not buildup on sharp corners or edges)

A. Tim,

Hard anodizing is an excellent insulator, my only concern would be the way that you join the 2 housings together. If this involves metal screws, or bolts, most hard coaters will mask these threads, as once they are anodized with a "thick" hardcoat, it is pretty near impossible to get a bolt into the threaded hole. So therefore, you will have continuity between the two housings, unless there is another insulator involved in your fasteners. Material selection of your housings is important also, as some alloys are very difficult to hardcoat...and being a defense contractor, your spec may call for 2024, or another high copper alloy, which, is difficult, but not impossible, to put a "thick" hardcoat on. Some aluminum castings are also very difficult to hardcoat, also. (When I say "thick" I'm referring to a coating that's .002" or more) One more small piece of advice would be to radius all sharp edges as much as possible (I believe 1/16" radius is recommended for a .002 hardcoat), as all sharp edges do not coat well, are very prone to micro-cracking during the anodizing process, and chip very easily after hardcoating, if not carefully handled. And since friction/wear does not appear to be an issue in this case, I would strongly recommend that your hardcoater seal these parts.

Ahh, fellow anodizers ... don't you like it when an engineer actually thinks "finish first"?

If only all engineers started from the finished product and worked backwards

A. There is a process called Akadize that has excellent electrical insulating properties plus a harder finish. I work with a company in Pomona that provides this service.

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Ed. note: Thanks Art! But, gentle readers, please try your best in this free forum of camaraderie & technical information exchange, which has been made possible for you by our supporting advertisers, to use generic & descriptive names for processes rather than the trademarks and trade names of their competitors. We certainly should now note that Luke Engineering & Mfg. Co., Inc [a finishing.com supporting advertiser] offers similar proprietaries in their services, and their support helps makes this site possible.

A. I don't know if a 'better' grade would solve the problem ... but if the problem is the hardness of the aluminium, then using 6082 in a harder temper (H24 for example), would increase the hardness by perhaps 10 to 15%, but would probably not give better results.
The 7000 series, (7075 for example) would increase hardness by maybe 50%, but the same might apply.
Could you spread the torque force by increasing the area of the washer, or reduce the torque, or use harder aluminium alloy washers?
Best of luck anyway.

A. Dale
The insulating layer on anodised aluminium is aluminium oxide. This is very hard but also brittle. Although you can control the torque of the capscrews, it is unlikely that their bearing surface is finished to a particular standard and there may well be areas where the contact pressure is very high and the underlying aluminium substrate will distort to accommodate this -- thus cracking the anodised layer.
Even if you can get a "better grade" of alloy, the underlying problem remains. Remember that even if you succeed, the first maintenance man to work on the machine may never have heard of torque -- the safety factor will be small.
I would suggest that you need an insulating washer. Thin PTFE should be acceptable in a pharmaceutical application.

Thanks for the replies. To give you some more information, the cap heads are selected so that the surface contains no burrs or pips even to the point where the technicians building the machine use a file to take sharp edges away. Should the machine need to be dismantled, the anodised bush will be replaced with another 'fresh' one and the same technicians are used each time so they know the job well.

After visiting the anodiser responsible for the coating, it was thought that the coating wasn't thick enough so a trial batch are being coated with around 50 microns thickness and some are also having PTFE applied. I just thought there may be a better grade available for an application like this as I'm sure some people will have experienced similar problems.

A. Dale,
I can suggest Keronite as a solution, a 40µm Keronite layer with an air dry PTFE overspray should give you a positive lasting fix.
Best regards

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Ed. note: Thanks Rod! But again, gentle readers, we ask that you please try your best in this free forum for camaraderie & technical information exchange, which has been made possible for you by our supporting advertisers, to use generic & descriptive names for processes -- like "spark anodizing", "microarc oxidation" or "plasma electrolytic oxidation" rather than the trademarks or trade names of our supporting advertisers' competitors :-)

A. Voltage?
Aluminum alloy (high Cu or Si levels reduce the insulating value of the anodize)?
Hard anodize or conventional?
Dissolved aluminum in the electrolyte (high levels increase sulphate incorporation into the anodize)?
Flat surface or wire? If wire, what diameter (smaller diameters cause more anodize cracks, so less insulating)?
Environment (low humidity, non-condensing, wet)?

Section 10 of Chapter 12 of "The Surface Treatment and Finishing of Aluminum and Its Alloys".
6th Edn., covers electrical properties of anodic films. Earlier editions have similar information.

Tests for breakdown voltage are given in EN 12373-17, BS 6161: part 15: 1987 and ISO 2376.

A. Hi Kevin. Unfortunately I am not familiar enough with the technology to understand the potential issue you are concerned about. Are you suggesting the possibility that a static charge could build up between the mounting feet and the frame which could damage the solar panel, or shock an installer? I can't picture that happening.

Regards,

A. Hi, Kevin,

It is virtually impossible to get any meaningful measurement of electrical resistivity. An allied property, impedance, is measured. The impedance is used as a means of testing the quality of sealing: the higher the impedance the lower the porosity of the anodic film. Sealing quality is usually the preserve of decorative anodizers interested in corrosion resistance.
Just to make life difficult, the result of the measurement is usually expressed as admittance rather than impedance. Admittance is the inverse of impedance.
The testing procedure is well documented in anodizing test methods.
The question you really ought to ask is why your customer wants to know. It might be possible then to offer some more specific guidance. I wonder, for example, if the interest is really electrical breakdown voltage. Maybe too, that is a hundred miles from what is wanted!

Harry

A. Hi Irem,

The answer lies in your question: buy a copy of the standard. Surely it must be a requirement of a defense contractor to keep up to date copies of the standards to which they are working?

A quick Google search said that the standard has now been superseded by BS EN ISO 2376. I doubt if the content of the standard has changed but is more to do with bringing old British Standards into line with modern ISO formats.

I was much involved with BS 1615 in the 1960s. It included the method of measuring voltage breakdown which eventually became this standard. It was intended as a method of determining anodic film thickness. For this purpose it was killed off by eddy current machines.

Best wishes,
Harry

A. Hi Nabin. We appended your inquiry to a thread where Harry Parkes has discussed the issue. Please see his posting, then ask for any necessary clarification. Thanks!

Regards,

A. An ohmmeter can be used to check for continuity.

A. If you wonder anodized profile's quality as conductivity you should implement "admittance test". One of the important characteristics of anodized surfaces is "dielectric", but this issue depends on quality of anodized surface (especially sealing quality is most important). So if you measure sealing quality, according to Qualanod book you should make "admittance test". If you read this chapter in Qualanod book you will understand your question; but also if you make this test you must buy "anotest machine". Also this test is not applicable for cold sealed profiles and coloured profiles (medium bronze, dark bronze and black).