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Dielectric strength of anodized coatings

Hello fellow finishers,

My question is regarding anodic films produced from sulfuric acid baths ( MIL-A-8625 / MIL-PRF-8625 [on DLA], Type II, class 2) on a 6061 T-6 alloy. Does anyone have information on dielectric strength of these coatings and how one would go about measuring this property? Thanks in advance for your help.

Keith Wicklund, CEF-SE
avionics - Minneapolis, Minnesota
1996

I recall seeing values for specific breakdown voltages for sulfuric acid films around 40 V/micrometer of oxide thickness. There is a commercial instrument to measure this, although I do not remember the vendor. It essentially consists of an electrode of known area, that is put onto the surface with a spring, so there is a constant force, and a high voltage AC power supply. The voltage is increased until a sudden voltage drop occurs which indicates breakdown of the film. See ASTM B110-45 (1973).

Gary A. Nitowski
Alcoa Center, Pennsylvania
1996

Thanks, Gary. That tells me what I need to know. Apparently, dielectric strength is not considered to be as important as other properties of anodic films because ASTM B110 has been discontinued with no replacement mentioned. The only other source of information I could find on the topic was the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] . The values they mention there seem to be consistent with those you mention. Thanks again for your time.

Keith Wicklund, CEF-SE
avionics - Minneapolis, Minnesota
1996

Get a copy of the mil spec for anodizing. it used to have methods and values. obviously it depends on the thickness and the density of the film. Even has the unit for testing. One square inch of copper under a given pressure and voltage. If you can not find it, write and I will look for it.

James Watts
- Navarre, Florida
1996

My previous comment blew it. It must have been in a cross reference document. If you will e mail me the ASTM number, I will see if it is in an old data base that I have access to. Sorry for the stupid booboo.

James Watts
- Navarre, Florida
1996

A. I have used a DC voltage from a short circuit protected power supply to test as a measurement of anodize quality. Low quality anodize will arc at voltages less than 300. Most good anodize will go to 600V or higher. It is a method that I use and it is not based on any particular standard, just years of experience with various suppliers and geometries. I just use simple voltmeter probes and just rub them on opposite sides of the part. Be careful not to become part of the circuit!

Matthew Thie
- indianapolis, Indiana USA
May 22, 2012