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"Aluminum conductivity, chromate coating, oxidation"


Q. I am familiar with chromate conversion coatings on aluminum per Mil-C-5541 [link is to free spec at Defense Logistics Agency, dla.mil]. We use and prefer class 3 for electrical conductivity. Since the coating is not conductive, conductivity is accomplished by pressing or wiping through the thin coating.
Question 1: if the coating is wiped away at a conductive contact point, will aluminum oxides form and consequently become a barrier for conductivity?
Question 2: I know the environment will affect the speed at which oxidation forms, but generally speaking, how fast will aluminum oxides form in a room environment? Almost immediately? hours? days?
Question 3: I have cleaned 5052 and 6061 sheet and 360 cast aluminum with scotchbrite pads and have then rubbed the "clean" area with a white towel. A black residue is removed from the bare aluminum. What is this black residue, aluminum oxide?

Jeff Beseth
electronics manufacturer - Olathe, Kansas, USA


A. Class 3 chromate is conductive, but requires some contact pressure. It should NOT be wiped off.

At room temperature, high-purity Al forms a 1-2 nm film of amorphous alumina in air ~instantaneously after cleaning. This film thickens to about 10 nm in a few weeks, then more slowly to ~100 nm over 5 years at ~55% relative humidity. The oxidation rate increases with humidity and is higher for alloys. In electrical connections, resistance can increase enough over time to cause heating, further oxidation and sometimes fires.

The black residue is hydrated alumina (aluminum oxyhydrate), with color due to oxidized alloying elements.

For documentation of chromate conductivity beyond that in MIL-C-5541E, see Table V of Mil-F-14072 [affil. link to spec at Techstreet]D FINISHES FOR GROUND BASED ELECTRONIC EQUIPMENT. Electrical contact (for RF only) for Class 1A coatings may require toothed lockwashers. Class 3 electrical contacts (power & RF) do not. A footnote adds

"Touch up may be required around areas where toothed lockwashers are used. Use additional chromate conversion coating (chemical film) applied via brush to seal outside surfaces of mechanical fasteners and all abraded areas."

To summarize: cleaned aluminum has an initially very low electrical resistance which significantly increases over time, becoming insulative. Class 3 coatings maintain a low resistance electrical contact.

Ken Vlach [dec]
- Goleta, California

contributor of the year

Finishing.com honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.



Q. Ken:
Many many thanks for your very informative responses. However, more questions!
1) Sorry to re-question your comments, but do you really, really mean that the chromate coating is a conductive material, or do you mean that it is conductive in the sense that it is so thin that it essentially allows for easy conductivity via contact pressure? It appears that the thicker the chromate, the harder it is to achieve conductivity. This leads me to believe the gelatinous coating is basically a thin insulator? The comments from Mil-F-14072D do not seem to necessarily contradict the idea that the chromate is a thin insulator, instead that different degrees of pressure are needed to press through to make contact. Also, I am no plater, but have heard that platers often use the same bath for class 1A and 3, but increase the time immersed to accomplish the thicker 1A, which makes me conclude thin allows for easier penetration.
2) Is the black aluminum oxyhydrate non-conductive?
3) If two pieces of bare aluminum in a ground based electronic enclosure are in contact in a mild environment, then it is NOT possible to expect long term conductivity between the contacting surfaces, right? Oxygen will be allowed between the mating surfaces, amorphous aluminum will form, thereby making the conductivity unpredictable.

Jeff Beseth [returning]
electronics manufacturer - Olathe, Kansas, USA


A. 1) Probably should say that the chromate coating is a thin, poor conductor. It's not a single phase but rather a mixture of oxide, hydroxide and chromate. The chromate anion and water content act as an electrolyte impregnating a relatively non-conducting matrix. Gels are all somewhat conductive, with conductivity increasing with ionicity.
Conductivity decreases if the coating is dehydrated by baking.

Pressure increases the contact area. Contact is improved by using (soft) tin-plated contacts which conform to the surface. For corrosion resistance, you don't want to break through the chromate coating (unless necessary for the amperage). From MIL-C-5541, the resistance with a flat contact is 0.005-0.010 ohm/sq. inch @ 200 psi. So a contact of 0.04 sq. inch would have 0.125-0.25 ohms resistance @ 8 lb(f). OK for grounding & instruments (low current), maybe not for high current in a confined space.

2) Not much info. The oxyhydrate is perhaps a semiconductor doped by cations of alloying elements. Rather crumbly, which lowers conductivity further. But, not as insulating as alumina.

3) Yes, bare aluminum contacts (both Al-Al & Al-Cu) continually oxidize and increase in electrical resistance over time. On-off cycling increases the wedging effect of the oxide between the conductors. This accounted for roughly 3000 residential fires and 200 fatalities in the late 1960s and 1970s.

Ken Vlach [dec]
- Goleta, California

contributor of the year

Finishing.com honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.



A. Hi -

You are correct, the chromate conversion coating is itself non-conductive, but in a class 3 coating it is applied so thinly that standard contact pressure is enough to allow conductivity.

Standard aluminum corrosion products are non-conductive -- therefore, as you thought, overtime you would loose conductivity.

Those two things being the case, you are best off applying the class 3 and NOT WIPING IT OFF - if you wipe it off you may have slightly better conductivity initially, but as corrosion builds you will have increased resistance and insulation. If you DON'T wipe it off, you will initially have slightly lower conductivity, but this will remain much more constant as the chromate will prevent future oxidation of the aluminum metal.

Good luck.

Jim Gorsich
Accurate Anodizing Inc.
supporting advertiser
Compton, California, USA

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A. If you don't have the need to qualify the chromate for class 3, I advise you to use a yellow chromate. the yellow is much more protective than the clear. The coating itself whether clear or yellow is non conductive. To make contact it is best to use screws, clamp, or the type of washers that cut easily through the coating. Yellow chromates will migrate over small scratches and continue to protect. Do not abrade the coating before making contact to get the best long lasting protection.

don baudrand
Don Baudrand
Consultant - Poulsbo, Washington
(Don is co-author of "Plating on Plastics" [affil link to the book on: Amazon or AbeBooks ])

April 19, 2011

Q. These responses were very helpful in my understanding of MIL-C-5541 (Alodine). What confused me was that I had read as pressure is increased between two BARE aluminum pieces that resistance goes down until the pressure is at 1200 psi. Any increase in pressure after that does little in reducing the resistance. Also the resistance across the bare aluminum pieces is about 1/10 of what it is with Alodined aluminum pieces -- well, initially. Then I understood that MIL-C-5541 is there for consistent resistance as bare aluminum will oxidize over time and the resistance increases dramatically. When I saw that MIL-C-5541 is tested at 200 psi giving a resistance of 5 milliohms per square inch, it made me wonder if there was any benefit in increasing the pressure to a higher value than 200 psi. Is there any benefit -- or is it maybe even detrimental? I wondered, too, because I have always heard that the Alodine coating is soft (although I may be mistaken -- maybe it is only soft until it dries)... does it tend to fill in the gaps between the two pieces at 200 psi and going to a higher pressure is of little value?

James Purdy
- Georgia

January 5, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi,

I just wonder how can we differentiate Chromate surface and Anodized surface electrically. i.e., by inspecting resistance value on multimeter.

We have got 50 different types of Aluminium components with Anodizing and Chromate finish.

Anodizing is as per MIL-A-8625F Type II, Class2 and Chromate finish is as per MIL-DTL-5541F, Type II, Class 3.

What are the quantitative measures to differentiate between anodized surface & chromate finish surface?

I usually use multimeter's "buzzer" to observe whether surface is electrically conductive (if buzzer beeps then surface is chromate finish) or non-conductive (if buzzer does not beep then surface is Anodized surface). But I believe this is really a crude way to inspect.

Is there any resistance value specified anywhere in MIL spec (or any other spec) which indicates the surface is anodized for > 'X' ohm and <'X' ohm for chromate finish?

This is an ongoing issue to determine correct (conductive or non-conductive) surfaces and this affects our end customers, some want conductive surface and some want non-conductive?


Mihir Rawal
Mechanical Component Engineer - Sydney, NSW, Australia

January 6, 2014

A. MIL-DTL-5541 states: Electrical resistance testing. When under a nominal electrode pressure of 200 psi, class 3 coatings are qualified under MIL-DTL-81706 to have a resistance not greater than 5,000 microhms per square inch as supplied and 10,000 microhms per square inch after 168 hours of salt spray exposure. In addition to the coating or coating thickness (see 6.1.2), other variables heavily influence resistance values when using the test method specified in MIL-DTL-81706 or other similar methods. The following two variables (see and may have a greater effect on electrical resistance values than the conversion coating thickness.

These values are an average over 10 locations (readings) per panel, and the number of panels (usually 5).

The above data covers chemical conversion coatings. I just read MIL-A-8625, and it does not have electrical resistance as a requirement.

Fauna Tester
- Seattle, Washington USA

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