Surface oxidation on Aluminum Alloy & electrical continuity

[rmain]

I'm using aluminum alloy (6061-T6 sheet at the moment) to construct a chassis for mechanical support of an assembly. This chassis also serves as part of an EMI mitigation system (RF, GHz range), so I need to ensure electrical continuity between the chassis and other components of the system. I was concerned surface oxidation would be a problem, but I've run some tests with a multimeter, and find that samples of material I've tested show close to zero resistance. I have been careful to lay the probes sideways on the material surface to ensure the sharp tips don't pierce the surface oxide. It's not clear to my why I'm seeing continuity under these conditions.

Some possible explanations:
- The material I'm using is NOT pure aluminum. Other metals in the alloy (6061 in this case) are exposed on the surface, and provide a path for continuity bypassing the Al Oxide layer.
- The oxide layer is somewhat 'porous'. Contaminants in these pores may provide the observed continuity. I did attempt to clean the surface with alcohol without observable change.
- The multimeter (Fluke 29) in resistance mode may apply a voltage sufficient to overcome the (very thin) oxide layer. I find this explanation unlikely, though I saw somewhere only several volts are required to 'overcome' the high oxide resistance. The mechanism of 'overcoming' the oxide layer was not explained. On the other hand, if this _is_ a factor, and a voltage drop will appear across the oxide barrier, that might have an impact on EMI mitigation, so I'll want to be aware of it.

When Aluminum wiring was first used in industry, there were significant problems especially where aluminum wires were terminated to copper wires & fittings, which was attributed to localized heating caused by resistance of an aluminum oxide layer. This was initially overcome by use of antioxidant paste, and mechanical connectors that broke through the oxide layer. My understanding is that the antioxidant pastes contained zinc particles that 'cut through' the oxide to provide continuity, and the greasy component of the paste prevented exposure to air (and subsequent re-oxidation). It appears antioxidant pastes are not as required with recent Al alloy wiring. Clearly the composition of aluminum alloy is important to mitigating surface oxides with respect to continuity & conduction.

In one location of my assembly, I plan on using a grease formulated with a built-in deoxidizer to help ensure continuity, while lubricating for ease of assembly. In another location, I plan on using conductive double-sided tape to help provide a mechanical bond, while ensuring continuity.

The same company that produces the grease above sells just their 'deoxidizer' in liquid form for cleaning & pre-treating the oxide layer. They claimed it would remove heavy oxide buildup. Since I haven't detected a continuity problem on samples of the material I plan to use, I have been unable to verify their claim.

I'd appreciate any feedback you can provide regarding the specific situation I've described above. I'm not interested in oxidation on exposed surfaces, etc., but in surface oxide as it pertains to close-contact electrical continuity.

 

[TugboatEng]

I would consider applying electroless nickel plating to prevent surface corrosive and eliminate the need for messy conductive greases.

 

[rmain]

Thanks for responding, Tug. I've seen your responses to similar questions on eng-tips.

We are already planning on applying electroless nickel plating on one of the key components.
The 'messy conductive grease' will be used to help assembly with a (conductive) o-ring (for pressure seal) & press-fit between two parts. We do plan on testing without the grease, but my current thought is similar to the electricians that look on NoALOX as 'insurance' against corrosion. If a thin film of grease helps to ensure no o-ring damage during insulation, helps with the pressure seal and helps to seal out corrosion in the 'press-fit' area, then I'm all for it! (Also, it will be assembled elsewhere, so the messiness is 'not my problem' , though I completely agree - it is messy).

It is possible to use electroless nickel on all the aluminum parts, but the cost does add up. If there is an easier/faster & cheaper solution now is the time to commit to it. Based on my tests, I don't see continuity problems, so we might just be able to use the parts 'as-is' & ignore the problem of oxidation.

 

[TugboatEng]

If your voltage is high enough it can break through the oxidation. Heating becomes a problem if the current is high.

 

[rmain]

What is 'high enough voltage' to overcome a typical (natural air-sourced) thickness of aluminum oxide?

I know anodized aluminum is essentially a 'thick' (up to 100s of microns) oxide coating, but as the oxide layer is also porous they also apply a sealer, which can also impact continuity. I did test anodized aluminum, and generally see zero continuity, and I can also pierce the anodization with the tips of my probes to achieve continuity. This is the behavior I was expecting to see with just normal air-sourced surface oxidation. I also tried heating a sample of 6061 plate with a heat gun (maybe 200C) for a short period to increase oxidation, but it had no appreciable effect.

 

[TugboatEng]

I think this may have what you are looking for. I'm traveling right now and don't have time to read it.

Electrical Conduction and Dielectric Breakdown in Aluminum Oxide Insulators on ...

https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=30168

 

[rmain]

Thanks for the link.
It's an interesting paper, but I don't know how much it applies.
They're intentionally growing thin aluminum oxide layers from Aluminum Nitride (on a silicon substrate) under controlled lab conditions.

I don't know if their 'lab purity' oxide compares in any way to oxide grown under natural (atmospheric) conditions on Aluminum alloys (like 6061, with it's components). They even comment that they aren't sure of the effect small amounts of Nitrogen present have on their oxide layers. Components of 6061 include Silicon, Magnesium, Manganese, Copper, Iron, etc.

 

[rmain]

I posted this issue to physicsforums, where there were some helpful suggestions & links.

I think the final answer is that the natural oxide layer _is_ present, but it only takes a few volts potential difference for the insulation to break down & conduct. You did mention earlier "If your voltage is high enough it can break through the oxidation.", I just wasn't expecting 2V(-ish) would be high enough.

 

[Gr8blu]

Just for reference - depending on specific model of (insert manufacturer's name here) multimeter used, the applied voltage for testing resistance can be 1.5, 3, 6, or 9 V. Something with more accuracy will likely test at one of the two higher voltages.

The "typical" aluminum oxide buildup is about 4 nanometers (40 angstroms), requiring a breakdown voltage in the 3.2 to 4.4 V range.

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