Surface oxidation of aluminum alloy and electrical continuity

[rmain]

TL;DR Summary

I need to understand why I see continuity on an aluminum (6061-T6) plate when checked with a multimeter, when I expected oxidation should prevent continuity, and define 'best practice to ensure continued continuity in an assembly including aluminum parts.

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.

 

[berkeman]

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.

You will get other good advice, but here are a couple thoughts:

** To have a good shield at GHz frequencies, you will need to have a pretty continuous weld along any seams. If you could post some drawings of the part(s), that would be a help.

** When grounding metal parts, you need two things: a) Multiple contact points and b) air-tight contact points. This generally means things like high contact forces and multiple "bite" gasketing or washers.

 

[Rive]

I think you could borrow some ideas from high current Al busbar connection systems. There are many manufacturers, usually with fairly detailed recommendations for the assembly process.

 

[rmain]

You will get other good advice, but here are a couple thoughts:

** To have a good shield at GHz frequencies, you will need to have a pretty continuous weld along any seams. If you could post some drawings of the part(s), that would be a help.

** When grounding metal parts, you need two things: a) Multiple contact points and b) air-tight contact points. This generally means things like high contact forces and multiple "bite" gasketing or washers.

Thanks for the input.
Unfortunately, I'm unable to post pictures.
A simplified description is a plastic box (sprayed with conductive paint), with a metal (6061) plate lid that drops into a groove at the top. The conductive paint at the groove will provide continuous contact with approximately 2mm of the bottom surface of the plate around the entire periphery. The plate will pressed against the paint with foam. Contact force will not be high, but the parts will be pressed together. It will be possible to add some form of 'conduction enhancement' at the edges of the plate if it is required.

There will be other breaches (holes) to this 'box', and I'm working to address each as required.

The wavelength of 1.6GHz in air is approximately 19cm. I'm expecting I should only worry about holes larger than 1/10th this wavelength, or 1.9cm.

Where wiring breaches the 'box', ferrites will be used to ensure conducted emissions are blocked.

 

[rmain]

I think you could borrow some ideas from high current Al busbar connection systems. There are many manufacturers, usually with fairly detailed recommendations for the assembly process.

Thanks for your input.
I am actually having one of the other aluminum parts nickel plated (using electroless nickel plating), not only for conductivity, but also for mechanical benefits. Surface treatments like plating or Alodine (chemical conversion) are also used to address oxidation in electrical distribution system such as busbars.

There are also some creative ideas (compression washers, slotted hole designs, etc.) with respect to physical alterations at connection points (like bolted connectors on busbars), that I don't think will translate to my design challenges, but it's great to have them in mind as I work through the design.

 

[Baluncore]

Where wiring breaches the 'box', ferrites will be used to ensure conducted emissions are blocked.

You will need a ferrite beads that operate above 1.6 GHz.
The ferrite beads and low-pass filter capacitors are usually built into a separate closed chamber in, or on, the side of the box.

 

[DaveE]

I am actually having one of the other aluminum parts nickel plated (using electroless nickel plating), not only for conductivity, but also for mechanical benefits. Surface treatments like plating or Alodine (chemical conversion) are also used to address oxidation in electrical distribution system such as busbars.

This is best practice for Al chassis. Anything else is prone to reliability problems. There are also manufacturability/QA issues with things like paste or star washers. Humans tend not to always do what they are supposed to.

Applications like house wiring with unplated Al rely on mechanical fastening systems that will pierce through the oxide layer and stay in place keeping oxygen out. This would be really hard to ensure along a long contact seam. Mechanical compliance is usually needed to prevent displacement due to TCE differences in the materials used. These are systems that usually aren't intended for service, disassembly, and reuse.

 

[f95toli]

The oxidization of aluminium is -unlike many other many other metals- self terminating. Unless you do something "extra" to the surface the oxide will always be quite thin. The oxide is actually very good (which is why it is used an an insulator), but the fact that it is so thin (~tens of nanometres) still means that is very easy to pierce mechanically. Hence, my guess is that the oxide is there; but that you are simply penetrating the oxide it when you test it with you mustimeter.

 

[marcusl]

If conductivity must be maintained, you need a surface treatment. A chromate plating (Alodining, e.g.) is standard but there are others available. The best choice depends on the environmental requirements and scratch resistance needed. Your multimeter test is irrelevant over the long term.

 

[DaveE]

I'll just add that the hardest engineering question is this:

"If I did something wrong, not "best practice" (or the normal way), or I want to cut corners; how bad will it be, what will happen?"

Most focused research is aimed toward better; higher performance, more reliable, etc.

Also, make sure you properly account for reliability costs throughout the product lifecycle. It's complex and specific to your product/market. But for durable goods, it's nearly always better to avoid problems than to fix them later. Are you like Toyota, or Yugo? Copy other's designs that are proven, spend your time and money on the unique stuff for your situation.

 

[rmain]

Mechanical compliance is usually needed to prevent displacement due to TCE differences in the materials used.

Mechanical compliance is present - the aluminum plate is compressed against the conductive paint with foam.

The plastic will have a CTE that is close to that of Aluminum so displacement will be minimal. If 'conduction enhancement' products prove insufficient to maintain continuity, we'll move on to surface treatment. Alodine is cheaper & would likely be sufficient, but electroless nickel is also an option since we will already be using it for other parts in this design.

 

[rmain]

The oxidization of aluminium is -unlike many other many other metals- self terminating. Unless you do something "extra" to the surface the oxide will always be quite thin. The oxide is actually very good (which is why it is used an an insulator), but the fact that it is so thin (~tens of nanometres) still means that is very easy to pierce mechanically. Hence, my guess is that the oxide is there; but that you are simply penetrating the oxide it when you test it with you mustimeter.

Mechanical penetration is clearly a possible explanation (given the thin layer), though I have attempted to be careful when applying the probes to only use the side & not apply significant pressure. I think if the oxide layer naturally present in atmosphere were so easily breached, terminating aluminum wiring would never have been an issue.

I did find a post citing a reference paper indicating there is a (poorly understood) mechanism which causes the insulation of thin-film (single digit to tens of nanometers) aluminum oxide to break down (conduct) under a few volts potential difference.

Referring back to the documented aluminum wiring issue, a few volts drop in a junction of wiring carrying tens of amps (for instance), could result in significant local heating.

 

[SredniVashtar]

Not sure if it's a urban legend because I did not run the numbers, but afaik the oxide layer is usually so thin that you can tunnel through it. No need to scratch it.

Anodization, on the other hand can build up an oxide later that will prove to be insulating. The colored aluminum containers for fruit mousse, for example are anodized on the outside and covered with an epoxy or polymeric layer on the inside. Both sides are electrical insulators.

 

[Tom.G]

Referring back to the documented aluminum wiring issue, a few volts drop in a junction of wiring carrying tens of amps (for instance), could result in significant local heating.

Yup!
That's why there were so many fires when Aluminium wiring for houses was first introduced. I don't remember the details, but eventually the design of some switches, outlets, etc were modified for compatibility with Aluminium wiring.

Aluminium wiring is rarely seen these days in the USA. One exception is in the long distance high voltage transmission lines, the ones on tall Steel towers out in the country.

There is even the same problem with high-current Copper busbars. Just bolting them together does not stop oxidation build-up. A joint compound loaded with Copper particles is used on the joints; I seem to remember the joint compound as being petroleum based, thereby excluding Oxygen.

A funny story:
I live a few blocks from the Pacific ocean, in other words a salt air environment is common. Several years ago there was a power outage in the neighboorhood. The power company came out and dutifully restored it.

A few hours later the power again failed, and was again restored. I managed a conversation with one of the linemen doing the work and asked what was happening.

Here is a synopsis with some background information added:
The 16kV distribution wires were Copper, as was all the low voltage (consumer) wiring. However, the clamps used for the taps and splices on the 16kV lines were Aluminium. Now, there are a few grades of Aluminium that tolerate salt air, but not these. The salt air started corroding the Aluminium taps, high resistance, high heat, faster degradation, failure.

They had not replaced all of the Aluminium connectors on their first visit. They did on the second visit.

Cheers,
Tom

addendum: In residential and light commercial areas, the three wires at the top of the poles carry 16,000Volts, Delta-connected. The transformers on the poles are fed by the 16kV and step it down to the common 120/240V 2-phase for residential use. For heavier loads there are three transformers stepping down the 3-phase 16kV to 120/208 3-phase Wye-connected.

@Baluncore, if I messed up anything here, please supply corrections!

 

[Baluncore]

The joints between aluminium conductors are made under an excess of dielectric grease that prevents oxidation of the junction by excluding air and water. The clamps are designed to bite through the hard sapphire Al₂O₃ surface oxide of all strands, while the junction remains immersed in the grease. The deformed surface of the aluminium junction remains bright, reflective and conductive.

Petroleum jelly was originally used as dielectric grease, but it deteriorates with heat, can burn, and forms hydroxides with water. Silicone grease is now formulated to be more stable and better reject water.

A funny story:
The Australian, copper, twisted-pair telephone network, was reasonably reliable until they reduced the number of skilled technicians in the 1980s. Apparently, silicone grease was good for protecting the copper terminations. After they started using silicone sealant to protect the copper network, it took a year for the brown copper acetate to form and intermittently insulate the connections. By then the system was beginning to collapse. Apparently, the reason Australia went digital so rapidly, "was to increase the reliability of the network".