A common requirement is to have parts of an assembly anodised (possibly for corrosion protection, electrical insulation or cosmetic reasons). However there is often a requirement to electrically bond parts together. There are a wide variety of reasons that an assembly might need electrical bonding, but some of the most common involve:
- earthing safety requirements
- prevention of stray voltages
- EMI shielding purposes
- lightning strike mitigation (particularly in aerospace applications)
Since the coating produced by anodising is made up of aluminium oxide, it is non-conductive. This poses a challenge for bonding parts together. There are a number of options for designers to consider. Recommending electrical bonding techniques is outside the scope of the service that Metal Finishings Ltd offer, but below are some of the approaches that are common.
Mating Faces
Electrical bonding is achieved by having a flat face on each part of the assembly that mates with another part. This might be raised, flush or inset - as long as reliable contact can be guaranteed.
This face is to be free of anodising. Either by masking or by mechanic removal (usually skimming or spot facing) or much more rarely by local chemical stripping.
Sometimes this face will be left in the bare condition when very low resistance is necessary, but this is risky because it may be liable to corrosion over time - which might compromise the overall corrosion resistance of the part or lead to increased electrical resistance on that face.
It is more common for a low resistance coating to be applied to the face that is free from anodising, such as Surtec 650V (RoHS compliant), Alocrom 1000 (not RoHS compliant) or another suitable conversion coating.
Another option when a conversion coating is not suitable or insufficient on its own, is to apply a conductive paint - for example, something from Henkel's Electrodag range.
Bonding Jumper
This is a similar concept to above, but tends to involve a small area (free from anodising) that can be bolted/clamped to a jump cable which will then connect to the rest of the assembly. This may leave non-conductive seams, so is often not suitable for EMI shielding purposes and may not be suitable for other purposes. Other concerns include that it may be a single point of failure.
Options Other Than Anodising
In some cases, designers may consider options that do not involve anodising. One that's commonly seen involves the whole part being coated with a conversion coating such as Surtec 650V and then selectively painted, leaving unpainted areas with a low resistance finish. Less commonly, a part may be plated all over with a metal that provides appropriate corrosion resistance and a low electrical resistance (although it's important to consider what will happen if the part starts to corrode).
In cases where some corrosion prevention is desirable but the assembly will be used within a low-corrosion environment (for example inside a data centre or a heated office) and it is not a cosmetic piece, it may be sufficient to apply a conversion coating only (as a final finish) and benefit from low electrical resistance across the part.
Disclaimer
This page is provided for information only, it should not be considered advice and we cannot accept any responsibility or liability for your use of the information on this page. The information on this page is used and relied on at your own risk and you bear the sole responsibility for any outcomes. E&OE.