Aluminium parts can require more preparation than steel parts before they can be electroplated, and this is a critical factor in determining the longevity of the coating.
Aluminium Needs a Zincate Pre-Treatment
Aluminium as a base material forms a natural oxide layer quickly when exposed to the air, which cannot be bonded by an electroplating process. The aluminium surface has to be treated with a zincate process prior to the plating process in order to remove the oxide layer and to create a thin and even layer of zinc on the surface of the aluminium, which then can be coated with the electroplating process.
Account for Galvanic Compatibility
Zinc-nickel is relatively close to aluminium on the galvanic corrosion series, which is why it is so widely preferred over pure zinc for aluminium substrates. Exposed aluminium at part edges, or even damage to the coating can corrode rapidly in wet conditions. So, good coating coverage is important. Guidance on the zinc nickel plating process reveals that the alloy composition is critical to ensure good galvanic corrosion protection and that typically this contains between 12% and 15% nickel.
Build in Dimensional Allowance
Zinc Nickel Plating can deposit at a thickness of between 8-25 microns and this needs to be allowed for in the dimensioning of close tolerance items such as bores and threaded features. It would be wise to discuss the required thickness with your supplier prior to issuing your drawings for production.
Specify the Right Passivation Layer
The specification for a suitable passivation layer such as trivalent chromate on aluminium parts coated with zinc nickel plating should be noted on drawings and included in the supplier’s process. More on Zinc Nickel Plating can be found at www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating/.
Check Supplier Approvals for Aluminium Work
A company that has previously worked with aluminium components will be able to confirm their processes have been validated for use on this material type. Ensure that supplier qualified test data for a sample part made from similar material to that to be processed is available to view prior to committing to production of parts to be processed.
It ensures the parts are produced correctly the first time and are able to perform as required in service.
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