Aluminium is a common base metal for Electroless Nickel Paring but needs to be processed slightly differently to steel.
Zincating Before Plating
Natural oxide layers on aluminium parts form very quickly and in most circumstances need to be removed prior to electroless nickel plating. A zincating solution is commonly used to remove the oxide layer and deposit a thin layer of zinc which the nickel can then adhere to. Adequate adhesion of the electroless nickel to the aluminium substrate is critical and without a ‘strike’ process such as zincating, adhesion failures will occur even with the best of cleaning practices.
Check Adhesion With a Bend or Scribe Test
It is worth ensuring that the process operator has specified the adhesion requirements for aluminium parts rather than referring to general quality standards. A simple scribe-and-tape test or a mandrel bend test can quickly reveal poor adhesion that has been missed by visual inspection. The supplier should be able to advise on the specific test they use and supply records of previous results.
Account for the Softer Substrate
As mentioned previously the Electroless Nickel Plating process provides a hard wear resistant surface. However the aluminium base material is a relatively soft material. The substrate will deform under high point loads and consequently the coating will crack or peel from the substrate. As with all hard surface coatings to the soft substrate material, thicker deposits do not solve this problem.
Thermal Expansion Mismatch
The thermal expansion of aluminium is greater than that of nickel, and therefore large temperature fluctuations can cause the coating to crack or lift from the substrate.
For Electroless Nickel Plating, visit https://www.poeton.co.uk/surface-treatments/plating/electroless-nickel-plating/.
Specify Alloy Grade and Temper
Different aluminium alloys have different properties and some are easier to plate than others. High copper alloys of the 2000 series can cause problems with adhesion. Also, materials that have been heavily cold worked can introduce high levels of residual stress into a part. It is very important that the correct material is specified on the drawing and that the plater has experience of processing this material. If you are processing the material in-house then also check the workplace exposure limits for nickel compounds as laid down in UK regulations.
Be sure to identify these issues on the drawings at the specification stage to avoid unnecessary rework.
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