Aluminium can be effectively zinc nickel plated provided it is properly prepared. The pre-treatment is more critical for aluminium than it is for steel in order to achieve a suitable bond between the coating and the substrate to provide a high corrosion resistance.
The Oxide Layer Problem on Aluminium Parts
The aluminium parts arrive with an oxide layer on them – a very thin stable layer that is naturally developed on the surface of the aluminium as soon as it is exposed to the air. This natural layer must be removed chemically before any coating can develop a strong bond to the surface of the aluminium part.
The Zincate Pre-Treatment Step
For treatment of aluminium prior to plating a zincate process is typically used. A zincate process removes the oxide layer on the surface of aluminium and then deposits a layer of very thin zinc that the plating solution can bond to very well. In many specifications, including ASTM B630, a double zincate is specified. This process involves stripping the first layer of zinc off and then applying another very thin layer of zinc. The double zincate process provides a very uniform surface for coating and typically provides the best adhesion on aluminium parts after plating.
Thickness Ranges to Specify
Aluminium is a softer metal than steel and typically has much tighter dimensional tolerances. When specifying a plating thickness for aluminium parts, 8 to 12 microns of zinc nickel coating is typical for good corrosion resistance in a salt spray test. Thicker layers of plating are possible but must be agreed with the plater and part dimensions checked after plating.
Limiting Bimetallic Corrosion
Where components are made of aluminium and are bolted or otherwise connected to steel parts (such as nuts, bolts, washers, etc) the differences in electrical potential can cause the aluminium to corrode at an accelerated rate. A Zinc Nickel coating can help to bridge the electrical potential differences to act as a sacrificial coating.
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Which Alloy Series Plate Well
6000- and 7000-series aluminium alloys can be successfully plated. 6000-series alloys such as 6061 and 6063 are typical for zinc nickel plating, being relatively easy to plate. 7000-series alloys can also be plated but generally require closer bath control and may require a double zincate pre-treatment. The 2000-series alloys are by far the most difficult to achieve good adhesion and generally do not make good plating candidates due to frequent failures of the plated coating to adhere to the surface of the part. Always verify the alloy for a part intended for plating with your supplier prior to processing.
If you are using mixed material assemblies, it is sensible to specify the alloy series that are to be plated to prevent time being wasted in rework.
