Aluminium parts can be satisfactorily chrome plated with the right preparation.
Apply a Zincate Pre-treatment First
Aluminium parts have a natural oxide layer on the surface that forms within seconds of exposure to air. This layer must be removed prior to plating, and a zincate solution is used to remove the oxide and deposit a thin layer of zinc on the surface. This layer provides a clean, reactive surface for the plating to key to, ensuring good adhesion.
Use a Copper or Nickel Strike Layer
When trying to get a good adhesion of hard chrome to an aluminium part, a zincate pre-treatment alone is not sufficient. An intermediate strike layer of copper or nickel is required. The strike layer can be deposited by electroplating and helps to get a good bond between the substrate aluminium part and the hard chrome coating. The risk of blistering or peeling of the coating is greatly reduced.
Allow for Thermal Expansion
For Chrome plated aluminium parts it is important to allow for thermal expansion. This means that parts can be dimensionally correct at room temperature and yet still not fit together as required when in service. Tolerances must be set to account for the greater expansion and contraction of aluminium compared to steel. Close tolerance parts such as bores or shafts are particularly sensitive to this effect.
Expect Realistic Hardness Figures
The hardness of hard chrome plated to aluminium would typically be in the order of 850 HV to 1000 HV. However, this coating is applied to a relatively soft substrate and thus is not designed for high load bearing. For more information on how electroplating can be used on a variety of base metals see TWI’s technical knowledge pages.
A useful reference for Chrome Plating is www.poeton.co.uk/advanced-treatments/apticote-100-hard-chrome-plating.
Check Adhesion and Thickness After Plating
When applying the chrome layer test for adhesion using a tape and also check the coating thickness using an eddy-current gauge. Parts that have been inadequately pre-treated will show signs of lifting in these tests and should be reworked before use.
Getting the preparation right is key to a part that lasts and one that fails.