While chrome plating steel parts presents few challenges, pre-treatment of aluminium prior to chrome plating is critical and must be done correctly to achieve adhesion.
The Zincate Step
Most chrome platers use a pre-treatment process called a Zincate on aluminium parts. The Zincate process dissolves the natural oxide layer that forms on the surface of aluminium when it is exposed to the air. A layer of zinc is then deposited onto the surface of the aluminium. This layer of zinc provides a surface for the chromium to bond to. Most chrome platers use a double Zincate process. This means that after the first Zincate process the zinc layer is stripped off and another Zincate process is applied to produce a very fine surface for the chrome to bond to.
Alloy Choice Matters
Some aluminium alloys are easier to chrome plate than others. The 6000-series (such as 6082) is suitable for chrome plating but some of the high-silicon casting alloys and most of the 2000-series are not. It is essential that the plater is informed of the alloy type of the aluminium part prior to processing to avoid possible problems.
Allow for Deposit Thickness
When you add a layer of chrome to a surface using Hard Chrome Plating, you are adding a large amount of material to the surface. For parts such as aluminium rods or bore, the chrome will have a typical thickness of 25-250microns, depending on the specific requirements. This means that parts like these will have to be machined to be slightly smaller than required to allow for the chrome to build up to the required thickness on all faces. Hard Chrome Plating is covered in more detail at https://www.poeton.co.uk/advanced-treatments/apticote-100-hard-chrome.
Masking and Fixturing
Aluminium is relatively soft and easily marked during stacking. So, as an alternative to nylon, plastic contact points can be used. When threads or bearing bores are to be plated, these must be masked using plugs specifically designed for the task. These are a lot more effective than tape which can lift off under the bath chemistry to leave unplated patches.
When Chrome Beats Hard Anodising
Although Hard Anodising is often the first solution to consider for wear resistant aluminium parts, Hard Chrome Plating has the advantage of producing a thicker, more durable protective layer with excellent corrosion resistance, particularly in sliding or fretting contact. From about 50 microns and above, Chrome becomes the better option.
Confirming your alloy and setting out your required tolerances prior to plating is all that is required to produce a chrome plated part.
