Initial surface preparation is a critical stage in the overall process to ensure that the chrome deposit will bond correctly to the base material and last for a long period of time. Failure at this initial stage will result in subsequent processes failing to deliver as required.
Degrease and Clean the Part First
Remove oils, cutting fluids, etc. from the part prior to coating. These can be removed using a soak cleaner (e.g. alkaline-based) and then ultrasonically cleaned and possibly subjected to an electrolytic degrease prior to coating.
Correct the Geometry by Grinding or Polishing
A chrome plated surface reproduces the peaks and troughs of the surface it is plating, so the surface must first be ground or polished to correct any geometry errors, such as pitting or high spots, and to produce a surface finish of around Ra 0.4 to 0.8 microns for hydraulic and bearing type components.
Strip Oxide Layers with Acid Activation
The steel surface will quickly form a passive oxide layer. Chrome will not adhere to steel in its oxide state. Short etch with hydrochloric or sulphuric acid immediately prior to plating is required to remove the oxide and produce a surface that is chemically active and ready for coating. For more on this, take a look at https://www.poeton.co.uk/advanced-treatments/apticote-100-hard-chrome.
Mask Threads, Bores, and Precision Faces
Plug, cap or lacquer mask off areas such as precision faces, threads, bores, etc. that are required to be chrome-free. Thread forms and tapped holes are prime examples of areas where unwanted plating can cause major problems with subsequent assembly.
Check Base Metal Condition and Hardness
Typically a minimum hardness of 58 HRC is required for chrome plating of this nature.
Rack the Part for Even Current Distribution
The points of contact need to be placed in such a way that there are no unacceptable witness marks. The part must also be positioned within the tank to ensure that all recesses and edges receive the necessary current. If a part is racked poorly then the thickness of the chrome will vary across its surface with edges being particularly thick and recesses being thin. This is why surface preparation is so important and why chrome plating is subject to strict control as outlined in the chrome plating guidance under UK REACH.
Each stage of preparation relies on the previous stages to be completed to an acceptable quality; if one stage is allowed to deteriorate then the whole process can be adversely affected.
