Hard Chrome Plating Inspection and Quality Checks
Inspection of plated components is key to avoiding problems further down the production line. Whether you are purchasing components which have been plated for you or specifying a job to be plated, it is wise to know what to check when the plated items arrive.
Check Thickness Against the Drawing
To confirm thickness, eddy-current or magnetic induction gauges can be used to read the chrome thickness on completed parts. Alternatively, cross-sections can be taken from witness pieces and then measured. A record of the thickness should be kept and cross-checked against the specified tolerance on the drawing (typical tolerance for chrome plating is 25 microns +10/-0 to 250 microns). The plater should also be able to supply a copy of the gauge’s calibration record.
Measure Surface Finish
Surface roughness is typically specified in terms of Ra or Rz values and is measured using a contact profilometer. Hard Chrome Plating typically produces a surface which can be ground or lapped to a tighter Ra value after deposition. Check whether the surface roughness figure quoted is before or after any post-plate grinding and check that the appropriate surface texture measurement standard has been used to ensure that repeat and accurate measurements have been taken.
Test Adhesion
Check for adhesion by trying to file the surface of the chrome. If the chrome starts to flake or lift then the chrome has not adhered properly to the substrate. For critical applications a coupon is plated and then bent. If the chrome starts to flake or lift then the plating process has failed.
Inspect for Cracks and Porosity
Hard chrome plating, as deposited, contains micro-cracks in the surface layer of the plate. In fact, the lubricating properties of chrome are greatly enhanced by these micro-cracks. However, the chrome can also develop macro-cracks, pitting and even patches of porosity visible to the naked eye. In the case of harder chrome plates ( Rockwell C or above), these defects can be revealed by means of a dye penetrant test on the surface of polished plates where such defects would otherwise go undetected.
Ask for Process Documentation
Look for process documentation – the plater’s bath records, job start and completion dates and operator signatures. In regulated industries, such as aerospace, traceability back to the specific plating run is as important as the visual inspection. Without it, a part that has passed the visual check is nothing.
A simple check list of Hard Chrome Plating inspection at the receiving end of a delivery of plated parts could prevent many problems occurring during assembly.