PITTING IN HARD CHROME PLATING

[mands]

Can anybody please give me some information on what can cause pin holes in hard chrome plating ?

The substrate is 17-4 PH

Thanks

 

[Linstrum]

Hi, Mandy: This phenomenon goes with the territory as is mentioned in the answer to your other question about crazing in hard chrome plating. The pin holing is actually taken advantage of and is used for the lining of engine cylinder bores, the pin holes acting as microscopic oil reservoirs to lubricate the piston rings and piston skirt. Porche is/was amongst the manufacturers who used "orange peel" chrome plated cylinder bores in engines. Chrome plating for corrosive environments, such as hydraulic cylinder piston rods, automobile bumpers, and decorative trim, is done on top of layers of copper and nickel plating. This is for several reasons, two of which are to chemically seal the substrate surface and to provide a "buffer" zone that is somewhat pliable (at least in comparison to the chromium). The copper is able to creep a little on a molecular scale and somewhat relieve the tension stresses in the chrome layer that are produced by its deposition. The copper does not eliminate the surface pin holes and crazing but can reduce their magnitude somewhat. As I recall, the copper layer is put down first then is followed by nickel, another layer of copper, then finally the chrome. More alternating layers of copper and nickel under the chrome are occasionally used. Cheap chrome plating like is used on Japanese motorcycles does not utilize the necessary layers of copper and nickel and consequently the iron immediately rusts underneath, which lifts the plating off. Richard Linstrum