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Blistering chrome on zinc cast parts

Having browsed this forum on a number of occasions to try and find solutions to various plating issues, I've come un-stuck with something and can't seem to find the answer.

We are an importer of light fittings that are manufactured in the Far East, among these we have a fitting that has a cast zinc fascia screwed in front of a 50 W halogen lamp.

After the light fitting has been switched on for a couple of hours, cooled to room temperature and switched on again, the chrome begins to blister.

They are very small blisters around 1/8" diameter.

Having scraped some of the chrome off the rear of a fascia we can see a copper coloured material beneath which, I presume is copper plating applied to the zinc pre-chrome.

My theory is that bi-metallic expansion is causing the blisters but if that is the case, how can it be resolved?

Phil Gooch
Product designer - Birmingham, UK
September 16, 2010

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Phil, your suggestion that it is differential thermal expansion is valid, but I am concerned that the blisters appear to be circular - if it was thermal expansion, I would expect to see some creasing in the chrome overlay. The other thing is you mention the substrate is cast zinc. I just wonder if the casting is very porous and traps some processing liquids and when the housing gets hot during use, the liquid boils off and causes small bubbles to form that show up as blisters. To check this, you will need to take some cross sections and examine them under a microscope.
In the interim, you could specify to the supplier that your parts are checked before dispatch by doing a thermal cycle on them.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
September 20, 2010