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Chrome plated sheet metal - corrosion on underside

I am a Conservation student and an object I am working on has a hollow chrome plated sphere, cut into half and hinged together. Imagine a hinged egg, but chrome plated on the outside and with a dull grey metal on the underside. Some of the grey metal has been rubbed off and shows copper underneath. My questions involves what kind of metals are layered together on chrome plated sheet metal. The chrome surface is in good condition, but I would like to know what kind of layers would be on the reverse, the 'non visible' side. I suspect it is a thin layer of steel, but the object seems very light to be steel. If anyone has any ideas, I would much appreciate it. Thank you.

chrome plated egg

Erin Secord
Cardiff University- Student - Cardiff, Wales, United Kingdom
2005

A. Several layers of metal on what looks to be printed circuit board material. Is this an artwork?
Steel wouldn't be used although iron is a possibility; but much easier to evaporate aluminum in vacuum. Possibly nickel or tin plating.

Best to have it analyzed by SEM-EDS. Since you've aroused our curiosity, please report your results.

Ken Vlach [deceased]
- Goleta, California

Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2005

A. Hi Erin,
I don't understand the pic -- is this highly magnified corrosion? And where is the grey metal you want to identify?

But to the general question, decorative chrome plated metal always has a layer of bright nickel plating under it, and often a layer of copper plating below that. But it is very possible for the nickel to not be bright & reflective on the inside of the "egg"; if you look at a chrome bumper you're likely to see the same thing on the non-visible backside, where some nickel plates out -- but rather inadvertently more than deliberately. On the inside of the egg, facing away from the anodes, the current density is probably quite low, like the back side of a bumper, and due to that low current density you get some coverage but not brightness. For lightness, the 'egg' could be made of aluminum rather than steel; but in that case there would be no rust. Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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