Plating mystery

[Waggy]

So I decided to plate a watch at the weekend, I polished the case back to brass as usual and placed it in my nickel plating solution. I then applied my normal 0.10-0.15 amperes and let the magic happen. Then after about 10 mins I noticed that the nugget of nickel was bubbling instead of the watch case.... Then I realised I had connected my leads backwards, making the case and not the nickel nugget the sacrificial metal. I pulled the case out and it had become very dark grey with a frosted look, it was not possible to see this in the green nickel plating solution.

Here is the strange bit, I decided to polish back the case to re-do the plating, but as I polished the dark grey took a polish and looked like gun metal or titanium. I polished a little more and the grey disappeared to reveal a silver metal (colour not the element) coating.

How/where did this come from?? Could I have pulled the copper out of the brass leaving behind the zinc???

I swapped back the leads to the correct configuration and plated the watch perfectly. But where did the silver come from when I had them backwards??

[RichardHarris123]

Not certain but you probably pulled the zinc out of the brass ( dezincification) leaving a zinc coating on the surface. Zinc is higher on the electromotive series. 

Magnesium is used as a sacrificial metal.