NiO black spots on gold coated pin
Black spots on a gold plated pin were analyzed with XPS and found to be NiO. Nickel is commonly plated onto copper wire before electroplated gold is applied over the nickel. If the gold electroplating incorporates too much contamination in the gold layer, easy diffusion paths for nickel to the surface through the low quality gold allow it to seek the oxygen nickel wants much more than gold does.
