[BCKTHUNd]

I did an experiment where I plated with different metals (Zinc, Copper, and Nickel), then submerged the coupons in a salt solution to test their resistance to corrosion. I noticed that after the pieces of metal have been submerged in the solution for a while, fragile black substance has formed on the surface that grow quite thick but come of easily when shook slightly. Does anyone have an idea what it might be? At first I thought it may be CuO but it also appeared on the other pieces of iron coated with nickel and zinc.

Also, another question I have is how normally the anode in an electrolytic cell is decomposed, but does this still happen if I use a graphite bar for the anode? Also, if it does decompose, what reaction is happening at the anode? I have tried looking it up but couldn't find a clear answer. Does the graphite participate in any reaction, or is it simply electrolysis of H2O?

Here are some images showing the black substance deposited on the coupons:
https://imgur.com/gallery/CEaKGdw
(File too large to be attached)

[Vidya]

Graphite can undergo combustion at high temperature to give CO2 gas
Check about Hall-Heroult Process