Can this electrochemical reaction be explained? Steel, copper and electricity

[MarkSheffield]

My son brought home a fourth-grade science experiment a few days ago: He was to make some sort of property change in a steel nail. He had decided that he wanted to make it rust, so we set about creating an electrolyte solution out of salt and water and leaving it in there. There wasn't noticeable activity there for a few days, so we decided to add some electricity.

I used a 12V/500mA wall wart as the source and a piece of copper as the cathode, a clip onto the nail for the anode, all immersed into a bath of salt (NaCl) water. After 10 minutes, the water had turned completely dark, almost black, with a brown scum floting on the surface.

I was shocked at the rate of the reaction. I ran this for a couple hours, there was quite a bit of current running through it as the wall-wart got very hot. When the nail was retrieved from the bath, it looked like it had been pickeled rather than rusted, as there was only a pitting of the outer surface (total loss of the original surface sheen) but not red/brown at all.

My question is: What is the process that occurred to the nail? I see two choices, maybe there are more:

An acidic bath of HCl from the liberation of hydrogen from the water due to the electrical current, combining with the loose Cl atoms from the salt.

Iron oxide in a very thin layer

Can anyone explain what went on?

 

[Borek]

No idea what have happened - but a lot may depend on the electrode material - read 'what the nail was really made of'.