Hi there! I'm just ended a simple experiment we discussed some posts ago: I tryed to load with hydrogen a solid Fe part, without co-depositing enything else on it. The (somewhat preliminary) result is that no voltage is generated when the pieces are used as WE.
I used two iron nails, one made of mild steel and the other of hard steel. Their diameter was about 2 mm, their length about 6 cm. I cleaned the nails and rubbed with sandpaper. Their appearence was silvery and bright. I set the nails in a small electrolytic cell connected to negative supply (cathode) and I used another piece of iron as anode. At first I wanted to use just water and some salt to increase conductivity, but then I decided to not increase variables and using the same ingredients I already used before. So I added to water a couple of drops of HCl. The current was set to about 10mA for about 8 hours (some regulation was needed to keep it steady). A costant flow of small hydrogen bublles was generated by the cathode. After 8 hours the nails were mostly shiny as at the beginning, so nothing was plated on the surface and no signs of oxidation/rusting.
I took the nails, rinsed and dryed, and inserted them in a small diameter brass tube (same material used for the LEC) with a couple of spacers. Then I measured the voltage: 0 mV in both cases.
So it seems that the co-deposition process is a necessary step: apparently the "magic" happen inside the co-deposited layer.
This experiment also gave me some important clues on the deposition process I used on my first LEC: after one hour or so, the bubling on the cathod decreased, the current also decreased. In order to increase the current level I increased the voltage (this is the thing that also a current generator would do). The current increased to the desired level, but not the bubbling. After some minutes a black deposit started to appear on a small spot of the cathod closer to the anode. This was the same thing that happened with my first LEC WE. Now it has an explaination: once the excess H+ (and Cl-?) ions are evolved, a new electrolytic reaction takes place (the one with activation energy just above the first). This probably involve the reduction of Fe ions at the cathod. These ions apparently are not in solution, but are probably "stored" at the anode as magnetite or something similar, as suggested by Alan Smith.
So the reaction has two different phases: in the first phase (typically voltage is <0.5V) excess hydrogen is evolved on the cathod and some form of Fe salt is formed on the anode. Then in the second phase (when voltage is increased to > 0.8V or no competing reactions can occur) the actual deposition happen. Probably starting with a higher voltage makes both reaction occur at the same time, but in this case it is difficult to keed the current low. Clearly this is a very delicate equilibrium to handle and for sure regular plating with Fe salts is more reliable and practical.