I got the black oxide bath to work. I didn't catch that it didn't work on stainless, but it definitely worked on magnetic coathanger wire. The only
problem I had was the solution boiling off! I guess I could use a reflux setup of some sort.
I've got a PWM and some H-bridges on the way
The solution evaporating is quite a normal effect. One can just turn down the heat and add DI or tap water to replenish the bath if the coating takes
a longer time to develop. I've used both and the bath does not seem to mind the accumulation of salts from tap water. Still it coats just fine.
The bath does not need to be on a rolling boil to operate....a slight simmer around 90+C will do just fine and keep the evaporation rate under
control.