"Ignoramus32654" wrote in message
...
I made a basic electrolytic derusting setup and am using my welder

http://igor.chudov.com/projects/Weld...berTig-Welder/

to run the current.

I am derusting a very rusty old railroad tie plate that spent a while
laying in my yard.

After a while of derusting at about 15 amps, 37 volts or so, the
surface of it turned black.

Should I expect it to become shiny metal after a while, by itself, or
is it my responsibility to clean off the black crud?

Could it be that I am basically done with derusting and simply need
to clean the black crud with a wire brush or some such?

Exactly. I've done quite a bit of this over the past few years and I've read
what more experienced people have to say about it. What you've done is to
remove much of one oxide, the red rust, and then convert some of it to the
black oxide. The black stuff *usually* brushes off nicely with a wire brush
(I use a stainless brush, but that really isn't necessary).

If you use phosphoric acid on something that's really rusty, you get a
similar layer of black oxide, but it's much more tenacious and difficult to
remove, in my experience. Hydrochloric acid gets rid of the black oxide, but
it may attack the base metal a bit and it may leave some salts that can lead
to more rust. My experience, again, is that this is not really a problem.

BTW, I use a car battery charger for a power supply, and a slab of Poco 3
EDM electrode graphite that I cut off of an old electrode with a backsaw, by
hand. I use the hand-swabbing method much more than the tank method.

--
Ed Huntress