Removing black-oxide rust?

[Jeff Cord]

I have an ancient Stanley #7 plane that I've de-rusted using citric acid.
The plane has cleaned up but now is covered by a black powder like coating (I'm assuming it's black oxide rust).
Anyway, I've tried to remove it using green scrubby pad but the surface of the plane is just slightly pitted and it's not cleaning up.
Is there a secret to removing the black coating, especially when it's in the little pits?
thanks,
jeff

[Darius Ferlas]

Soak the whole thing for a few hours in coca cola or fresh lemon juice.

[Jeff Cord]

I had soaked it over night in citric acid to remove the rust.
The red rust is gone, but now I've got this black stuff on it.
Did I soak it too long?
After soaking it did I do something to cause the black rust?

[James G. Jones]

I use a brass wire brush chucked in my drill press. My only word of caution would be to keep a firm grip and wear safety glasses when cleaning the small parts. Don't ask me how I know this.

James

[Peter Evans]

Jeff, you left the plane in too long. Citric is great and cheap (I get bulk lots from a cleaning supplies wholesaler in 25kg bags, and share around) - however works best if you pay attention.

Acid does not stop with the rust, but keeps on going into the steel (much less of a problem with cast iron) - therefore put the tool into a strong hot solution (a mixer will speed up the action as well as heat), and take it out 10-30 minutes later and use a scourer to scrub off the converted rust. If there is any rust left, put it in again, and check.. and so on. If there is a patch of very bad rust, do not leave the tool in the solution, but hit the bad area with a wire brush in a drill, and perhaps place a bit of towel kept damp with citric solution just on the bad spot until you are sure the rust is gone. A damp towel is also a way to remove rust from a large saw - I use mechanical rust removal for saws.

When rust removed, dry tool and hit with a hair dryer until very warm - this drives the moisture out of the tool, then put in sun for a few hours in summer, or in front of a fire or heater in winter. The process will sometimes leave a greyish tinge to the tool - remove this mechanically, wet&dry paper with WD40, a buffer or deburring wheel etc. You may then use your preferred rust preventer on the warm tool; the preventer (light oil, wax) goes on best if the metal is warm, and kept warm for a period.

If the tool is grey/black probably only mechanical means will work. A short acid bath might help to soften the coating.

A test I used was to place 3 rusty tent pegs in the solution, and take them out at different times. The peg taken out quickly was perfect after scouring and heating. The peg left in for a day had mild pitting after scouring, the peg left in for a week had considerable pitting and needed W&D paper to clean off the coating. This acid etching is what sharpens files/rasps.

Cheers
Peter in Sydney, perfect early autumn weather

[Dominic Greco]

I use a brass wire brush chucked in my drill press. My only word of caution would be to keep a firm grip and wear safety glasses when cleaning the small parts. Don't ask me how I know this.

James

Oh yeah, I hear you on that! I usually install a brass wire wheel brush in a chuck at my lathe. Nothing like having a variable speed wire wheel! But I always wear a face shield when I'm doing this! Plus I've taken to wearing a leather turners apron/smock to help reduce me getting impaled by tiny brass wires

I recently cleaned off a 605 1/2 and 606 using Evaporust and had to deal with the gray finish left by this stuff. A short session with the wire wheel brush did the bob and now these planes are nice and clean.

[Jeff Cord]

That sounds exactly like what happened.
I left in too long and now the iron is a bit pitted (not bad but just a bit) and I have a bit of black rust.
I didn't consider that the acid would eat into the metal.
Sounds like the brass wheel is my best bet.
thanks again.
jeff

[Bob Strawn]

Black rust is a better rust crystal. Red/brown rust opens pores and helps spread more rust destroying the tool. A lot of old timers used to encourage the black/grey rust. The crystal does not expand, it tends to self heal, and it gives a fair advantage over shiny for protection from further rust.

Pitted however is bad. Pits increase surface area, and often are more porous than the buffed shiny surfaces.

Light rust boiled in water can become black rust, acids, such as vinegar or citric acid can start black rust.

Be careful about boiling, boiling can soak water into pourous structures making a freeze risk, Some metals can lose temper or deform due to the heat.

Bob