Four related questions re etching of steel / Chemistry / Forums

Forums

4hv.org :: Forums :: Chemistry

« Previous topic | Next topic »

Four related questions re etching of steel

Move Thread

LAN_403

Thingmaker3

Fri Jun 30 2006, 03:55AM

Registered Member #124 Joined: Fri Feb 10 2006, 01:30PM
Location: Portland Oregon
Posts: 35

I've been reading a little about etching steel with ferric chloride, vinegar, etc. and I have a few questions...

1) When etching with vinegar, does the acetate "recycle" the way chlorides do when etching with FeCl3?

2) Is there a convenient way to promote Fe3O4 in preference to Fe2O3 or FeO when etching? (Is there any easy way to limit the O2?)

3) Is it possible to "neutralize" a Lewis acid in the manner of a Arrhenius acid? Rephrased: Is the formation of a coordinate covalent bond called "neutraliztion?" If not, what is the proper term?

4) When finished etching is there a way to insure that I've got rid of all the Cl from the surface of the steel? Some kind of aluminum compound maybee?

Some additional notes which may or may not make a difference: FeCl3 is about 1/4 x to 1/5 x the concentration used for circuit boards. Vinegar is about 1.5 x the concentration used for cooking.

Electroholic

Fri Jun 30 2006, 04:31AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

as i remember*
Fe3o4 is just a mixture of fe2o3 and feo.
Fe only have 3+ and 2+ ions.

Thingmaker3

Fri Jun 30 2006, 10:05PM

Registered Member #124 Joined: Fri Feb 10 2006, 01:30PM
Location: Portland Oregon
Posts: 35

Fe3O4 is "black oxide" which sticks doggedly to the steel underneath and provides the dark part of dark/light contrast. Properly done, the Fe3O4 will lie in a depression just beneath the unetched masked surface. Fe3O4 is the whole point of etching steel.

FeO converts quickly to Fe2O3. Fe2O3 is rust. FeO and Fe2O3 are undesirable in this context.

Fe3O4 forms preferentially in lower O2 environments. How do I safely get such an environment in my garage?

Bored Chemist

Sat Jul 01 2006, 11:18AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I don't know a lot about etching but I know some chemistry so I hope this helps.
FeO, Fe3O4 and Fe2O3 are all unstable in the presence of acids.
The reaction of the metal with the acid will generate hydrogen so, locally, the oxygen concentration will be low.
Fe3O4 is a compound in its own right- it isn't a mixture. For example it is strongly magnetic whereas FeO and Fe2O3 are not.

What are you trying to achieve by this etching?

Eric

Sat Jul 01 2006, 05:42PM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116

I think the etching and blackening processes would be different. If your etcher blackened, it would tend to halt the etching since Fe3O4 is a protector. There are several different processes for chemical blackening, just google blackening or 'black oxide'.

The traditional 'hot' method uses a boiling mixture of 700-950g/L NaOH, 20-30min. I would guess that in the process it would remove any Cl still on the surface from etching.

Thingmaker3

Mon Jul 03 2006, 04:00AM

Registered Member #124 Joined: Fri Feb 10 2006, 01:30PM
Location: Portland Oregon
Posts: 35

Specific application of interest is either enhancing contrast of pattern welded blades, or decoration & hallmarking of blades. My understanding (which could easily be flawed) is that several things are happening at once. Ferric Chloride & water make for a dilute HCl. The HCl converts iron from the steel into Ferrous Chloride and Ferric Chloride. (The chlorine gets "recycled.") Meanwhile, iron is also combining with oxygen at the reaction sites, making FeO, Fe2O3, and Fe3O4. In a pattern welded blade, the higher carbon zones are eroded more than the low carbon ones (don't know why). For decoration or hallmarking, a mask is used. The black oxide is not "fixed" at this point, and will rub off easily.

Boiling the blade in washing soda after etching is one way to 1)get rid of the etchant and 2)set the black oxides. (Heat "fixes" the black oxides so it won't rub off.) If baking soda is used instead, the blade will be more likely to rust - UNLESS the baking soda is in boiling solution after a quick dip in trisodium phosphate. Other folk use weak ammonia rinse to clear out the FeCl3 and then use a blow-dryer for heat.

Anyway, with pattern welded blades, a light sanding or buffing takes the oxide off the physically higher low-carbon zones to make them shiny againg, contrasting sharply with the oxide left in the physically lower high-carbon zones. With decorations or halmarks, the mask is removed to reveal the unaffected bright steel in contrast to the dark markings.

All this is a wee bit over my head, as chemistry is one of my weaker subjects. I just want to minimise risk of anerobic rusting after etching by minimising residual FeO and Fe2O3. I figured lowering the available O2 might do that.

ragnar

Mon Jul 03 2006, 04:22AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Is this related to / would you consider chemical blackening by sodium sulfite treatment?

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob