Thread: More metal bluing

Originally Posted by Stustoys

Sorry Bob I must be thicker than usual tonight

"Magnetite is moderately more protective against rust than plain iron and especially but it wear off and will still eventually convert to rust."

"Magnetite is moderately more protective against rust than "regular orange rust" and especially but it wear off and will still eventually convert to rust."

Still not getting it

Paying attention because I want to try it. You know who's getting the blame if it doesn't work


Stuart

you might be thick so i must be jungle thick n dense

when you figure out the vest way Stuart let me know...

OK There are a number of possibilities to start with to guard against a (red/orange) rusted object form more rusting.

1) remove old rust i.e. get back to Plain-raw iron

2) Leave the iron rusty with orange/red oxide

3) convert the rrusty with orange/red oxide to black oxide

4) coatings and other conversions

etc

Of the first three, 3) is much better than the other two and the least effective is to leave the rust there - a rusted object has a greater surface area exposed and being more porous stays wet for longer hence rusts quicker than removing the rust and getting back to a flat dry surface.

So Bob, if you just boiled a red/orange rusted item in distilled water, would that be enough to convert to black rust? (as per post 28).
From what I've read so far it seems to me that the the fuming is to establish a uniform coat of rust on the steel, so if it is there already?
Michael

Originally Posted by Michael G

So Bob, if you just boiled a red/orange rusted item in distilled water, would that be enough to convert to black rust? (as per post 28).
From what I've read so far it seems to me that the the fuming is to establish a uniform coat of rust on the steel, so if it is there already?
Michael

You are correct about the fuming being to establish a uniform coat of rust on the steel - but as important is that it is a THIN and smooth coating bonded to the base metal.
Only a thin outer layer of red rust is converted by the boiling, so if the red rust layer is too thick it won't convert and it leaves a irregular layer of black oxide over the remaining unconverted rust which is porous and relatively un-protective.

Remember once the water converts the outer layers of red to black oxide you end up rubbing most (99%) of the black oxide off so only the layer bonded closest to the metal remains. If that layer is not bonded to the metal it will bonded to underlaying red rust which is not a strong bond and will come off - if not now then later.

Best (i.e. longest lasting) results are obtained with the smoothest surfaces since this can produce the thinnest layers.

It's tempting to let the fumes rust the metal for longer to form thicker coatings of red rust.
When I have done this the result was a thicker patchy layer of rust and resulting soot most of which come off but leaves a more patchy finish - more, shorter exposures, will be better than a fewer longer fuming exposures.

When the weather is hot a 4-6 hour fume exposure is plenty. when its colder I use 8-10 hours. The worst finish was when I accidentally left something for 24 hours and the resulting coating was rough and irregular. If this happens early the resulting top layers will get increasingly rougher. If it happens in the second last or last pass it will still be protective but just may not look as as good.
If this happens in the first 3-4 passes I recommend going back to bare metal and starting again

In general the coatings on scotchbrite (matt) finished steel have not been as good as buffed to a mirror finish steel.

Here's some pics after the final pass (9 passes in all) through the fuming tank and a bath in warm engine oil..
The final pass included an accidental 24 hour fume in the fuming tank.
This has resulted in a rougher finish but I'm not that concerned about these pieces having a real fancy finish as long as they are reasonably well protected.

Photographic black things at night is difficult at the best of times but it is what it is.

This is with led fluoro lighting - it's hard to see much on these surfaces as they are too dark.
The glint you can see is a tiny bit of remaining oil


Now with flash only
You can see a bit clearer that they have a matt finish


Here are the pieces all assembled into the bearing housing.
For comparison the surrounding steel has two coats of matt epoxy-enamel black


In this one I have pushed the exposure in Photoshop so you can more easily .see the rougher texture on the edges - its nowhere near as bad as it looks


For comparison purposes here is the bluing of the small rotary table mounting plate and tool post I made a year or so back.
This had 10 passes though the tank.


.

Originally Posted by BobL

You are correct about the fuming being to establish a uniform coat of rust on the steel - but as important is that it is a THIN and smooth coating bonded to the base metal.
Only a thin outer layer of red rust is converted by the boiling, so if the red rust layer is too thick it won't convert and it leaves a irregular layer of black oxide over the remaining unconverted rust which is porous and relatively un-protective.

I read all this information with great interest, and you seem to know a lot about it. You are undoubtedly right about more thin layers instead of one thick, but I can't understand what happens when you repeat the fuming/boiling several times. Wouldn't that form a thick layer also? (be it more uniform) that would be polished off? Reading the previous posts I assume not, but can you explain what happens?

Peter

Originally Posted by WoodBee2

I read all this information with great interest, and you seem to know a lot about it.

Don't be fooled by appearances. I have undertaken the process ~7 times in ~3 years and during that time blued/blackened maybe ~30 pieces.
What I do know something about is inorganic chemistry as it used to be part of my day job but I'm not a corrosion expert.
Corrosion science is very specialised and as complicated as or maybe more complicated than metallurgy.
However, I can read and mostly understand scientific papers on corrosion.

You are undoubtedly right about more thin layers instead of one thick, but I can't understand what happens when you repeat the fuming/boiling several times. Wouldn't that form a thick layer also? (be it more uniform) that would be polished off? Reading the previous posts I assume not, but can you explain what happens?

The soot formation only forms where there is red rust.

However, the red rust layer formed is not uniform either, instead it consists of a very thin layer over most of the metal and then patches of mounds, lumps, bumps and other structures most of which are fragile and porous. Provided all these red rust structures are thin enough they will be converted to black oxide but the fragile rust also becomes the soot which must be rubbed off to allow the fumes to get access to the untreated metal underneath.

The soot comes off easily but the very thin layer of black oxide that is hard bonded to the metal and to itself is much harder to remove. In the first couple of passes, after removal of the soot the amount of black oxide that remains hard bonded to the metal is so thin and so sparse that the metal may only look like a dark grey.

*********************************************************************

If you want to know more read on
OK lets take a step back and try to understand rusting.

The simple view about rust formation is that air (oxygen) attacks the surface directly and eats holes in metal.
Iron that has been kept away from water (including any condensation from humid air) can stay rust free for years and when it does rust it does not rust anywhere near fast forming a smooth layer - this is rare because there is always water in the atmosphere.

What really speeds up corrosion is water, even the small amount in dry air is enough. When water droplets condense on cold iron the water already has dissolved oxygen in it and this is much more reactive than oxygen gas in the air. The dissolved O attacks the iron more on some places on the iron surface than others and forms an iron rich solution in the water drop.
When the iron oxidises into the water it releases electrons into the metal which move to other local parts of the metal forming a slightly reducing environment in that area which pulls some of the iron out of the water droplet. This is why you get pitting, on some parts of the metal surface and in others you get mounds.

This is a picture of bare steel left outside out of the rain - note the patchiness of the rust.



Here is a schematic rom http://chemwiki.ucdavis.edu/Analytic...dies/Corrosion of what happens

Because of the water, and that oxygen from the air can continually dissolve into the water, the small amount of reduced iron also deposits large variable amounts of iron oxides and water (embedded in the solid structure) which is what rust is and that is why it is rarely a uniform colour. It's not really a well defined material and is not considered a material at all in a chemical sense, instead it's considered a solid mixture. The deposit is loosely bound to the metal and unlike some other metal oxide layers does not form a barrier against further corrosion at this is why rust is so friable.

Now what you don't want with metal bluing is any visible pitting or for the layer of rust to build up into large lumps which is happens when the metal is exposed to the acids for too long.
You want the pits and the deposition spots to be so tiny as to be to be invisible and so all the red iron oxides to be converted to the black iron oxides. If the lumps are too big the red oxides will not be converted into black oxides and the surface will not be as strong as a solid black oxide only layer. What happens after boiling and when the soot is removed is you can reduce the high points but the pits generally retain some soot which is not desirable.

I have never hear of this but I reckon that the brown hues sometimes seen during metal bluing is due to incomplete reduction of the red to the black rust. A very small amount of red rust inside the black rust can probably be tolerated.

In my last pass on the parts I blued in this thread I did leave the parts too long in the fuming chamber and there were larger patches of rust formed.
To show this effect I pushed the exposure in Photoshop in the picture below to show the speckled outer edges of these discs.
The highllghts you see are not shiny metal but the polished tops of the converted or black oxide mounds that are reflecting the flash compared to the surrounding darker areas.

This is why if you want a smooth finish, starting with mirror finish is essential because it's all downhill from then on.

Hi Bob,
I have been on holiday for a while, but have just read your thorough but understandable (to me) explanation. Thank you very much for the effort to write this all down!

Peter

Originally Posted by BobL

For Concentrated HCl I used Diggers Spiirts of Salts from Bunnings.
Nitric acid is not so easy to get but Sulphuric acid can be substituted for nitric..

Diluter forms will also work but take longer although some reduction in time can be achieved by then using a smaller chamber/

Hi Bob,

Bunnings dont seem to have Diggers anymore. They have Chemtech brick and tile cleaner, 95g/L Sulfamic acid and 53g/L Hydrochloric acid. Is that acceptable?

Thank you.

Stuart

p.s. the funny thing is I have a blacking kit in the shed I havent even tried yet

Originally Posted by Stustoys

Hi Bob,

Bunnings dont seem to have Diggers anymore. They have Chemtech brick and tile cleaner, 95g/L Sulfamic acid and 53g/L Hydrochloric acid. Is that acceptable?

Thank you.

Stuart

p.s. the funny thing is I have a blacking kit in the shed I havent even tried yet

Sulfamic is probably not oxidising enough.

What about this mob.
http://www.anpros.com.au/contact-us/

Originally Posted by BobL

Sulfamic is probably not oxidising enough.

What about this mob.
http://www.anpros.com.au/contact-us/

Considering they're about a 10 minute walk from my place they will do just nicely. Thank you.
Don't they have lots of nice stuff! Though it is tempting to buy everything 2.5L at a time (which granted would be more then a little silly)

Stuart

Originally Posted by Stustoys

Considering they're about a 10 minute walk from my place they will do just nicely. Thank you.
Don't they have lots of nice stuff! Though it is tempting to buy everything 2.5L at a time (which granted would be more then a little silly)

Stuart

It's not worth keeping large amounts of nasty stuff in a domestic environment. Before we had an accountable acid disposal system at Uni I acquired a 20L carboy of used high purity aqua-regia. When I say used it was used, it had been used for cleaning labware that was already way cleaner than would be the case in most labs. It was going to be disposed of so I grabbed it. I used some of it in a plastic bucket to dissolve Zinc plating off some steel pipe - veritable little volcano that turned out to be leaving a 5ft clean ring of brickwork on the back pergola paving. When the black plastic 20L carboy started to blister I got worried and I didn't want to keep it in the shed or under the house for obvious reasons so it lived on the back veranda for a while - bit of risk when kids came over to play. I did some steel etching with it - nothing fancy and not that good either. With summer coming I transferred half a litre to an FEP teflon container and used the rest to clean the remainder of the pergola and driveway brick paving. It worked great - killed all the weeds, moss etc and nothing grew in between the bricks for a couple of years.