Removing black scale

[paul]

Is there any way to remove the black scale that forms on BMS after heating other than emery/file/steel wool? (I'm thinking delicate surfaces or difficult to reach corners etc)

TIA

[tbsteam]

acid?

[steammadman]

ELBOW GREASE ! ! ! !

[SteveW]

Paul,

One trick I've heard of is to not let is get blackened in the first place. If you can exclude oxygen (air) during the heating process it'll stay clean.

I think there is some stuff that works for this but I don't recall what it's called maybe the original snowpake correction fluid.

[modeng2000]

Paul, I have heard that a layer of soap can be used to prevent the oxidation.

John

[Tel]

Soft Soap for preference, if you can still get it

[paul]

Sorry I should have made it clearer - silver soldering was involved. A layer of soap on the non soldered parts might help. Experiment time!

[ron]

Paul
Bead blast? also leaves a nice matt finish,
Ron

[paul]

I love experiments. Here we see (top to bottom)

1 stub of mild steel part coated with 'Dove' hand soap (!)
2 job at red heat with nice crusty appearance on soap end (yes I know it doesn't look red hot but that's cameras for ya)
3 job after cooling and rinsing in water

The soap had precisely the opposite effect to that required; the left, coated, half was almost uniformly black whereas the uncoated part was at least still shiny in places.

I guess it's the oils and other additives that do it. Maybe I should try some 'Simple' soap!





Ron, I was thinking more of very small parts with awkward little corners - maybe a bit too big for beads?

[ron]

Paul
Beads will get into the smallest corners, they are only a few thou in dia, less after they've been used a few times and smashed up! Unfortunately my blast cabinet ended up in the dump due to overuse, it was a very handy piece of kit.
Ron

[freddo]

Paul

Try Alan Stepney's anti-rust technique - washing soda and a battery charger - get the full recipe from his site. Even takes the surface coats off stainless if you leave it long enough, so watch it like a hawk and remove when appropriately cooked.

Freddo

[peterseager]

I asked the same question on another site and got much the same response. In the mean time I have been advised to use hydrochloric acid and it works. I had some 16% acid used for cleaning stone and brick work and diluted it with an equal amount of water.

I was lucky and was taught metalwork at school. One thing I remember being told was that the pickle we used was a mixture of sulphuric acid and hydrochloric acid. That's obviously why it worked with copper or steel. The pickle was in a stoneware bowl perched on a lab stool near the brazing hearth!

[jgb7573]

For removing the scale from black (hot rolled) mild steel I use a 10% solution of phosphoric acid. Take the metal out of the pickle, wash it with clean water and the steel starts to rust it's so clean! I would certainly try that for small parts.
By the way, the ordinary soaps found in chemists are not soft soap. If you ask the chemist for some, he will give you a jar of green jelly like stuff (assuming he's still got some). I got a small jar about 20 years ago and still use it when hardening silver steel. It works well.

[joegib]

May 4, 2008 8:13:38 GMT jgb7573 said:

By the way, the ordinary soaps found in chemists are not soft soap. If you ask the chemist for some, he will give you a jar of green jelly like stuff (assuming he's still got some). I got a small jar about 20 years ago and still use it when hardening silver steel. It works well.

Would you mind explaining exactly how you use soft soap please? I remember reading that this could be used to prevent oxide formation when heating tools way back in the 70s when I first got into model engineering. I duly obtained a very dusty jar from my local Boots and, as you say, the contents had a green jellylike appearance. Trouble was, this substance was about as yielding as a jellybaby — in its neat state it just didn't have the stickability to adhere to metal. I tried mixing a portion with water to make it smearable but no joy — the stuff seemed totally insoluble in water! In the end I just assumed it was so old it had degenerated and was no longer fit for this use.

Anyway, in those days GH Thomas was a regular attender on the SMEE stand at the ME Exhibitions (I think I picked up the tip from his writings). In a quiet moment that year, I buttonholed him about it. He gave a throwaway answer along the lines that you just smeared the soft soap on the tool prior to heating, him eyeing me as though he'd just encountered the local village idiot. I came away with the conviction that the soft soap of his universe was quite different from mine.

So, jgb7573, when you say your supply is jelly like, is it smearable like petroleum jelly, or is the consistency different? This query still sounds stupid but after 30 years I'd love to solve this mystery.

[jgb7573]

Hi Joe,

My pot contains a stiff jelly-like, or maybe waxy substance. If you push a bit of silver steel into it a little bit sticks to the steel, but it's not a lot. I frequently try and smear a bit more on with the finger. I then heat up the job slowly and the soap melts, bubbles up and then blackens. Once it's got to this stage, I heat up as normal. When you dip the bit of steel into water, most of the remains of the soap drops off and you're left with a nice clean bit of steel.

John

[joegib]

OK, thanks for that. I'll give it another try sometime.

[Noddy]

Do yourself a favour and give the HCl a miss, It'll have everything for miles around rusting for ever more!

The electrolytic cleaning (attach to negative (black) lead of batery charger and use piece of scrap steel as positive electrode in weak sol of washing or caustic soda) is good, but don't get solution too strong, it's unpleasant because of the caustic fizz when its working, and if too concentrated, the resistance gets too low and the fancy battery chargers will automatically switch off. Don't use it on highly stressed parts, as you may cause hydrogen embrittlement (bake at 200 to 400 celcius for 24 hours if you must use it on stressed parts...).

Phosphoric is good. perhaps use it stronger than 10%. you get it from farm supplies places as milk scale remover... it attacks scale far faster than the underlying metal.

Keith

[paul]

As ever lots of good advice and a few warnings! Thanks all for the tips, I hope others find them useful too.

[dickdastardly40]

Keith,

just bought some soda crystals (sodium carbonate & decahydrate) for 79p on the off chance this is the right stuff for electrolytic rust removal, what do you reckon? (on the stuff not the price)

Al

[peterseager]

I've not found rust to be a problem with HCL, but then I never use it in the workshop and it always goes back into a marked, closed container after use. It's the same way as I use citric acid solution on copper. The advantage is that stone/brick cleaner is available from a builders merchant.