Descaling Hot Rolled Mild Steel. Success!!

[Richard T]

Having spent a couple of years wrestling with grinding and filing black steel plate, I finally had another go at Googling an easier solution.

I got the usual industrial answers involving a specific type of hydrochloric acid and the counter active alkaline spraying that appears to be essential to the coating/painting industry.

But on one forum from more than ten years ago, an odd, ignored post from a small time Americorn blacksmith said that a saturated solution of salt in vinegar was enough if the steel was left in it for 36 - 48 hours. I didn't believe it. This stuff is really difficult to shift; it coats angle grinder disks in minutes and just gets smeared around thereafter, blunts files, drills, etc. .... but I gave it a go.

I put a small offcut in said solution in a plastic food box and waited. After 24 hours, no change; hard as ever. After 48 hours the scale just fell off. :shock: I used a scourer under the cold tap but the scourer was hardly needed. The metal was unscathed. Bright and shiny.

Since, I have bought some square section plastic guttering with end pieces, cut a 4' 2" length and sealed it up with bathroom filler (seems to work so far) so I can put a whole 4' length in at a time. Works a treat. Now I have a second length in the same solution to see if it can be reused.
It's great - it can just sit in the garage doing its stuff while I can get on with other things. Things less noisy and less hard on the wrists.

 

[siggy_7]

Eeek - that'll make me think twice the next time I'm down the chippy!

Great tip, thanks for sharing.

 

[Racers]

Hi, Richard

That's filled away for use later, I have had the same problems getting through the stuff my self.

Pete

 

[spinks]

can you not just flap wheel it??

 

[Cheshirechappie]

Now that IS interesting! At long last, the chance to work stress-free clean steel without bothering with heat-treatment (and subsequent cleaning).

I wonder what the salt does? Any chemists care to comment?

 

[dickm]

My chemistry knowledge is 50+years out of date, but presumably in effect the sodium chloride in the presence of acetic acid gives hydrogen ions and chloride ions, i.e. the hydrochloric acid Richard T. mentions for industrial de-scaling, along with acetyl and sodium ions. But that's probably a gross oversimplification.............
What is the scale, anyway?

 

[AndyT]

This interweb thing is rather good for finding out stuff, isn't it!

One thought - instead of a long trough taking up floor space - if you bought a length of downpipe and sealed one end (might need some ingenuity - do drainpipes ever need stop ends?) you could stand the whole thing upright and drop your steel in like umbrellas into a large umbrella stand. It would take up less space and you'd not lose so much by evaporation.

 

[Richard T]

Thanks CC - I think it might be magic.

Dick, the scale is apparently iron oxide formed in a completely different way to that of rust. This is according to Peter Ross (blacksmith late of Historic Williamsburg and good buddy of Roy Underhill). Whenever iron is made red hot, the black scale forms immediately it cools from red. When forge welding Borax is used to either stop it forming or to disperse it from the weld.

Andy, good idea but the down pipe is just too narrow. Fortunately I can shut the garage door and it can pong away all it likes.

 

[Richard T]

I've just been out to see how the second piece of steel in the same solution got on after 48 hours.



This is the side that was facing upwards in the trough



Dead easy - and this was the side that was face down:



Considerably harder to shift. I am puzzled as to why this should be. Is it because it was not facing the air or not facing the light ...? It must be one or the other, both sides are equally covered in the solution as the sheets are 70mm wide and do not sit flat on the bottom of the trough so I'd say that there is about 1/2" of solution on either side.
Anyway, it's not that hard to get off the underside so I'm not complaining.

 

[marcros]

Thanks CC - I think it might be magic.

Dick, the scale is apparently iron oxide formed in a completely different way to that of rust. This is according to Peter Ross (blacksmith late of Historic Williamsburg and good buddy of Roy Underhill). Whenever iron is made red hot, the black scale forms immediately it cools from red. When forge welding Borax is used to either stop it forming or to disperse it from the weld.

Andy, good idea but the down pipe is just too narrow. Fortunately I can shut the garage door and it can pong away all it likes.

soil pipe? that definately has stop ends available.

 

[AndyT]

Thanks CC - I think it might be magic.

Dick, the scale is apparently iron oxide formed in a completely different way to that of rust. This is according to Peter Ross (blacksmith late of Historic Williamsburg and good buddy of Roy Underhill). Whenever iron is made red hot, the black scale forms immediately it cools from red. When forge welding Borax is used to either stop it forming or to disperse it from the weld.

Andy, good idea but the down pipe is just too narrow. Fortunately I can shut the garage door and it can pong away all it likes.

soil pipe? that definately has stop ends available.

Just what I was going to say!

I keep thinking of the 'comic' potential of you walking into the garage and accidentally stepping on one end of a four foot trough of dirty vinegar!

 

[Richard T]

:lol: There's every possibility.

I'm going for a third time in the same dip. It has to run out of umph sooner or later ...

 

[mind_the_goat]

" I am puzzled as to why this should be."
If there is any gas created in the reaction then it would escape from the top but be trapped on the bottom, leaving less fluid in contact with the metal. Any generated heat would aid fluid circulation on top (the warmer less dense fluid would migrate to the surface) but not so much on the bottom.

 

[dickm]

You can also get access plugs to close off the ends of smaller (40mm/68mm) plastic piping should you not need the volume of soil pipe.

Thanks for the elucidation of scale, Richard. From hazy memory of school chemistry, there are indeed several forms of iron oxide, even within valency states. But details are even hazier!

 

[AES]

Excellent Richard T, thanks. That's another one bookmarked for when needed.

Krgds
AES

 

[Richard T]

Update: It looks like twice in the same solution is the limit.

On the second dip (which is the one I photographed) the underside took some more vigorous scrubbing but the third time, there were patches that just wouldn't shift.

Ah well ... vinegar is cheap

 

[mickthetree]

Excellent, thank you for sharing your experiments Richard.

 

[AES]

Thanks for the update.

BTW it's ages since I was living in UK (where I remember that vinegar was usually the malt variety - brown - for fish n chips, etc) but here (Switzerland) we also get something called "putz essig" - that's literally "cleaning vinegar". It's completely clear like water, cheaper than "ordinary" vinegar, and when I've used it for de-rusting (after an electrolytic bath) it seems somehow "stronger" than normal vinegar.

Perhaps you can get it in UK too now?

Krgds
AES

 

[Richard T]

AES - I've been using pickling vinegar that is spiced. 8) (The steel tastes that much better.)

I only got that because it was the only type I could get by the gallon in a little village shop we stopped at. I noticed that it says it is 5% acid; I'm sure that stronger can be got it's just that this spiced stuff was convenient to try the other week.

 

[AES]

Richard,

OK fine - whatever works ;-)

We can't get spiced cleaning vinegar here as far as I know!

AES