# Descaling Hot Rolled Mild Steel. Success!!



## Richard T (25 Apr 2013)

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.


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## siggy_7 (25 Apr 2013)

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

Great tip, thanks for sharing.


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## Racers (25 Apr 2013)

Hi, Richard

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

Pete


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## spinks (25 Apr 2013)

can you not just flap wheel it??


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## Cheshirechappie (25 Apr 2013)

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?


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## dickm (25 Apr 2013)

My chemistry knowledge is 50+years out of date, but presumably in effect the sodium chloride in the presence of acetic acid gives hydrogen ions and chloride ions, i.e. the hydrochloric acid Richard T.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?


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## AndyT (25 Apr 2013)

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.


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## Richard T (25 Apr 2013)

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.


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## Richard T (26 Apr 2013)

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.


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## marcros (26 Apr 2013)

Richard T":qlu3gn2t said:


> 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.


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## AndyT (26 Apr 2013)

marcros":1eka4bmv said:


> Richard T":1eka4bmv said:
> 
> 
> > Thanks CC - I think it might be magic.
> ...



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!


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## Richard T (26 Apr 2013)

: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 ...


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## mind_the_goat (26 Apr 2013)

" 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.


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## dickm (26 Apr 2013)

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!


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## AES (28 Apr 2013)

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

Krgds
AES


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## Richard T (2 May 2013)

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


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## mickthetree (2 May 2013)

Excellent, thank you for sharing your experiments Richard.


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## AES (2 May 2013)

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


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## Richard T (2 May 2013)

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.


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## AES (2 May 2013)

Richard,

OK fine - whatever works ;-)

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

AES


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## Cheshirechappie (2 May 2013)

Now I know why the industrial process of descaling metals by immersing them in various acids for various times is known as 'pickling'. I wonder if they use spice additives in those? Perhaps they all have their secret blend, like sausage makers do!


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## sdjp (3 May 2013)

Hi chaps, bit of a lurker to date, but this is relevant to my interests, so to speak! (I've done degrees in Chemistry and Material Science)

Forge scale is iron oxides - with an emphasis on the plural. Whilst there might be subtle differences due to the exact forging environment, it's typically a layer of haematite, magnetite and wustite, with the hematite as the layer between the iron and scale. The rough thickness of each are in a 1:3:100 ratio, with the haematite as the thinest. 

In the context of hot rolled steel, the rolling action of the forge ensures that the scale is cracked.

Based on the photo's of the wiping off of the scale, I suspect that the picking solution is preferentially attacking the interfacial layers, and thus the primary mechanism is not attack on the wustite. Either that, or it is the magnetite layer that is attacked, and the remaining haematite layer is so thin to be invisible - but I suspect not in this case. The preferential attack of the interfacial layers is dependant on the cracks in the scale to allow the solution to penetrate, at least that will have a strong effect on the time required.

(By the way, Richard, are you sure the two photos of the steel in the trunk haven't been switched? The second one (of the underside) has a remaining scale pattern that appears to be an exact match of the first one (of the upper side), a situation that seems to have a vanishingly small probability ... )

Chloride ions act as a catalyst for oxidation of iron - to rust or Fougerite (green rust) without oxygen present. Green rust formation is slow, so that strongly suggests that if oxidation of iron is relevant, then it's an oxygen dependant process. This idea would mesh with the observation that the underside cleaned slower than the upper side - and is the only air dependant process I can identify that might be occurring.

This effect of chloride ions is not going to deplete them, therefore we can reasonably assume that the decline in effectiveness is due to the vinegar becoming neutralised (or, possibly, some mechanism sequestering the chlorine somewhere - but chloride salts are usually highly soluble, in particular the iron ones are; so I discount that). Therefore the action of the vinegar is crucial - but even with the chloride catalyst, an acid environment will attack iron faster than water will, so that doesn't rule out this from being a primary effect.

Rob points out about gas production reducing contact with the solution. It is possible to get a small amount of hydrogen produced, but I can't see it being enough to have a major effect - but then, if it's forming in the micro cracks, then might just be enough. The concentrations involved seem too low for significant gas production to me. However, there's a way to tell, and cover that in a moment.

Let me distill that down to a summary, then I'll make some concrete suggestions:

Suggested mechanism: Vinegar attacks the oxide layers, probably preferentially the layers next to the iron, the chloride ions oxidise the iron underneath the scale - this is what loosens the scale. This process would require oxygen from the air.

For the vinegar, look for 'non-brewed condiment' - it's purer and cheaper (and less tasty!) than actual vinegar.


To establish the exact mechanism (which is, frankly, not really needed, but the scientist in me has to document this stuff!):

Place two pieces of scaled steel in a pickling bath, one raised up on blocks to just under the surface, and the other directly beneath it, at the bottom of the tank. This then gives 4 surfaces exposed to pickling, which we can label from the top as A, B, C and D. Then pickle for however long, and check the under side of the top piece (side B) against the top side of the lower piece (side C). If there is a gas dependant mechanism, then if it relates to gas escape, then side C will be better pickled (and clean easier) than side B; whereas if it related to gas reaching it from the air, then side B will be clean easier than side C. Note that the placement of the top piece above the bottom amplifies the effect of any gas production, so aught to give a clear result in that case. If both are the same, then that implies that there is both a gas production, and an air requirement - or neither. Comparison with side A and D can separate these - if A cleans easier than D, then there _is_ a gas requirement, if they are the same, and the same as B and C, then there is neither.

In the event that there is neither effect, then that would suggest that the reason that the underside was more difficult to clean was something to do with the steel itself, and not the pickling.

If there is a gas production effect, that implies that the optimal geometry of the work piece to be picked is to have the main surfaces vertical (rather than the more natural horizontal). Also, if there is a gas production effect, that implies that it should _not_ be used in a sealed container.

Equally, if there is a gas requirement (which is almost certainly oxygen), that also suggests a sealed container would be less efficent.


As I said, I'm not really expecting anyone to actually do that experiment, unless you're _really_ keen, but it's a habit to document this stuff these days!

I can go further into the structure of forge scale if anyone wants - I spend quite a while doing computational modelling of wustite a while back (although that was for the magnetic properties, rather than chemical) ... but I've probably already gone past the 'useful' level of detail. Still, I hope that's some useful stuff in there for people.

Stuart


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## Richard T (3 May 2013)

Blimey Stuart, that's what I call a first post. Welcome to the forum.  

"_a layer of haematite, magnetite and wustite, with the hematite as the layer between the iron and scale. The rough thickness of each are in a 1:3:100 ratio, with the haematite as the thinest. 

In the context of hot rolled steel, the rolling action of the forge ensures that the scale is cracked.

Based on the photo's of the wiping off of the scale, I suspect that the picking solution is preferentially attacking the interfacial layers, and thus the primary mechanism is not attack on the wustite. _"

This explains a lot - the way that it comes off is as though it has been released from its adherence to the surface of the steel. The structure of the flakes is still hard and crispy they just are no longer attached.

Though it is a while ago, I think that the photos are in the right order - being stills it's hard to see the extra vigour needed for the underside. I'll try to take some more photos next time. 

I don't think that my needs are anything like as exacting as those of the industries that need very clean surfaces, I just need to be able to carry on with further work without blunting tools and have nice, flat surfaces to mark and take accurate measurements from. 

However it works, it certainly beats angle grinding followed by draw filing flat again.


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## AndyT (3 May 2013)

Richard T":2gsaumq1 said:


> Blimey Stuart, that's what I call a first post. Welcome to the forum.



Seconded! Information is worth so much more when it is shared.


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## mind_the_goat (7 May 2013)

So actually pumping air into the bath might help? Counter intuitive to remove oxide layers but you can't argue with the chemistry.


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## sdjp (7 May 2013)

mind_the_goat":2s1r5isk said:


> So actually pumping air into the bath might help? Counter intuitive to remove oxide layers but you can't argue with the chemistry.



*Might* help. It depends on the relative reaction rates of the oxide dissolution, versus the formation of hydrated iron oxide (rust) that will force the delamination of the oxides. 

Since my earlier musings, I did a bit of digging. According to Sidhu et al, Clays and Clay Minerals, Vol.29, No. 4, 269-276 (1981); the presence of the Cl- ion accelerates the dissolution of various iron oxides, and, in particular, magnetite is faster to dissolute than haematite - at the test concentration they used by a factor of around 60! (The test conditions were 0.5 mol per litre HCl; which is _roughly_ similar to commercial 5% vinegar + salt).

That implies that there would be a thin layer of haematite left behind - but this would be practically invisible (and probably beneficial, as it would act as a protective layer, although probably removed by a simple scrub.)

Sidhu et al also noted that, "The effect of temperature on the initial dissolution rate can be described by the Arrhenius equation". Which, given that they give the activation energies (about 80 kJ mol-1), lets one do some relative rate calculations. Importantly: each 10 degree Centigrade change in temperature will nearly _triple_ the rate of reaction (and winter/summer can easily be a 20 degree shift) - that's something to keep an eye on.

The problem with all those observations is that is that they were done on pure minerals, and not a mixture of iron and iron oxides (as one might infer from the name of the journal!) They also do not explain a gas dependant mechanism (either in production or consumption) which still leaves an observation unexplained - although it might not be gas dependant.

I've a project in the offing that'll need me to get some sheet metal (type immaterial) - so I might see if I can pick up some hot rolled steel (which would do the job fine) - assuming I can sneak it into the shed past SWMBO! That'll let me do the experiments, and get some clear answers on this one. If so, I'll update, but I'm afraid it's just theories at the moment.


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## rafezetter (5 Jul 2013)

I've got some experience of plastic extrusions, so an alternative to 110 soil pipe you might also want to consider square pipe for air ducting - like this:

http://www.screwfix.com/p/manrose-rectangular-flat-channel-1000mm/14118


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## Richard T (6 Jul 2013)

Thanks rafezetter, that would be very useful if I needed to descale wider bits.

As it is, what I have done up till now will keep me busy for a looooong time.


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