Rust removal with citric and ascorbic acid

[Sonic EscapeParticipant @sonicescape38234]

I’m always looking for new chemicals for my restoration projects so I rubbed a vitamin C pill to a rusted metal part. The result was promising so I bought a few kg of citric and ascorbic acid in powder form. They are very cheap. The citric acid is ~2 euro/kg in small quantities from food suppliers. Now I understand why the put so many additives in food.

I diluted about 0.3kg of mixed acids in 2.5l of water and give it a try with these two parts. Before I clean them with sodium hydroxide.

One interesting thing is that the cone part reacted strongly with the solution. First I thought that maybe there is sodium hydroxide left on it. But the reaction continued for hours!

I let it overnight and this is the result:

The metal looks a like it was sandblasted with a fine powder. Maybe I let it for too long? I tried some Autosol paste on a small area it seems to restore the metal color. From what I understood these acids are transforming the rust in a water soluble substance. It is working also if you use the citric acid undiluted as a cleaning powder. The bonus is that it is also a little abrasive. I think this option is worth considering if you want to clean big parts that would require large quantities of Evaporust.

 

 

[JAParticipant @ja]

I tried “strong” citric acid on slightly rusty cast iron a couple of weeks ago without success. I am about to try baking soda (easier to obtain than washing soda) and an old fashioned battery charger.

JA

[BazyleParticipant @bazyle]

First time I have heard of ascorbic acid as an option. Used vinegar from pickled onions also works but the most important thing is preparation by oil removal and initial mechanical removal with a power wire brush.
I only soak things for more than an hour when I see the vinegar is almost exhausted and if using something more powerful (phosphoric acid is my preference) I paint it on for economy and wash off within 15-30 minutes.

[Bazyle]

On Bazyle Said:

First time I have heard of ascorbic acid as an option. Used vinegar from pickled onions also works but the most important thing is preparation by oil removal and initial mechanical removal with a power wire brush.
I only soak things for more than an hour when I see the vinegar is almost exhausted and if using something more powerful (phosphoric acid is my preference) I paint it on for economy and wash off within 15-30 minutes.

But phosphoric acid actually remove the rust? Because I saw that it is the main ingredient of some rust converter products. I thought that it just transform the metal oxide into a more stable substance.

[VicParticipant @vic]

All acids etch the surface to some degree so I use Electrolysis. Just an old battery charger and washing soda. It often doesn’t take long either.

[BazyleParticipant @bazyle]

Yes the phosphoric produces phosphates some of which you can then wash off and some remains as a protective grey colouring. Sometimes if I don’t get the part dry quick enough or it is humid it gets a light ‘tea stain’ of flash rust which can be removed if aiming for shiny with Scotchbrite or a vinegary cloth. I try to get the Waxoyle protection on as quick as possible.
Because of the above I only do this kind of work on sunny summer days. These chemicals are too weak to affect skin (eye precautions though) and I stay away from hydrochloric.

[Vic]

On Vic Said:

All acids etch the surface to some degree so I use Electrolysis. Just an old battery charger and washing soda. It often doesn’t take long either.

Vic’s electrolysis suggestion is also far more effective than Citric Acid or Ascorbic Acid could ever be.

Citric, Acetic, and Ascorbic Acids are all Organic acids, which means they are weak.  So weak that food products are full of them, and they can be safely eaten.   Ascorbic Acid is Vitamin C.  All safe as a rust removal agent, but slow, if they react at all.

Inorganic Acids react far faster, but require protective clothing and careful handling.   Concentrated Nitric Acid is best performer, but not used much for ordinary work because Sulphuric Acid is considerably cheaper and works nearly as well.  Unfortunately Nitric Acid and Sulphuric Acid are controlled in most countries because they’re used to make explosives, so buying them isn’t straightforward – buyers are potential terrorists.   Sonic may find it’s still possible to buy concentrated Sulphuric Acid in Rumania without fuss – it’s a common industrial chemical with many applications, and having to control sales means lots of costly admin.    Hydrochloric Acid is cheap and generally available but it’s not particularly suitable for rust removal or pickling.

Phosphoric Acid doesn’t remove rust, rather it converts rust into to solid stable Iron Phosphate, filling gaps and making it hard for rust to form underneath.   Works quite well, except Iron Phosphate isn’t strong – think paint and putty!

Dave

 

 

[SillyOldDuffer]

On SillyOldDuffer Said:

 

Unfortunately Nitric Acid and Sulphuric Acid are controlled in most countries because they’re used to make explosives, so buying them isn’t straightforward – buyers are potential terrorists.   Sonic may find it’s still possible to buy concentrated Sulphuric Acid in Rumania without fuss

 

 

 

Yes, here 5 liters cost around 30 euro, 98% concentration. What else do I need for the bomb? 🙂

 

[JAParticipant @ja]

Comments on Iron Phosphate

1. There is more than one Iron Phosphate. That should only interest chemists
2. Iron Phosphates are porous and can hold oil. Steel tubes about to be drawn down to a smaller diameter were frequently phosphated (treated with Phosphoric Acid) and then dipped in oil. The retained oil then lubricated the drawing process
3. Phosphating has been used as a surface preparation prior to painting. It gave a corrosion proof, relative, finish. I don’t know if it is still used but it was associated with stoving paints.

JA, who should be in the workshop

[MikeKParticipant @mikek40713]

This has me thinking…

I ferment vegetables regularly (trying to help my arthritis via the gut biome).  I wonder if the brine that I always pour down the drain has use for rust removal or pickling soldered things?

Mike

 

[Sonic Escape]

On Sonic Escape Said:

On SillyOldDuffer Said:

 

Unfortunately Nitric Acid and Sulphuric Acid are controlled in most countries because they’re used to make explosives, so buying them isn’t straightforward – buyers are potential terrorists.   Sonic may find it’s still possible to buy concentrated Sulphuric Acid in Rumania without fuss

 

 

 

Yes, here 5 liters cost around 30 euro, 98% concentration. What else do I need for the bomb? 🙂

 

Concentrated Nitric Acid. Any good organic chemistry text book will give details.

HOWEVER – DON’T DO IT

It the UK, concentrated H2SO4 + concentrated HNO3 = Bomb squad + police (if you are lucky).

When I did A level we did lab work making and using nitrating solutions. I doubt if that is done these days. It is a process that killed quite a few chemists two hundred years ago.

I would not be surprise if this is moderated.

JA

[Sonic EscapeParticipant @sonicescape38234]

What concentration of sulfuric acid is enough to dissolve rust? The battery electrolyte solution is enough?

[pgk pgkParticipant @pgkpgk17461]

‘one shot’ drain cleaner is 91% sulphuric acid. I last bought some 6 or 7 years ago without any hassle. But take handling it seriously.

[pgk pgkParticipant @pgkpgk17461]

And no-one has mentioned molasses. I scrounged a bucket from neighbour farmer last year and it failed to remove rust from mild steel immersed for 3 weeks.

[bernard towersParticipant @bernardtowers37738]

Phosphoric acid is the main ingredient in aluminium cleaners used by truck cleaners

[noel shelleyParticipant @noelshelley55608]

I thought Lactic acid was an ingredient of ali truck body cleaner ? Noel.

[bernard towersParticipant @bernardtowers37738]

Not the ones my lot used.

[Sonic Escape]

On Sonic Escape Said:

What concentration of sulfuric acid is enough to dissolve rust? The battery electrolyte solution is enough?

Any concentration will do something, it is just a matter of time. If you can get sulphuric then you can get Phosphoric. Sulphuric will be no better than the citric you have or the vinegar you can get and splashes and fumes may be more corrosive. Phosphoric has the added advantages given above.

[Sonic Escape]

On Sonic Escape Said:

What concentration of sulfuric acid is enough to dissolve rust? The battery electrolyte solution is enough?

Battery acid concentration is a reasonable starting point.  One of my books has numbers, I’ll see if I can find it.  Too strong isn’t as nearly as effective as dilute acid, but too dilute will slow it down.  Don’t expect instant results,  leave it to soak.

Not sure about Drain Cleaner acid as a rust remover.  Mostly strong Sulphuric Acid, plus unidentified additives.  I’ve no idea what the additives do.

Even in dilute form, Sulphuric Acid is much more potent than Citric or Vinegar,

Dave

PS I know it’s been spelt Sulfuric for about 50 years.  Can’t help it – I’m old-fashioned and British!

 

[CHAS LIPSCOMBEParticipant @chaslipscombe64795]

pgk-pgk. Something strange here! Molasses is very widely used in the Australian old vehicle restoration process and it results in clean, bright steel (until it rapidly re-rusts). A week is often sufficient. Molasses is also popular amongst the American firearm lobby for rust removal and they probably thought of the idea of using it in the first place. Sure to be a reference on the web somewhere.

[Kiwi BlokeParticipant @kiwibloke62605]

Yep, molasses works for me, too – but it is very slow. It’s a good idea to remove as much rust as possible, by other means, before soaking in molasses for a fortnight. Dilution doesn’t seem to be critical. There are some opinions that it works better once it has started to ferment (and smell disgusting), but this is a myth.

Molasses (or a component of it) is a chelating agent: oxalic acid is too, and also works as a rust-remover, with little risk of damage to the non-rusted material.

However, I must get around to trying electrolysis – it seems to be the answer. In a book from the 1920s, it was suggested that if a rusty steel object were electrically connected to zinc, and both were in an electrolyte, the electrochemical cell formed would ensure electrolytic rust removal, without external power. Simpler, but probably much slower.

[Kiwi Bloke]

On Kiwi Bloke Said:

 

However, I must get around to trying electrolysis – it seems to be the answer. In a book from the 1920s, it was suggested that if a rusty steel object were electrically connected to zinc, and both were in an electrolyte, the electrochemical cell formed would ensure electrolytic rust removal, without external power. Simpler, but probably much slower.

I have not heard of directly removing rust with Zinc as the cathode and no external electricity but Zinc cathodes are certainly used in that way to prevent corrosion. The Zinc is sacrificial and is corroded away.

JA

[CHAS LIPSCOMBE]

On CHAS LIPSCOMBE Said:

pgk-pgk. Something strange here! Molasses is very widely used in the Australian old vehicle restoration process and it results in clean, bright steel (until it rapidly re-rusts). A week is often sufficient. Molasses is also popular amongst the American firearm lobby for rust removal and they probably thought of the idea of using it in the first place. Sure to be a reference on the web somewhere.

Problem is ‘Molasses’ isn’t a single consistent product.  Unfortunately the same name is used to cover a range of by-products from the  sugar industry, and what’s in it varies by region.  What’s sold as ‘Molasses’ in the USA is likely to have very different properties from the ‘Molasses’ available in Australia, and both could be different again from what’s available in Europe.

A further twist is that ‘Molasses’ sold for human consumption is considerably refined compared with Cattle Food Molasses.   Never seen a good rust report about the ‘Molasses’ sold by UK food shops, whereas the US variety found in farms as a cattle supplement seems effective.   I guess whatever is removed from human-grade Molasses is what works on rust.

For rust-removal, user has to get the right sort of Molasses, and it’s not clear what that is!   Suck it and see.

As inconsistency is always a problem with folk-remedies, I prefer to buy commercial products. Though a bit spendy, they work, and aren’t hazardous!   Might be a surprise, but chemists always do a better job than untrained traditionalists.

Dave

 

[JA]

On JA Said:

On Kiwi Bloke Said:

 

However, I must get around to trying electrolysis – it seems to be the answer. In a book from the 1920s, it was suggested that if a rusty steel object were electrically connected to zinc, and both were in an electrolyte, the electrochemical cell formed would ensure electrolytic rust removal, without external power. Simpler, but probably much slower.

I have not heard of directly removing rust with Zinc as the cathode and no external electricity but Zinc cathodes are certainly used in that way to prevent corrosion. The Zinc is sacrificial and is corroded away.

JA

Could be made to work, but creating electricity with a home-made Zinc Battery is gob-smacking expensive compared with mains power.

More sensible to use Vic’s method.  Washing soda provides the electrolyte and the electricity comes out of a transformer.   Lots of it!

Dave

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