PING: Chemists / metallurgists.

[T i m]

Hi All,

I've been experimenting with de-rusting of 'stuff', initially using 'electro', then chemical with Deox-C and now citric acid.

So, not sure where to start concentration-wise I mixed 1kg of citric acid power with about 20 l of hot water and then put my test parts in to 'soak'. A fairly rusty screwdriver was pretty clean 24 hours later so I added and removed various bits from then on with generally very good results (nice clean steel / iron).

However, I left a few bits in for a few days and today I've noticed that there is a distinct 'crust' on the uppermost surface of the objects, *possibly* the citric acid that has re-crystallised (if that's what it could do)? It can be scraped off with a suitable tool to varying degrees of ease (easy from chrome, less easy from more rough / pitted parts).

So, remembering my 'O' level 'science', is it possible I made a 'super-saturated solution' when I initially used hot water and now it has all cooled down that some of the citric acid has re-crystallised out?

Would it do so (mainly) on the horizontal surfaces of objects in the bath?

The second question, is there anything I could use to chemically remove this buildup but that wouldn't dissolve my fingers or the various metals please (and be ideally readily available and cheap)? If citric acid is an acid are we looking at an alkali of some sort? A quick Google suggests caustic soda (sodium hydroxide) but if so what concentration (roughly) would I be looking at please? Would 'washing soda' do?

The third question is of 'hydrogen hardening'. I think I understand this is the penetration of hydrogen into the material, (eventually) wreaking it to some degree (to varying levels depending on the material).

How likely is this to affect the strength or reliability of everyday items? (ie, not aircraft / F1 components). ;-)

Cheers, T i m

[Andrew Mawson]

I use citric acid extensively for rust removal and find it a very benign system. The concentration can be varied very widely without a problem, and it is quite safe to put your hands in. If the solution is too weak it just takes longer, and if you put too much in you get the crystals forming that you mention.

Like any chemical reaction it is temperature dependant so works faster warmer. I have successfully de-rusted several large pieces of engineering machinery with total success.

The huge advantage of citric acid it that the rust is converted into (I believe) iron citrate, which is water soluble. So if a part like a nut and bolt is frozen solid by rust, slowly but surely the rust is removed and the resulting iron citrate goes into solution freeing the part. Contrast this with phosphoric acid, where the iron phosphate is insoluble and remains on the part. So in the case of the nut and bolt, likely it will stay frozen even though the volume of iron phosphate is less than the volume of rust converted. (rust is 7 times the volume of the original steel)

AWEM

[ss]

On this subject I am trying to loosen the screw inside a tap in order to replace a washer. The screw is locked solid, would squeezing in some lemon juice free the rusted screw?

[Ferretygubbins]

...

It will do if it is a super saturated solution. However if the crystals are just around the water level of the solution it's probably just due to evaporation of the solvent.

As these crystals are most probably just citric acid crystals you'll be able to dissolve them with water (as you did when you made up the acid). Just rinse everything with fresh clean water and you'll be fine.

Cheers

Mark

[T i m]

Ah, ok ta.

About how long do you expect it to take (with your solution) please?

The next thing to go in is a HD trailer jockey wheel where the thread is frozen (rusted) into the nut.

I think it's a similar thing with the electro-de-rusting process where it's line_of_sight etc.

So, do you neutralise the citric acid before painting and if so how please?

Cheers, T i m

[T i m]

Are you sure it's steel and therefore 'rusty'? I would have thought anything intended to be that 'wet' would be brass or some such?

If it is all steel then given time (and the acids access around the thread) it might well help.

Cheers, T i m

[Andrew Mawson]

I usually only have to leave bits overnight. Solution strength can be a bit haphazard, I just toss some citric acid powder into whatever vessel I'm using and top up with buckets of hot water from the tap. I have a couple of heavy pvc cattle troughs that I use for bigger bits. Longest I had to leave something was three weeks, with several changes of solution. It was a

60 ton hydraulic press that had been through a severe fire. The platen is raised and lowered on four huge screws which are interlinked and driven by chain above the press. They run in massive bronze nuts each about as big as two fists. The nuts were badly seized onto the rusted screws - it had been left outside for 6 years after the fire, which was sufficient to melt the motors. I arranged for each nut/screw to dangle into a bucket of citric acid and eventually freed them off such that I now use the press with the original bronze nuts and screws!

AWEM

[T i m]

Funnily enough the surface of the tank was about the only place the crystals weren't. There were some on the horizontal surfaces of everything in the tank and also on the (vertical) sides of the (plastic) tank itself? I'd best describe is as similar to lime scale you would see on the inside of a kettle.

Hmm, I scrubbed with a nylon brush and rinsed with a garden hose + jet nozzle but this stuff is generally too hard to move easily. Even in a bucket of hot tap water it wasn't moving on it's own or even with said scrubbing. It did break away from some rubber easier though.

Hence why I was hoping for some reactant (ideally a liquid bath or tank) that I could use to remove any of this 'crust'.

I think next time I'll make the solution up cold and that way I will be able to tell how much citric acid the water can support (when cold).

Cheers, T i m

[T i m]

Ok, thanks.

I like it. Like yer grans cooking. ;-)

With the Deox-C the solution went from clear to dark as I cleaned things. This citric acid seemed to remain pretty clear for quite some time but tonight I noticed it was fairly dark.

A good way to strip any protective paint of such things ;-(

Ouch.

Result. ;-)

Thanks for the feedback etc.

Cheers, T i m

[newshound]

On 17/10/2013 18:45, T i m wrote: > Hi All, > > > The third question is of 'hydrogen hardening'. I think I understand > this is the penetration of hydrogen into the material, (eventually) > wreaking it to some degree (to varying levels depending on the > material). > > How likely is this to affect the strength or reliability of everyday > items? (ie, not aircraft / F1 components). > > Cheers, T i m > Hydrogen cracking is something which can affect high strength steels but AFAIK is more commonly associated with electroplating or possibly the electrolytic rust removal technique normally used with Sodium Carbonate, rather than what you describe which is a form of acid pickling.

Electrolytic rust removal appears to be remarkably effective, from what I have seen via Google.

[Chris Hogg]

Citric acid is very soluble in water. The MSDS below gives it as

59.2g/100g, so your 20 litres of water should be able to dissolve approaching 12 kg of citric acid. See

I don't know what is the precipitate you are finding on all the surfaces, but it sounds as if something is crystallising out, but what that might be, I'm not sure, except that it's unlikely to be citric acid. Rust is usually a mixture of mostly hydrated ferric oxide with a lesser amount of hydrated ferrous oxide, and although ferric citrate is apparently soluble (1)(2), ferrous citrate apparently isn't (3), so perhaps it's that. However, the Wiki reference says that ferrous citrate is soluble in ammonium hydroxide, so washing the pieces in a solution of household ammonia might remove the deposit, if it is in fact ferrous citrate. The ammonia will at least clear your head!

(1)

and scroll down to Hydrolysis. (2) (3) in the side panel.

As for hydrogen in steel, see

but as I'm no metallurgist, I can't really comment as to how important it is for everyday items, but at a complete guess I would say not very.

[ss]

I am not sure Tim but all else has failed so nothing to lose....I will give it a go.

[Martin Brown]

Rinsing in warm clean water is probably good enough, but dilute sodium bicarbonate aka baking soda will be as good as anything.

[T i m]

Thanks for that Martin.

Cheers, T i m

[T i m]

Oh.

(handy, thanks)

No, quite, from what you say above (with my solution 1/12 of what it could have been!).

Well, it's the best theory I have so far.

Well, it will be an easy / cheap experiment in any case.

Hehe. This is all being done outside of course. ;-)

Interesting. I have some EDTA which I bought to try recovering some lead acid batteries from sulphation.

Printed.

Hmm, I have some acetone here so I could try that first.

I think it *could* be one of those things that can become a bit of an urban myth (when referring to 'everyday stuff'), depending on the sort of things you deal with (of course). Like, I recovered a couple of very rusty trailer brake drums (before I had heard of this hydrogen hardening) but some have mentioned the issue isn't so bad (if at all) with cast iron. I have seen the mention of heat being use to drive out the hydrogen and so I put one item (a motorcycle front axle) in the over for a couple of hours at 220 DegC last night just_in_case.

If you are interested in this subject I found a fairly interesting article that examines the effects in the aircraft industry:

Or

I think I was surprised to read that even once the found signs of this hardening (as you say, possibly as the result of plating in one case) and after some mechanical failures they continued using these components in service because the lead time on replacements would be too long and / as the aircraft were due to be retired from service 'soon'?

P21- Section 20.5.2 (20.5.2.4)

I found out citric acid attacks (reacts strongly against) galvanised surfaces and I've since read that they have to watch out re the acids in fruit when they are being stored or processed.

Thinking out loud, because my current citric acid cleaning 'trough' is outside (all be it potentially covered, it's not impossible that some rain has got into it but I wouldn't have though at sufficient quantities to have cause this strange reaction.

OOI, an old rusty panel saw is now so flexible it can be rolled up but that seems to be the opposite of 'hardening'. ;-)

Thanks very much for all the info. I'll let you know if anything dissolves this 'crust'. ;-)

Cheers, T i m

[T i m]

Understood.

Yes it is. I have 'processed' quite a few things now using a PC PSU (stripped of all but it's +12V outputs and those put on longer and fused output cables), and it is indeed very effective.

This was one hub from a trailer that had been left on its side for some years and was full of water.

I don't think I had a 'before' of the inside of the drum (but you can probably imagine what it was like) but this was after a relatively short time in the electro tank).

They were left 'bare' in the garage and so have gathered some more rust so I might treat them in the citric acid tank (some fresh stuff) and then rinse / neutralise and paint / protect straight away.

Cheers, T i m

p.s. I did get your email version of this but assumed you sent it that way by mistake?