Alternatives to Shield Technology Restore Rust Remover?

On 30/12/2010 09:45, Cliff Coggin wrote: ... snipped

It's because salt and vinegar go with chips, not ships ;-)

(Sorry, I'll get my coat)

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In article , Cliff Coggin writes

Cliff,

I doubt if H+ ions penetrate much into the metal lattice - and if they do, they would almost certainly be rapidly changed into H2 molecules. That of course can cause hydrogen embrittlement in many metals, including some high-strength steels. Once the metal is removed from the solution, the supply of further H+ ions is removed.

I rather suspect residual NaCl is more likely to be a cause for concern. Corrosion requires the existence of corrosion cells, with some material to provide the cathodic reaction of absorbing the electrons given up by the M -> M+ reaction at the anode. This is usually (but not always) oxygen. It also requires some source of potential difference (dissimilar metals, oxygen concentration, differential stress and grain boundaries are important examples). Ships (and offshore steel structures such as oil production facilities and pipelines) have the huge disadvantage that they (a) spend most of their lives in highly conductive seawater (aids operation of corrosion cells, and also induces chloride stress corrosion in stainless steels), (b) often have dissimilar metals in contact (potential difference), (c) almost always are partially immersed, partly exposed to air (source of O2, and provides oxygen concentration potential differences), (d) are often welded structures, with all the potential that gives for stress and corrosion nodes, and (e) are often relatively highly stressed (induces stress corrosion, and makes the consequences of corrosion penetration more serious).

Also, the rust removal treatment we are discussing is a one-off process, and one which merely removes the side-effects of corrosion for cosmetic effect. Marine structures require constant protection, to avoid the loss of material and strength (and to avoid creating stress-generating pits) in the first place.

The kind of things most of us fabricate are neither so highly stressed, nor subject to such a harsh environment. We do tend to over-engineer much of our output (and I speak of model engineering - the group title is a clue here - and exclude those who may make real life-critical parts) and treat it fairly carefully (cleaning, keeping dry, oiling or painting, avoiding dunking in seawater for several years etc.). I doubt if Mike's recipe, if followed up with some modest after-care, will cause serious problems.

However, the test is in trying it. I think I will pinch some salt and vinegar from the kitchen and have a go! Will take some time to see the results of any post-treatment corrosion though.

Interesting discussion though!

David

Reply to
David Littlewood

The two key items:

a) make sure no more salt can be dissolved. It will start settling on the bottom and no amount of stirring will get it into the liquid.

b) start with enough vinegar to completely cover the metal.

It may be of benefit to wiggle the metal back and forth once or twice during the day, to mix the active ingredients and displace any small bubbles which may or may not settle on the surface.

A good experiment would be to find two identically rusted bolts and treat only one. That way, there's a control mechanism which may be used to satisfy the scoffers. It also makes good photographic evidence.

This subject seems to have taken on a polarization bordering on the religious. Please DO try it on normal backyard iron/steel...it's common and well known to most. You results should dispel any doubts.

mike

Reply to
m II

Amusingly, I had been using a length of 20SWG piano wire to lift parts out of a bucket of 10% phosphoric acid a month ago. I'd normally use my hands, but they had a cut on them and the acid was stinging. Any road up, I'd hooked one batch out and put another batch in to soak. An hour later I used the wire to hook out the latest batch and it promptly snapped.

It turned out that the wire that had been in the acid had got a case of hydrogen embrittlement. All the wire that had got the acid on it wouldn't bend past 45 degrees, whereas the wire that stayed out of the acid would bend double. Unfortunately, I didn't experiment with the effects of baking it for a while to drive the hydrogen out. That's an experiment for another day.

The warning is that _any_ acid cleaning may cause hydrogen embrittlement and it'll be noticeable on thin stuff.

PS:- salt in vinegar forms a buffered hydrochloric acid and sodium acetate solution.

Mark Rand RTFM

Reply to
Mark Rand

  1. I have used cleaning vinegar which is brewed from mollasis no salt added. It seems to be stronger than normal vinegar. I soak my metalworking files in it and it seems to make them cut better. Available in the laundry section at Woolworths.
  2. Electrolysis is also good ,a cheap battery charger and a bucket of water with washing soda added works for heavy rust . Only removes the rust and does not attack the parent metal.
  3. Citric acid bath , citric acid is used in cooking ,and is available from the super market ,it's the stuff that makes sweets fizzy to the tounge.
  4. I bought a bottle of R20 sold here as a rust converter ,smells suspiciously like mollasis thinned down , it claims to convert rust to a protective coating . I tried it on a piece of rusty steel and a few hours later the rust had turned to a dark black purple colour .The steel has been left in the weather for around 6 months with no evidence of rust forming again where it was treated.
Reply to
Kevin(Bluey)

I think you have missed the point Mike. Nobody doubts that acids dissolve rust. That fact was discovered hundreds of years ago by the likes of Lavoisier and other early chemists, so you are not revealing a great secret. The trick is to do it without damaging the article or leaving it prone to further corrosion, that is the difficult and expensive part of the process, and where this thread started a month ago.

Cliff.

Reply to
Cliff Coggin

Sorry I have to dispute that statement unless new research in the 12 years since I retired from teaching chemistry has fundamentally changed the nature of solution chemistry.

1) A buffer solution maintains a constant pH by "removing" added hydrogen ions (ie if acid is added) and "removing" added hydroxide ions (ie if alkali is added or hydrogen ions are removed). Whilst the above mixture will maintain the pH if alkali is added as some of the large excess concentration of undissociated ethanoic acid (acetic acid "vinegar") molecules will dissociate into ions and the hydrogen ions thus formed will neutralise the added hydroxide ions forming water. However this mixture has no mechanism for removing extra hydrogen ions if an acid is added so the PH would drop. To make it into a buffer solution forget the sodium chloride and add excess ethanoate ions in the form of sodium ethanoate (sodium acetate), the ethanoate ions would join to the added hydrogen ions forming ethanoic acid molecules and thus bring the pH back up to close to the original value. - in general a buffer solution contains a weak acid with the sodium or potassium salt of that acid added to it. 2) I would also suggest that including the term hydrochloric acid in the description is misleading as (as well as a very high concentration of water!) the mixture actually consists of a very high concentration of sodium ions and chloride ions (ie sodium chloride), a high concentration of undissociated ethanoic acid (ie acetic acid) and a very low concentration of hydrogen ions and ethanoate ions. (if my memory is correct, Ka for ethanoic acid is about 1.7x10 to the power of -4 giving a pKa of 4.8). So whilst whilst this mixture contains chloride ions and hydrogen ions the concentration of what might be called hydrochloric acid is very small. 3) I think that mentioning hydrochloric acid in a description of this method of removing rust is a red herring. If anyone wants to use hydrochloric acid for this process you would do better to put your fingers down your throat and vomit on the piece of rusty metal - the concentration of hydrochloric acid is much higher (and thus pH much lower) in stomach acid than in this mixture. Better still use cat sick - I've always found that it is excellent at cleaning stains in carpets and furniture if washed off after a few minutes! 4) I would have thought that if it is the addition of a strong electrolyte such as sodium chloride to the acetic acid which has speeded up the rust removal then it is not just a simple acid reaction but the mechanism is more complicated involving Redox reactions - the addition of the large number of charged ions will significantly increase the electrical conduction of the mixture and the variation in electrode potential between the rusted and non rusted parts will drive the reaction.

I've enjoyed trying to remember and apply chemistry again, it's more than

10 years since I last did and of course memory plays tricks and it may be a complete load of bollocks :-)

Alan

Reply to
Alan P Dawes

I couldn't have put it better myself.

Happy New Years, scroats :) --

Chris Edwards (in deepest Dorset) "There *must* be an easier way!"

Reply to
Chris Edwards

In article , Alan P Dawes writes

Alan,

I was about to put down some of these points in response to Mark's comment earlier today, but had to take a sick cat to the vet instead (no, seriously, and I didn't get any cat sick to test either). You've saved me the trouble, and said it far more clearly and comprehensively than I would; as far as I can see you are spot on.

David

Reply to
David Littlewood

Just checked in a Book of Data. My memory was a power of 10 out, the pKa should be 10 times smaller 1.7x10 to the power of -5 but the pKa value is correct - I should have noticed the discrepancy between my 2 remembered values.

Happy New Year Alan

Reply to
Alan Dawes

That'll teach me to post while merry (mind you, it's usually when I'm merry that I'm most lucid!).

Note, that the bit above the off the cuff stupid remark (the bit about piano wire suffering hydrogen embrittlement in phosphoric acid) was the genuine article. I was quite shocked by the experience and will be far more thoughtful when using acids or electrolytic cleaning on thin or stressed articles in the future.

Regards Mark Rand RTFM

Reply to
Mark Rand

That's very interesting! I gave up using Phosporic acid in particular (ye old navel jelly!) because it leaves the metal in such a porus and reactive (smelly!) state it seemed to defeat the object, and of course it disolves the metal too. I also started wondering about using phosphric acid in case I was inadvertantly generating small amounts of organo-phosphate compounds which can have nasty neurological effects at very low concentrations ( many insecticides, herbicides, and nerve gases are organo-phosphates), but that might just be something akin to hypochondria!

Reply to
Scrim

I wonder if it was hydrogen embrittlement or or just that the acid etched surface had lots of good places for cracks to start, unlike the shiny new piano wire? Your idea of cooking it in the oven might clear that up (if you still have some handy) so long as it isn't hot enough to change the temper of the steel.

Reply to
Scrim

Rust 'Converters' are another thing altogether. They leave a kind of plastic layer behind that prevents further rusting. I can't find 'R20' but this Rust converter is described as a mixture of Tannic Acid and "2-Butoxyethanol, an organic polymer".

Reply to
Scrim

Same here. I am now searching for better finger protection, as the chain mail glove was way too irritating to the cat's esophagus. I'm also looking into a feline diet with a built ejection system.

mike

Reply to
m II

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