Metal de-coroder

[Mark RandParticipant @markrand96270]

Posted by Kiwi Bloke 1 on 26/06/2019 12:03:17:

Thanks Martin. Well, we're getting somewhere… But questions remain: why do chelators prefer to bind the Fe from the oxide rather than from the lump of metal? Is it because the oxide is far more soluble in the chelator solution than is the elemental iron? Will chelatorsattack the base metal significantly / at all?

Possibly because rust is a porous powder loosely mechanically bonded to the surface where as iron is a (poly)crystalline solid.

[Anthony KnightsParticipant @anthonyknights16741]

I have a tin of black treacle in my kitchen cupboard. Perhaps i should relocate it to the workshop.

[pgk pgkParticipant @pgkpgk17461]

My chemistry is too outdated and limited to really break down an answer but it occurred to me to dig about as to what is in molasses.

If you 'chemical composition of molasses' and view the images then there is a legion of charts some of which show a significant difference between cane and beet molasses. This link for instance shows a signiicant difference with pH with their sample of cane molasses at ph5 and signiicantly that sample also has a substantial level of reducing sugars. The old school biology test for glucose will precipitate copper salts from benedicts solution for instance.

This link which happens to be beet molassses also gives an indication of the variety of minerals within the complex (not really a suprise since we're looking at organic life).

So….(and this is a simpliied 'stretch&#39.. it's seems reasonable to assume that cane molasses would have enough acidity to break down some of the iron oxides into a soluble form as well as perhaps some indirect reduction. (perhaps the complex of chemical composition having an effect analagous to a catalyst?). It may/my not be that the effect is more pronounced on the oxide than crystalline iron (and anyway the reaction is slow enough that users stop when the rust has gone. A test with leavng a piece of clean iron for months?).

Because of the complex of organic compounds there may well be a binding effect on free iron to other compounds conveniently called chelation here.

I other words everyone is correct (or the soup of molasses is too complex to unravel)

pgk

[Kiwi BlokeParticipant @kiwibloke62605]

My experience is that months of immersion in molasses doesn't cause the base metal to be etched – but the rust goes.

It does rather seem that 'chelation' is a term that gets used without being strictly defined. Oxalic and citric acids get called chelators. Both remove rust. Both are acids and, as far as I know, form iron citrate and iron oxalate. Is this chelation? Are these relatively simple metal salts of organic acids chelates? I'm not getting much further in this particular quest for knowledge…

[pgk pgkParticipant @pgkpgk17461]

Just shows how rusty my chem is…
If you look up the definition of chelation and recall that iron can be Iron 2 or iron 3 and is quite reactive it easily forms bond within a ring structure. The image of Iron Citrate shows it doing just that here so officially it is chelation (and i was wrong). But as I stated originally my profession used the term as a way of pulling out heavy metals and manipulating them to an excretable form and I wonder which science appropriated the term first.

I still think using that term here, however technically correct, imbues the use of such de-rusters as having a magical property to the average shopper.

[Martin WParticipant @martinw]

A further bit of digital digging has shown that chelation is a very specific process where a large molecule will form at least two bonds to a metal ion. One definition found is :

• A chelate is a chemical compound composed of a metal ion and a chelating agent. A chelating agent is a substance whose molecules can form several bonds to a single metal ion. In other words, a chelating agent is a multidentate ligand.

Also an entry can be found in Wikipedia here . There are many other descriptions of chelation but they all describe the same process

Hope this helps.

Martin

[Tim StevensParticipant @timstevens64731]

The term chelation is used in gardening – it is a good way of supplying chemicals (such as iron) to ericaceous plants (eg heathers and rhododendrons) when they are planted in soils which are not acid enough.

It is also the case that many organic acids serve as reducing agents (ie they reduce ferric iron Fe+++ to Ferrous iron Fe++), and this is much easier to dissolve. Thus ferric oxide (rust) becomes ferrous, and is dissolved by the acid. Ferric compounds tend to be yellow or brown, ferrous tends to be green. A plastic bottle of citric acid which has been used for rust removal will start off green-ish, and after a week or more, will become yellow and pull in the sides of the bottle. This is because the dissolved ferrous iron absorbs oxygen from the air in the bottle, producing ferric iron and lowering the air pressure (as the oxygen is no longer there).

I hope this does not add to the confusion (if any).

Cheers, Tim

Edited By Tim Stevens on 28/06/2019 15:15:57

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