Why does this electrolytic derusting trick work?

Blackstrap Molasses

on eBay or

Amazon

(affil links)

A. The rust on your tools is FeO, or iron oxide. When you run the electricity through the tools the electrons reduce the FeO into Fe and O2, oxygen gas. Every time an equal part of the galvanized bucket which is Zinc is oxidized into ZnO or Zinc Oxide, a white powder. So every time you do this, a little of your bucket is blown away, watch for holes! haha

A. Getting rid of rust can cost a fortune in chemicals but if you have the time and can fully submerge the item to be de-rusted, here is a cheap way to do it. Submerge it in a black molasses and water solution. 1 part black molasses to 9 parts water. I have 40 liters of the stuff mixed up.

descendants of the mutineers

Where I used to work on Norfolk Island (in the South Pacific, halfway between Australia and Fiji - the place where the majority of the descendants of the mutineers from the Mutiny on the Bounty live), we had a 200 liter (44 Imperial gallons, 55 US gallons) drum mixed so we could dip big stuff. Like any small island, because of salt spray, rust is a real problem over there.

Really severe rusting will need a week or so in the solution. I recommend removing the job after a few days and wire brushing to remove the loosened scale, a quick hose down and return to the pot for another go. It is essential that the steel is fully covered. Unprotected steel will corrode very badly at the air/solution interface where it come out of the solution. Note: Some alloys e.g., zinc will be severely corroded by this stuff so keep it for iron and steel only or test it first.

I bought 5 liters of blackstrap molasses (stuff they feed horses) from my local feed and produce store. It is quite cheap.

A. Jim,

I can answer some of your questions. Sodium gluconate + sodium hydroxide used with current is an excellent deruster for mild steel. No cyanide is needed. Main problem is that when the rust is removed there is a pit left in the surface. The derusted surface is not blemish-free. The best I can recall of the operating conditions is: 8 oz/gal of sodium gluconate, 8 oz/gal of sodium hydroxide, 140 °F and 5 Volts anodic.

Hope this helps,

----
Ed. note: Anodic means the work is attached to the positive, as opposed to the negative proposed above by Adam H. Neither is wrong; sodium gluconate is a chelating agent which dissolves the rust.

A. Hi Rob. Pat's answer is probably apropos. The caustic eats some paints, the gluconate dissolves rust, and the electricity generates scrubbing bubbles of oxygen.

Acid (low pH) is the opposite of caustic (high pH), and I imagine that you used the term as slang rather it's scientific meaning.

Regards,

A. I think your teacher was trying to claim that the direction you have the battery leads hooked up meant you were plating onto your part rather than dissolving metal off of it, Carmi. But while that might be "sort of" true from a hypothetical viewpoint, because plating is done with the workpiece cathodic, actually you are correct and your teacher is incorrect!

It is true that in electroplating you make the workpiece cathodic (connected to the negative pole of the battery). Then, if you have the right operational conditions this surfeit of electrons at the workpiece can attract and reduce the positively charged ions in solution to metal and have them deposit on the work as metal (electroplating).

However, de-rusting can be done with either polarity. If you use the same polarity as electroplating (the work cathodic) and the right conditions, the rust Fe⁺⁺⁺₂O₃, reacts like this:

Fe⁺⁺⁺ plus electrons => Fe⁰

In fact -- although it's not foolproof and certainly not easy -- this process has actually been used to "run the film backwards", restoring corroded artifacts closer to their original condition.

You are not necessarily electroplating just because the workpiece is cathodic, nor necessarily "de-plating" just because the workpiece is anodic. If you don't have the right conditions, electroplating won't occur nor will de-plating occur -- there's more to it than just the polarity. Most every industrial plating line in the world has an electrocleaning step early in the process; maybe half are anodic and half are cathodic … and none are intended to either de-rust or electroplate, and they don't :-)

Demand an A+ !

A. Dear Michael!
If you are art restorer I think that you must know that electrolytic de-rusting cannot be used for plated objects of any sort. Second problem with this process is hydrogen embrittlement. Best process for removing of thin layers of rust is ammonium citrate(5% solution, pH 3,5). Blued or chemically coloured surfaces can be cleaned only with oil based solutions (Balistol, WD-40 ⇦ this on eBay or Amazon [affil links] , petroleum).You can visit ICOM-CC metal working group webpages. Conservation Online web site is very good too.Good luck!

A. Hi, Joe. Electroplating wastes are not regulated based on their actual hazards, but "categorically". What that means is, if it was generated by a commercial electroplating shop it's regulated, regardless of what it is. I doubt that there are any regulations covering your situation, since it is neither an industrial process nor a highly defined one, nor does it start with nor generate hazardous chemicals. And I do not see any environmental impact if you put the residue into your toilet and down the sewer. washing soda ⇦ this on eBay or Amazon [affil links] and iron are quite innocuous stuff for a sewer system. Good luck.

My niece was an actress at Surflight for a couple of summers a long time ago; I hope all is well on LBI.

Regards,

A. Hi Jerald. It's not called "electroplating" in that case, and it doesn't actually involve electroplating -- but it's the same equipment and sort of the same wet chemistry. We appended your inquiry to a thread on the subject. Good luck.

As an update to some previous responses: although de-rusting can apparently be done with either polarity, I have confirmed that the work should be cathodic (negative / the same as in electroplating) for the archeological restoration method of converting corrosion back to metal (as opposed to just removing it). I don't have this book, and haven't read it, but Amazon's "Look Inside" feature revealed that much. .

Regards,

A. Hi Bob. It can't electroplate iron onto the part because there is no iron in solution. If you proceed exceptionally slowly and are exceptionally lucky, some of the rust might be converted back to iron. But in the general case, the excess of electrons causes water to separate into hydrogen gas bubbles on your part and oxygen gas bubbles on the anodes. These "scrubbing bubbles" assist in the cleaning.

Regards,

A. Hi ishmael. Please try your best to be clear about what happened before you ask people to account for what happened :-)

I understand that you had 75 grams of iron "wall plates"; you connected them as the anode; over the course of an hour they lost a total of 5 grams. I don't know what you mean by "the amount of the solution was far more than 5 grams". Are you saying that you measured the weight of the solution before and after the electrolysis and the solution gained far more than 5 grams in weight? Yes, that certainly sounds highly improbable to me.

Regards,