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topic 671

Chart of Activity Series



A discussion started in 1996 & continuing through 2017

(1996)

Q. Where can I find a chart the lists or groups metals in anodic/cathodic order? I am an engineer that often calls out fasteners and hardware that may be subjected to potential differences.

Doug Dina
instruments - Duluth, Georgia


(1996)

A. Hi Doug. These charts are published in numerous books available at the library. Supplementary ones have been published to draw finer distinctions between alloys and deposition conditions.

But I think what you want to search for on the internet is the "Sea Water Series", as opposed to the "Galvanic Series", since the potential differences you are looking for depend on field conditions, and the "Sea Water Series" is supposedly a fairly good 'real world' arrangement of the potentials. Good luck.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(1996)

A. Doug,
I assume that you are interested in the galvanic series of metals in seawater? There would be a different galvanic series for different environments.
Two good sources of galvanic series data are Mil-STD-889 [link is to free spec at Defense Logistics Agency, dla.mil] and "Corrosion Handbook" by F.L. LaQue. The International Nickel Co. also published some interesting literature on this subject.

Stephen P. Gaydos
McDonnell Douglas Aerospace


(1996)

A. Another good source of information is Table VI in Mil-F-14072 [link by ed. to spec at TechStreet] ("Finishes for Ground Electronic Equipment") which shows compatible couples.

Also, there was a paper written by J.R. Crum and R.C. Scarberry of Inco Alloys International, Inc. Huntington Alloys, Huntington, WV 25720 (sorry, I do not have the exact reference) entitled "Development of Galvanic Series in Various Acid and Water Environments" which provides numerous anodic/cathodic charts relative to the conditions under which the metals were tested.

Blair Smith
aerospace - Windsor Locks, Connecticut


(2000)

A. I found a nice chart online at: www.corrosionsource.com/handbook/galv_series.htm

Paul Maxted
design - Round Rock, Texas


(2001)

A. I FOUND THIS NICE CHART: www.corrprev.org.au/Galvanic.htm

CARLOS A. GARCIA
aerospace - TULSA, OKLAHOMA



Precipitation of metals from aqueous solution

October 26, 2017

Q. I have metals from e-waste in aqueous solution and would like to know which metals precipitate out higher metals and in what order. Can I suspend a Silver Ingot in the solution to precipitate out higher metals?(platinum, palladium, gold, etc,)?

Pastor David Jonsson
- Trinidad, California USA


wikipedia
Galvanic_series

Recovery And Refining Of Precious Metals

November 2017

A. Hi Pastor David. The "galvanic series" offers the general info you are seeking and, yes, as a general rule you can put a silver ingot into an aqueous solution to precipitate out more noble metals like gold and platinum through displacement. But few things are as simple and universal as we wish they were, and you will probably find that this precipitates some but not all of the more noble, more precious metals. Good recovery of precious metals from wastes can be a complicated undertaking. Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


November 7, 2017

A. To work, the metal you use must be able to dissolve in the particular solution you have. If your solution is acid and, if it contains any chlorides, the silver will not dissolve and, therefore, it will not reduce any Au, Pt, or Pd out of the solution. Actually, to initially dissolve those 3 metals, about the only acidic solution that will work is aqua regia, which contains HCl (high chlorides). If that is the case, silver will not precipitate those metals from that solution.

Copper will precipitate those 3 metals from aqua regia at essentially 100% efficiency. However, if there is any unused nitric acid in the aqua regia, it will dissolve copper until is is consumed before it starts precipitating any of the 3 metals. On copper, the precipitated Au, Pt, and Pd will appear as a fine black powder, with maybe some very dark brown mixed in.

If you have a cyanide solution, you might have a little better luck using silver but I doubt if anything close to 100% of the 3 metals will drop out. In cyanide, you might try 325 mesh zinc dust to drop out the metals.

Chris Owen
- Benton, Arkansas, USA


November 9, 2017

A. This is a little better (and, windier) explanation to my post of Nov 7.

The precipitation or reduction of metals in solution, with a more active metal higher up (more active) on the electromotive series scale, is called "cementation" in the precious metals refining industry. The reason is that, when silver is reduced from a nitrate solution with another metal, the reduced silver metal looks like wet cement. The cementation of silver from a nitrate solution is probably the most common cementation method.

Metallic silver scrap, like sterling silver or 90% silver electrical contacts, can be purified to about 99.0% by first dissolving it in a 50/50, 70% nitric acid/distilled water solution and then cementing out the dissolved silver with copper metal. Higher purity (99.99%) can be obtained by putting the 99% silver through an electrolytic silver cell. In order to cement the silver with copper, some of the copper must dissolve. Look at it as an exchange system. The copper and silver trade places. The copper metal dissolves and the dissolved silver becomes a metal (powder).

In a pure silver nitrate system, with no free (unused) nitric acid present, about 1 gram of copper will dissolve for every 3.4 grams of silver cemented. When silver is dissolved in nitric acid, one should attempt to use ONLY ENOUGH NITRIC to dissolve the silver and no extra. The reason is that when you cement the silver with copper, any extra nitric will first dissolve copper until the nitric is used up and only then will it start cementing the silver. For reference, about 4.15 ml of 70% nitric plus 4.15 ml of distilled water will dissolve 1 gram of copper.

I used the simplest system I know of, silver cemented with copper, to explain this process. Although the same basic rules apply, your system is more complicated. In an acid system, I chose copper to cement the Au, Pt, and Pd because of its position on the EM Series and, also, because it's essentially 100% efficient. About the only metals below copper on this scale are the precious metals and mercury (which you probably don't have). The rule is that a metal will drop any and all dissolved metals below it (less active) on the EM scale, assuming the metal chosen is soluble in the solution. You could use scrap iron (maybe) or zinc instead of copper but either will drop not only the precious metals but all of the dissolved contaminants, like nickel or copper, below the metal being used. Dropping these along with the precious metals will just complicate the final separation and purification of the 3 precious metals.

Here again, as with the silver/copper process, if you used a lot of extra acid to dissolve the 3 metals and some acid remains unused in the solution, it will dissolve copper until it is used up, before it starts to cement the 3 precious metals.

I might mention that, in an aqua regia solution, instead of cementing out the 3 metals, selective chemicals and processes could be used to drop all 3 metals, one at a time. That way, they are already separated, a process which will have to eventually be done by somebody.

If this is a cyanide solution, zinc dust, with a lot of stirring, works best because it is nearly 100% efficient. It usually takes from 1 to 2 grams of zinc per gram of your precious metals. Most of the zinc will dissolve. From my experience, I would guess that copper isn't nearly as efficient in the cyanide matrix as zinc dust.

If there are still questions, please ask.

Chris Owen
Gold/Silver Refining Consultant - Benton, Arkansas, USA



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