MadSci Network: Chemistry

Re: Forcing copper to patina using a sacrificial anode.

Date: Wed Oct 3 13:21:02 2001
Posted By: Tracy Cheatham, Faculty, Chemistry, Central Carolina Community college
Area of science: Chemistry
ID: 1000679664.Ch


The quick answer to the question is yes you can attach copper to a more cathodic (meaning it more readily donates electrons) metal to accelerate corrosion (or oxidation).

Let’s take a look at the oxidation process. When a piece of iron (Fe) is left exposed to the elements, it will combine with oxygen in the air and form iron oxide, which is rust.

Fe + O2 --> Fe2O3

The iron loses electrons, the oxygen gains electrons in this reaction. So we say the iron is oxidized (lost electrons) and the oxygen is reduced (gains electrons). Iron has to lose electrons in order to form rust.

When iron is in contact with another metal, who loses the electrons and oxidizes? That is determined by an activity series. Iron is higher on the activity series than zinc, so iron is stronger (holds onto its electrons) than zinc, so zinc will more readily oxidize. That means that if iron is in contact with zinc and oxygen tries to take some of iron’s electrons to form iron oxide, Iron (Fe) will steal some of zinc’s readily available electrons and give them to oxygen. That is why zinc is called the “sacrificial” anode. It sacrifices electrons to protect iron, and forms zinc oxide (ZnO) in the process. When iron is in contact with copper, the copper is stronger and takes irons electrons to protect itself. Iron provides cathodic protection for copper, at its own expense. The metals at the top of the activity series ruthlessly steal electrons from weaker metals to protect themselves.

So what metals are stronger than copper on the activity series? What metals will copper sacrifice its own electrons to protect? What metals will steal electrons from copper and force it to corrode? The answer to all the questions are: Silver, Mercury, Platinum and gold. All of these are very expensive and some are hard to come by. That makes them poor candidates to patina copper.

There are ways to force a patina other than corrosion. Several commercial and home-grown methods are available to view at They also have a search engine to find alternative methods for patinas and literature searches. I consider it an excellent resource site for elctro- finishing.

I hope this answers the questions. If not, send in another one and we’ll give it a whirl:)

Any college chemistry book (and most high school) will have an activity series of metals in it. I used:
Brown & LeMay, Chemistry: The Central Science, Prentice Hall, 7th edition, pg.131.

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