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Message:: [QUOTE="NSP, post: 2999041, member: 74849"]I’m currently pursuing a degree in corrosion engineering. At one point I thought I may be able to understand how toning occurs, but then I started learning the material and realized how truly complicated it is. I suppose it’s one of those things where the more you know about a subject, the more you realize how little you actually know about it. I invite anyone to post comments, corrections, clarifications, questions, suggestions etc. regarding what follows. Corrosion requires four things to occur: an anode (a site where an anodic reaction occurs), a cathode (a site where a cathodic reaction occurs), a metallic connection between the anode and cathode to allow electrons to move to where they want to go, and an electrolyte with some sort of ions in it to balance out the movement of electrons (i.e.- to complete the circuit). In an anodic reaction, something is losing at least one of its electrons (this is called oxidation). In a cathodic reaction, something is gaining at least one electron (this is called reduction). I believe that when toning occurs, there’s probably a very thin layer of water present on the coin (on the order of microns) that helps the reactions take place. What reactions take place? There are handy tables that contain a bunch of Standard Reduction Potentials. These tables contain many half reactions in their reduction direction. Each half reaction has a certain electrochemical potential (E) associated with it expressed in volts (V). An example, where M is a generic metal, M2+ is a positively charged ion of that metal that had two electrons removed from it, and e- is an electron: M2+ + 2 e-  ⇌  M   E = number If the value for E is positive, it means that the half reaction wants to proceed from left to right (forward direction). If E is negative, it means the half reaction wants to proceed from right to left (reverse direction). If E is relatively large, it means it REALLY wants to proceed in its desired direction. Here are some possible reactions involving copper, oxygen, hydrogen, and water, and ions related to them (I couldn’t find any that included CuO): 2H2O + 2 e−  ⇌  H2(g) + 2 OH−    E = −0.8277 V Cu2O(s) + H2O + 2 e−  ⇌  2 Cu(s) + 2 OH−    E = −0.360 V 2 H+ + 2 e−  ⇌  H2(g)    E = 0.0000 V Cu2+ + e−  ⇌  Cu+    E = +0.159 V Cu2+ + 2 e−  ⇌  Cu(s)    E = +0.337 V O2(g) + 2 H2O + 4 e−  ⇌  4 OH−(aq)    E = +0.401 V Cu+ + e−    ⇌  Cu(s)    E = +0.520 V O2(g) + 4 H+ + 4 e−  ⇌  2 H2O    E = +1.229 V Source: <a href="https://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)" target="_blank" class="externalLink ProxyLink" data-proxy-href="https://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)" rel="nofollow">https://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)</a> Note that I have been referring to these as “half reactions”- that is because they’re only half of a reaction. You cannot have electrons magically floating around by themselves; they must come from another chemical. So what ends up happening is you have to reverse the direction of one of those half reactions and couple it with another half reaction for things to work. When you reverse a half reaction, the sign of E changes as well. Say you want to combine the following two half reactions, for example: 2 H+ + 2 e−  ⇌  H2(g)    E = 0.0000 V Cu2+ + 2 e−  ⇌  Cu(s)    E = +0.337 V I will reverse the second reaction: Cu(s)  ⇌  Cu2+ + 2 e−    E = -0.337 V Note that I changed the sign of E. Now I will combine them: Cu(s) + 2 H+ + 2 e-  ⇌  Cu2+ + 2 e- + H2 (g) The electrons can be canceled because they are present on both sides: Cu(s) + 2 H+   ⇌  Cu2+ +  H2 (g) To find the E for the reaction, you take the reduction (forward) half reaction’s potential and add the oxidation (reversed) half reaction’s reversed potential: E = 0.000 V + (-0.337 V) = -0.337 V Since this is negative, this particular reaction would not want to occur in the forward direction. This is why a copper Lincoln cent won’t do anything if it’s dropped in an acid solution containing H+ ions, like hydrochloric acid. Back to toning: all of these half reactions can occur in many different ways with each other and form all sort of corrosion products (toning) on the surface of a copper coin. It’s also worth noting that this doesn’t take hydrogen sulfide (H2S) into account, which also likely reacts with copper in multiple ways. Also, copper coins could be alloyed with other metals like zinc, tin, etc., and each of those can participate in a whole host of reactions. Temperature, concentrations of reactants, pressure, etc. also affect these reactions. Long story short: toning is a very complicated process with MANY variables involved. TL;DR: toning is very complicated[/QUOTE]

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