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<p>[QUOTE=&quot;NSP, post: 3264481, member: 74849&quot;]This is a follow-up to the post I made several months ago following the completion of my High Temperature Corrosion class. As I suspected, everything changes when heat is involved. I will do my best to explain it in a way that makes it somewhat clear. Before I begin, I need to provide some background information.</p><p><br /></p><p>When the coins you tested sat out in open air for many years, they developed corrosion products on their surfaces, like Ag2S, Ag2O, etc. These corrosion products are ionic compounds and on average have a regular crystalline structure. They are not perfect, however. Atoms can be missing (“vacancies”), atoms can be sitting in places out of the regular crystalline structure in gaps where they shouldn’t be (“interstitials”), etc. All of these anomalies are referred to as “defects” in the corrosion products.</p><p><br /></p><p>Atoms can migrate through the corrosion product layer via these defects. The interstitials can move around, and vacancies can swap positions with atoms and effectively move around. At high temperature, defects are able to move a lot more freely because they have more energy to do so. More mobile defects means reactants for the corrosion reaction can move more freely. This means oxidation can occur more rapidly. To summarize, high temperature = reactants can move through the corrosion product layer more easily = more oxidation.</p><p><br /></p><p>When exposed to a high temperature oxygen atmosphere, most metals will end up forming a scale that slows down further oxidation. Everything changes when sulfur is involved. I believe that at 350 °F oxygen and sulfur would react to form species like SO2 and SO3. Both sulfur oxides are not nice to metals or their protective scale of oxidation products. In the presence of sulfur oxides, metal oxides and metal sulfides simultaneously form at a fast rate. The sulfides and oxides do not provide any protection after they form and are full of defects. It is thought that sulfur oxides can permeate through defects in the corrosion products, meaning that corrosion occurs deeper and deeper into the metal. This is why your coins didn’t stand a chance despite the presence of Ag2S, Ag2O, etc. on their surfaces: the sulfur oxide goes right through the corrosion products and destroys everything in its path.</p><p><br /></p><p>The effect would be even worse at higher temperatures… 350 °F isn’t terribly hot. At lower temperatures, the coins probably wouldn’t be so destroyed. At room temperature, this likely wouldn’t happen at all because sulfur oxides would not form as readily as they do at high temperatures and the defects would not allow for much movement of sulfur oxides.</p><p><br /></p><p>It is worth noting that the mechanism I described here is the current understanding of how this occurs... that understanding could change in the future.</p><p><br /></p><p>I suppose this begs the question about how people can successfully artificially tone their coins in an oven without destroying them. If I had to guess, they probably don’t use a pile of pure sulfur because the resulting cloud of warm SO2 gas would not be beneficial to the appearance of the coins.[/QUOTE]</p><p><br /></p>

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