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<p>[QUOTE=&quot;-jeffB, post: 1565492, member: 27832&quot;]Here&#039;s why I was asking about penetration depth.</p><p><br /></p><p>I&#039;ve bulk-&quot;restored&quot; a bunch of dateless cull Buffalo nickels in white vinegar (5% acetic acid). Looking strictly at the activity series, it should preferentially attack the nickel, sparing the copper, although I realize it&#039;s probably not that simple in an alloy.</p><p><br /></p><p>I left a few of them in for a number of months. When I checked them, they had indeed turned dark coppery brown, and the supernatant liquid&#039;s color looked more like Ni2+ than Cu2+. I believe they were visibly thinner, although I don&#039;t think I actually dried and weighed them; I should go ahead and do that (they&#039;re in water now).</p><p><br /></p><p>So, if a CuNi coin sustains damage in an acid environment, I&#039;d expect the surfaces to be enriched in copper. Based on my own limited experimentation, it can produce the color change you&#039;ve described as well. What I don&#039;t know is how the composition changes with depth. I&#039;d be happy to provide a sacrificial nickel for you to <i>grind down</i>, measuring composition as you go, or cross-section and do a profile across the interior (if your spot size is small enough).[/QUOTE]</p><p><br /></p>

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