|MadSci Network: Chemistry|
The corrosion of iron is well-studied, with many books published - so (university) library! Rusting is subject to considerable experiental manipulation. Were all variables in your comparision rigrously identical except for the presence of salt vs fresh water? Rusting may require oxygen in air. Having the nails fully immersed versus partially immersed can make a big difference. The amount of air in the water also makes a difference, boiled out versus shaken up with air. Were all the nails identical? Alloys, physical treatment during manufacturing, and surface treatments vary- even to scratches! Do either 7-UP or vinegar dissolve rust? Will dissolved organics adsorb on the metal surface to protect it? Consider oleoyl sarcosine used to protect iron against attack in hydrochloric acid pickling baths (to remove oxide scale). There are no simple answers, which is one reason why experimental chemistry is interesting.
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