In nature, water can be acidified by sulfate and chloride salts, dissolved carbon dioxide, and certain types of bacteria. And acidic water can corrode metal equipment that is exposed to it for long periods of time, like in heat exchangers used to extract geothermal energy from underground fluids. Sami Khan demonstrated what corrosion looks like on this 500 nm thick layer of iron coating a silicon substrate by dropping hydrochloric acid (pH 1.5) onto it. Almost immediately, the very thin layer of iron began to crack and corrode revealing the silicon beneath it (blue under this polarizing filter). Khan, a PhD student at the Massachusetts Institute of Technology working in the lab of Kripa K. Varanasi, studies coatings that could save ferrous alloy materials, like steel, from long-term corrosion.
Submitted by Sami Khan and Kripa K. Varanasi
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