Corrosion inhibition of steel by aspartic acid (C4H7NO4), an amino acid of low molecular weight, was found to depend strongly on pH. At a pH less than the ionization constant at ~ 9.5 to 10 (measured at 25°C), C4H7NO4 appeared to accelerate corrosion. Above that pH, it acted as a corrosion inhibitor for steel. A specially constructed potential-pH diagram for iron (Fe) that incorporated C4H7NO4 showed the change in behavior was accompanied by the most stable thermodynamic state changing from an iron aspartate complex to iron oxide. Polymerized C4H7NO4 (polyaspartic acid) behaved in a similar manner. Some other amino acids of low molecular weight behaved similarly.
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Kalota et al. (1994) studied this question.
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