Effect of pH on Corrosion Inhibition of Steel by Polyaspartic Acid

Polyaspartic acid, a polymeric form of aspartic acid (C₄H₇NO₄), was examined as a corrosion inhibitor for steel as a function of pH, temperature, and hydrodynamic conditions. The temperature ranged from 25°C to 95°C, and the concentration ranged from < 1 wt% to ~ 10 wt%. Experimental procedures included electrochemical impedance spectroscopy (EIS), the rotating cylinder electrode (RCE), and coupon immersion. At low to neutral pH values, polyaspartic acid increased the corrosion rate of steel. At high pH (< ~ 10), polyaspartic acid was a reasonably robust corrosion inhibitor. Between pH 7 and 10, corrosion in the presence of polyaspartic acid was a complex function of temperature, concentration, water quality, and hydrodynamic conditions. By combining corrosion potential measurements with speciation diagrams obtained by titration, a reasonably cohesive explanation of the behavior was developed.