Bis-Mannich bases as effective corrosion inhibitors for N80 steel in 15% HCl medium

Three novel bis-Mannich bases (named as M-n-M) have been synthesized by condensation reaction of acetophenone and formaldehyde with different aliphatic diamines (n represents the number of carbon atoms in the diamines). Their corrosion inhibition behaviors for N80 steel in 15% HCl solution were evaluated through weight loss measurements, electrochemical tests, scanning electron microscope (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and contact angle measurements. The results showed that the inhibition efficiency was increased with concentration as well as the alkyl chain length of the inhibitor, with the best inhibitor efficiency obtained as 98.20% for 0.3 % M-8-M in 15% HCl at 90 °C. The electrochemical tests revealed these inhibitors are mixed control type. SEM, AFM, XPS and contact angle measurement results revealed that the inhibitors form a compact adsorption layer on N80 steel surface. Adsorption mechanism of the inhibitors obeys the Langmuir isotherm, and physical adsorption and chemical adsorption exist simultaneously. M-8-M was compared with two commercial inhibitors at elevated temperature and gave a satisfactory corrosion rate of 15.13 mm·y−1 on N80 steel in 15% HCl at 120 °C.