Amphoteric surfactant of octadecyl dimethyl betaine as an efficient corrosion inhibitor for cold rolled steel in phosphoric acid solution

The inhibition effect of amphoteric surfactant of octadecyl dimethyl betaine (BS-18) on cold rolled steel (CRS) in H3PO4 solution was studied by weight loss, electrochemical technique, surface analysis and quantum chemical calculations. The results show that BS-18 acts as an efficient inhibitor on CRS in 1.0 M H3PO4 medium at 20–50 °C, and the maximum inhibition efficiency is 87.1% with 50 mg L−1 at 20 °C. Its adsorption on steel surface obeys Langmuir adsorption isotherm, which is endothermic process accompanying with the increase of entropy. Compared with uninhibited solution, apparent activation energy, pre-exponential factor, activation enthalpy and activation entropy become higher in inhibited solution, while both reaction constant and rate constant decrease after adding BS-18. Inhibition performance remains more stable from 3 to 48 h, and fluctuates slightly with the acid concentration from 0.5 to 5.0 M. BS-18 is a mixed-type inhibitor through “geometric blocking effect”. EIS is a time constant with one depressed capacitive loop in Nyquist plot and one phase angle in Bode phase plot, and the charge transfer resistance turns to become more larger in the presence of BS-18. SEM and AFM micrographs of inhibited CRS surface appear little corrosion, and its contact angle is increased. There are functional groups of C–N and C–N+ in the adsorptive film of BS-18 on CRS surface using XPS analysis. Critical micelle concentration (CMC) of BS-18 is about 20 mg L−1 from surface tension measurements. Quantum chemical calculations confirm that the active adsorption site of BS-18 is the hydrophilic group of N+ and –CH2COO−.