Nonionic surfactant of coconut diethanolamide as a novel corrosion inhibitor for cold rolled steel in both HCl and H2SO4 solutions

Abstract

BackgroundNonionic surfactant has the combined merits of easy production, low toxicity and high surface activity, which can be deemed as the potential inhibitor with good prospect. The nonionic surfactant of coconut diethanolamide (CDEA) has no cloud point comparing with other nonionic surfactants, and the acylamide group of O=C-N in its hydrophilic head can make the good hydrolysis resistance of CDEA. MethodsThe inhibition effect of CDEA was studied as a novel corrosion inhibitor for cold rolled steel (CRS) in both HCl and H2SO4 solutions for the first time by weight loss, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) methods. The micro-morphologies and chemical compositions of CRS surfaces were fully characterized by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), atomic force microscope (AFM), confocal laser scanning microscope (CLSM), X-ray photoelectron spectroscopy (XPS) and contact angle measurements. The critical micelle concentration (CMC) values of CDEA in HCl and H2SO4 media were determined by surface tension measurements. Moreover, quantum chemical calculations and molecular dynamic (MD) simulations were used to theoretically study the adsorption sites and modes of CDEA. Significant findingsCDEA acts as an efficient inhibitor, and the maximum inhibition efficiency of 50 mg L−1 CDEA is above 95% in 1.0 M HCl, and 92% 0.5 M H2SO4 from weight loss method. The adsorption of CDEA on CRS surface completely obeys Langmuir isotherm, and the standard adsorption Gibbs free energy is around -40 kJ mol−1. CDEA can be arranged as a mixed-type inhibitor in both acids, while exhibits more anodic inhibition in HCl solution. The presence of CDEA significantly increases the charge transfer resistance, while decreases the double layer capacitance. Inhibition efficiency follows the order: 1.0 M HCl > 0.5 M H2SO4. A series of micro-morphologies of CRS surfaces by SEM, AFM and CLSM confirm efficient inhibition of CDEA for steel in strong acids. EDX and XPS provides the distinct proof of adsorption of inhibitor molecules. The inhibited CRS surface is turned to be hydrophobic with an obtuse contact angle. The adsorption reaches saturated state at CMC of 0.10 mM. The hydrophilic groups of —OH and —CON— are the active adsorption sites of CDEA, while the hydrophobic chain of —C11H23 extends to the aqueous solution phase.