Theoretical and experimental investigation of two alkyl carboxylates as corrosion inhibitors for steel in acidic medium

Abstract

Two alkyl carboxylates namely sodium octanoate (Na-Octa) and sodium dodecanoate (Na-Dodeca) have been studied as corrosion inhibitors for X60 steel in 0.5 M HCl solution using theoretical calculations, weight loss and electrochemical methods. A density functional theory (DFT) approach was first utilized to calculate their electronic properties to establish the relationship between inhibitive effect and molecular structure and to predict the most efficient inhibitor. DFT at the B3LYP/6-311++G (d,p) level was used to study the electronic properties of the isolated neutral and protonated forms of the two molecules. Additionally, their adsorption process on Fe metal was simulated using cluster models of Fen(n = 1,2,3,and 4) attached at the carboxylic O atoms of the two molecules. The B3LYP with LANL2DZ/SDD basis sets for Fe atoms and 6-311++G(d,p) for other atoms were used for the adsorption studies. Electronic properties, NBO data and plots of total and partial density of states (TDOS and PDOS, respectively) for the isolated and adsorbed inhibitor were performed and analyzed. Experimental results confirm that the two alkyl carboxylates act as corrosion inhibitors for X60 steel in 0.5 M HCl. Although the Na-Octa exhibited better corrosion inhibition behavior, increased concentration lowers the inhibition efficiencies of both carboxylate, and temperature increase is highly detrimental for Na-Octa but beneficial for Na-Dodeca only at 10 mM concentration. The concentration effect is attributed to increased particle interaction towards micelle formation. The temperature effect is attributed to enhanced Na-Dodeca solubility at high temperature.