Synergistic inhibition effect of walnut green husk extract and sodium lignosulfonate on the corrosion of cold rolled steel in phosphoric acid solution

The synergistic inhibition effect of walnut green husk extract (WGHE) and sodium lignosulfonate (SLS) on the corrosion of cold rolled steel (CRS) in 3.0 M H3PO4 solution was firstly studied by weight loss, electrochemical technique, surface analysis, quantum chemical calculation and molecular dynamics (MD) simulation. Results show that WGHE acts as a moderate inhibitor with a maximum inhibition efficiency of 73.4%, while SLS is poor inhibitor with the maximum inhibition below 40%. Mixing WGHE with SLS produces strong synergistic inhibition effect, and can significantly improve the inhibitive performance with a pleasant maximum inhibition efficiency of 97.2%. The synergism parameter is high than unity, and increases with the inhibitor concentration, but decreases with an increase of temperature. The adsorption of WGHE, SLS or WGHE/SLS obeys Langmuir adsorption isotherm. Individual WGHE or SLS mainly retards cathodic reaction, while WGHE/SLS mixture drastically inhibits both anodic and cathodic reactions simultaneously. The charge transfer resistance is drastically increased for the WGHE/SLS mixture, but the double electric capacitance decreases to more extent. The inhibited CRS surface by WGHE/SLS clearly appears little corrosion from SEM graph, and is of hydrophobic nature with an obtuse contact angle. Two components of naphthoquinone and 2-phenylchromone in WGHE exhibit synergistic inhibition with SLS. Quantum chemical calculations are well accordance with the expected inhibition order of WGHE/SLS > WGHE > SLS. The WGHE/SLS hybrid co-adsorb on Fe (001) and Fe (110) surfaces in the nearly flat manner, and the adsorption energy is more negative comparing with individual WGHE or SLS. (C) 2020 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.