Synergistic Effect of Walnut Green Husk Extract Complex Inhibitors on Steel in Citric Acid

Walnut green husk extract(WGHE) was prepared by reflux method using forestry and agricultural residue of walnut green husk as the raw materials. The synergistic inhibition effect of WGHE and the anionic surfactant of sodium dodecyl benzene sulfonate(SDBS) on the corrosion of cold rolled steel(CRS) in citric acid(H₃C₆H₅O₇) solution was studied by weight loss, potentiodynamic polarization curves, electrochemical impedance spectroscopy(EIS) and scanning electron microscope(SEM). The results show that individual WGHE or SDBS can retard the corrosion of CRS surface in 1.0 mol/L H₃C₆H₅O₇ solution to some extent. However, the maximum inhibition efficiency is below 60% for either individual WGHE or SDBS. Incorporation of WGHE with SDBS can obtain efficient inhibitive action, and the maximum inhibition efficiency can reach as high as 94.2%. All synergism parameters are higher than 1, and so there is a true synergistic inhibition effect between WGHE and SDBS. The synergistic inhibition increases with an increase of WGHE concentration, while decreases with an increase of temperature, and reaches a peak value with the acid concentration of 1.0 mol/L and the immersion time of 72 h. No matter whether WGHE is mixed with SDBS, the adsorption behavior on steel surface obeys Langmuir adsorption isotherm. The adsorption equilibrium constant of WGHE/SDBS mixture is increased evidently comparing with individual WGHE or SDBS. WGHE/SDBS is a mixed-type inhibitor, which shifts both cathodic and anodic branches to negative corrosion current densities. EIS has only one time constant, and its Nyquist spectrum exhibits a depressed capacitive loop. The charge transfer resistance of steel/solution interface follows the order: WGHE/SDBS > WGHE > SDBS. SEM and AFM micrographs confirm that WGHE/SLS can efficiently alleviate the corrosion degree of steel surface in citric acid media, and the surface roughness is significantly dropped. There is a synergism between SDBS and the major components of rutin, quercetin, 1-methylnaphoqinone and juglone in WGHE, but their synergistic inhibition is below than the mixture of WGHE/SDBS.