Depolymerized Lignin Products From Oil Palm Fronds (OPF) As Potential Corrosion Inhibitors For Mild Steels In Hydrochloric Acid Medium
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Date
2017-08
Authors
Mohamad Shah, Affaizza
Journal Title
Journal ISSN
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Publisher
Universiti Sains Malaysia
Abstract
Corrosion control is important to prevent metals and its alloys from continuous corrosion attack. The use of green inhibitors is one of the best options to protect metals and its alloys against corrosion because they are biodegradable, environmentally friendly, ecologically acceptable, cheap and readily available. Although several researches have been done on corrosion inhibition by lignin, reports on the detailed mechanisms of the adsorption process and identification of the active constituents are still unclear. Therefore, lignin from oil palm fronds (OPF) was depolymerized via nitrobenzene oxidation before it was employed as corrosion inhibitor for mild steel in acidic medium. A rapid reversed-phase high performance liquid chromatographic (HPLC) method was developed and validated for the determination of seven monomers that are present in depolymerized lignin products (4-hydroxybenzoic acid (4-HBA), vanillic acid (VA), syringic acid (SA), 4-hydroxybenzaldehyde (4-HB), vanillin (V), syringaldehyde (S) and 4-hydroxyacetophenone (4-HA)). Using a mobile phase of water: acetonitrile: methanol: acetic acid (79: 6: 13: 1) at a flow rate of 1.0 mL min-1 with UV-Vis detection at 280 nm, all seven products were isocratically separated, especially V and 4-HA, in about 14 minutes. All the analytes showed good linearity with squared regression coefficient (R2) ranging from 0.9993 to 0.9995. The limits of detection and quantification were in the range of 0.10-0.38 and 0.31-1.15 mg L-1, respectively. Good reproducibility of peak area (RSD ≤3.46%) for inter-day and intraday was found. Satisfactory recoveries for all samples were obtained (91.4 to 110.6%). This method was applied to determine phenolic constituents present in the depolymerized OPF and standard alkali lignin (OSL, OKL, OOL and OAL). It has been found that S is the main compound in all samples analyzed except OAL and 4-HA was identified for the first time in depolymerized OPF lignin. The inhibitory effect of all depolymerized lignin and its phenolic monomers were investigated on the corrosion of mild steel 1 (S1) and mild steel 2 (S2) in 1 M HCl medium. The corrosion inhibition behavior was evaluated by conventional weight loss method, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) studies. The inhibition efficiency increased with increasing inhibitor concentrations (37.1-98.3%). The inhibitory performance of depolymerized OPF lignin products for acid corrosion of S1 and S2 from the weight-loss measurement was better than OAL. The trend of inhibition efficiency of depolymerized lignin products was in the following order: OAL < OKL < OOL < OSL. While the trend of inhibition efficiency of phenolic monomers was in the order as follows: 4-HBA < V < S. The potentiodynamic polarization results for S1 and S2 reveal that all inhibitors act as mixed type inhibitor except for S1 in the presence of phenolic monomers that have shown to act as cathodic inhibitor. The EIS measurement results show that the charge transfer resistance increased as the inhibitor concentrations increased indicating the increased adsorption of inhibitors on S1 and S2. The OSL, OKL, OOL and its monomers were found to well fit with various adsorption isotherms such as Langmuir, Temkin, Freundlich and El-Awady. The ΔG°ads values between -14.55 and -36.30 kJ mol-1 proved that physisorption dominated the adsorption process. SEM results disclose that OSL, OKL, OOL, OAL and S transformed the S1 surface to smoother surface, while to the lesser extend for S2 that is consistent with the lower inhibition efficiency for S2 compared to S1 from all evaluation techniques. The developed HPLC method was used to analyze the content of the depolymerized lignin products before and after corrosion processes (from weight-loss technique). The analysis revealed that only phenolic aldehydes (S, V and 4-HB) gave significant contributions to the corrosion inhibition for S1 and S2 in 1 M HCl that do not depend on the total initial concentration of S, V and 4-HB in the depolymerized lignin products. In addition, the consumption percentage of corrosion inhibitor for a single monomer is always higher when used alone than when it is in a mixture. The results from the HPLC analysis corroborates well with the weight-loss measurement results.
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Keywords
Lignin from oil palm fronds was employed , as corrosion inhibitor for mild steel in acidic medium.