Water-based acrylic copolymer as an environment-friendly corrosion inhibitor onto carbon steel in 1 M H2SO4 in static and dynamic conditions
© Dinmohammadi et al.; licensee Springer. 2014
Received: 23 April 2014
Accepted: 27 October 2014
Published: 21 November 2014
The green corrosion inhibitors serve recently as a source of environmental friendly materials in the corrosive media. In present study, corrosion inhibitive performance of a new environment-friendly acrylic copolymer (methyl methacrylate/butyl acrylate/acrylic acid) on the mild steel in 1 M H2SO4 solution in the quiescence and hydrodynamic conditions was investigated.
Corrosion studies was performed by potentiodynamic polarization, electrochemical impedance Spectroscopy (EIS), quantum chemical calculations, optical microscopy and rotating disk electrode (RDE) techniques.
Polarization experiment showed that the inhibition efficiency increased with increasing the inhibitor concentration and the inhibitor acted as mixed–type controls predominantly cathodic reaction in whole conditions. The thermodynamic calculations showed that the inhibitor obeys Langmuir adsorption isotherm. The increase in the inhibitor concentration and immersion time had a positive effect on inhibition efficiency while temperature had a negative effect. In the hydrodynamic conditions in 600 ppm (optimum inhibitor concentration), the most efficiency was at 1500 rpm rotation speed which was attributed to the enhance mass transfer of inhibitor to the metal surface and then decreased through the high shear stress. Theoretical Density function theory (DFT) calculations reveal that the unshared pairs of electrons on oxygen atoms in functional groups such as OH, C=O and also π electrons in double bonds in structures of three monomers are suitable centers to adsorb. Finally, optical images showed that the presence of inhibitor decreased the corrosion attack sites on the surface.
Acrylic copolymer (methyl methacrylate/butyl acrylate/acrylic acid was synthesized and it is suitable inhibitor for mild steel in 1M H2SO4 solution in static conditions and increasing its concentration increases its inhibition efficiency.
In order to remove the undesirable scale and rust of metals, acidic solutions such as hydrochloric acid and sulfuric acid are the most common acids which are extensively used in the most industries, including chemical industries, petrochemical process, water cooling system, pickling system, boiler system, acid descaling industries, heat exchanger system, etc. (Schmitt 1984; Bentiss et al. 2007; Li et al. 2008; Abboud et al. 2006; Mernari et al. 1998; Singh et al. 1995; Muzaffer et al. 2008; Nable et al. 2008). Thus, in order to protect metal, using inhibitors for reduce the corrosion rate against the aggressive ions acid is one of the practical methods (Abd El-Maksoud and Fouda 2005; Wang 2001). Most of the well-known acid inhibitors are organic compounds containing nitrogen, oxygen and/or sulfur atoms, heterocyclic compounds, and π electron which cause improvement in the process inhibition efficiency (Liu et al. 2009; Ahamad et al. 2010; Foud and Ellithy 2009; Obot and Obi-Egbedi 2010; Soltani et al. 2010; Mahdavian and Ashhari 2010; Lowmunkong et al. 2010; Kertit and Hammouti 1996). The inhibition efficiency of the inhibitors is substantially defined based on their adsorption properties. According to studies which carried out organic compounds, it has been reported that the adsorption of organic inhibitors mostly depends on some physiochemical properties of the molecule. The existence of functional groups in the organic inhibitors causes steric effects and electronic density of donor atoms. Adsorption also depends on the present double bonds having π electrons in the inhibitors that cause interaction between π orbitals of the inhibitor with vacant d orbitals of the surface metal atoms which induces greater adsorption of the inhibitor molecules on the surface of mild steel (Bentiss et al. 1999; Lukovits et al. 1995; Quraishi and Sharma 2002). Organic molecules could be adsorbed on the metal surface by one of the four following mechanisms: (a) electrostatic interaction between charged surface of the metal and the charge of the inhibitor, (b) interaction of unshared electron pairs in the inhibitor molecule with the metal, (c) interaction of π electron with the metal, and (d) a combination of the (a to c) types (Bentiss et al. 1999; Naderi et al. 2009). In recent years, some organic polymers were accepted as corrosion inhibitors which have environment-friendly characteristics and they have hydrocarbon chain length, less solubility, and also do not have toxic properties. Thus, the green inhibitors serve as a source which is environment-friendly in the inhibition corrosion process (Ali and Saeed 2001; Umoren and Open 2009). There are few studies in literatures about the inhibitor performance of organic and inorganic inhibitors on corrosion metals and alloys such as copper in the hydrodynamic conditions and also the effects of hydrodynamic conditions on the inhibitor performance under laminar or turbulent flow (Geler and Azambuja 2000). When the rotating speed is sufficiently high, rotating disk electrode (RDE) could also generate transition and relatively turbulent flows. The hydrodynamic effects such as flow velocity and shear stress at the surface influence fouling. The high shear stress causes separation of the protective layer from the metal surface, and also, it can influence the inhibition performance in the corrosion process (Jiang et al. 2005; Tian and Cheng 2008).
The objective of this study is to investigate the influence of solution in quiescence and hydrodynamic conditions on the corrosion behavior of st52 steel in 1 M H2SO4 solution in the absence and presence of copolymer acrylic (methyl methacrylate/butyl acrylate/acrylic acid). The presence of the copolymer is emulsified in aqueous solutions. This water-based copolymer was already used as an inhibitor in simulated sour petroleum solution in stagnant and hydrodynamic conditions, as an inhibitor in hydrochloric acid media, and also as an adhesive commercial resin in coating industry, and no scientific research has carried out on inhibitive performance of this copolymer as a corrosion inhibitor in sulfuric acid media so far. In this work, electrochemical methods including potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) and quantum chemical calculations were used to model the structures of major constituents of the copolymer. These tests have been carried out in 0 (static), 500, 1,000, 1,500, and 2,000 (rpm) RDE in the absence and presence of 50, 200, 400, and 600 ppm inhibitor concentrations. The surface morphology of the polished mild steel in acid solution in the absence and presence of the inhibitor in the static and hydrodynamic conditions was investigated through optical microscopy imaging technique.
Preparation of copolymer inhibitor
The invasive solution of 1 M H2SO4 was prepared by dilution of 98% H2SO4 (Merck, Whitehouse Station, NJ, USA) using distilled water. The concentration range of the copolymer employed was varied from 50 to 600 ppm concentration.
Preparation of specimen
Chemical composition of mild steel samples
Electrochemical experiments and corrosion tests were carried out using an IVIUM potentiostat (IVIUM Technologies, Fernandina Beach, FL, USA) connected to a computer through a USB cable and a conventional three-electrode cell. The cell was equipped with a steel electrode as the working electrode (WE), and the counter and reference electrodes were platinum wire and saturated calomel electrodes (SCE), respectively. In order to establish a steady state from open circuit potential (OCP), the working electrodes were first immersed into the test solution for 30 min. The potentiodynamic polarization curves were recorded at a scan rate of 1 mV/s in the potential range from −150 to +150 mV relative to the OCP. For linear polarization resistance measurement, the potential of the electrodes was scanned from −15 to +15 mV around OCP with 0.5 mV/s scan rate. The EIS measurements were performed at corrosion potentials over a frequency range of 100 KHz to 0.1 Hz with AC signal amplitude perturbation of ±10 mV. The electrochemical measurements in the hydrodynamic conditions were measured using an electrode controller (model: AFMSRCE, PINE Instruments Co., Durham, NC, USA) to control the rotation speed in the range (500 to 2,000 rpm) in the absence and presence of the inhibitor of various concentrations. EIS Analyzer Software was used to fit the experimental results of EIS measurements using appropriate equivalent circuit. Each measurement data point as shown in the figures and tables was performed at least three times, and reproducibility was satisfactory.
Quantum chemistry analysis
In order to ascertain about molecular structures of monomers, quantum chemical study was carried out. The molecular structures of monomers were drawn using the Gauss view 5.0. The optimized structures of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the three monomers were geometrically calculated by density functional theory (DFT) method using B3LYP level and 3-21G* basis set with Gaussian 98 software.
Optical microscopy was employed to investigate the effect of the inhibitor on the corrosion attack morphology in the absence and presence of the inhibitor. The specimen surfaces were mechanically polished down to 0.05 μm alumina slurry both in static and hydrodynamic conditions. The specimens were immersed in 1 M H2SO4 solution in the absence and presence of the inhibitor optimum concentration for 120 and 20 min in static and hydrodynamic conditions at room temperature, respectively. After bringing out from the solution, each specimen was washed and cleaned by ethanol and then dried with flow of dry air.
Results and discussion
Effect of inhibitor concentration
Potentiodynamic polarization study
Polarization parameters and the corresponding inhibition efficiency of mild steel corrosion in 1 M H 2 SO 4 containing different concentration of inhibitor at 25 ± 2°C
Ecorr(mV vs. SCE)
Electrochemical impedance spectroscopy study
Electrochemical parameters of impedance for mild steel in 1 M H 2 SO 4 solution in the inhibitor different concentrations at 25 ± 2°C in static conditions
where Roct and Rct are the charge transfer resistance values of mild steel in the absence and presence of the inhibitor, respectively. As is observed, the inhibitor efficiency value in optimum concentration 600 ppm by charge transfer resistance is in close correlation with those obtained from polarization results.
Polarization resistance study
The polarization resistance obtained values of mild steel in 1 M H2SO4 solution in the absence and presence of the inhibitor were listed in Table 3. Appraisal of Table 3 data shows that the Rp values were increased with the increase in the inhibitor concentrations; it can be explained that the adsorption of the inhibitor is effective, and it inhibits corrosion by blocking the active sites of the metal surface. As seen, the results of LPR confirm the inhibitive behavior of the copolymer, and it is in agreement with the results obtained from potentiodynamic polarization and EIS measurements.
Effect of immersion time
Electrochemical parameters of impedance of mild steel in 1 M H 2 SO 4 solution in time immersion conditions in the presence of optimum concentration of the inhibitor at 25 ± 2°C
Effect of temperature
Polarization parameters and the corresponding inhibition efficiency of mild steel corrosion in 1 M H 2 SO 4 in the presence of optimum concentration of inhibitor at different temperatures
Ecorr(mV vs. SCE)
Thermodynamic parameters for adsorption of inhibitor on mild steel surface in 1 M H 2 SO 4 solutions
Thermodynamic calculations of the inhibitor adsorption
ΔH0ads and ΔS0ads values are given in Table 6 by using Equations 11 and 12 which can calculate the values of thermodynamic parameters for the adsorption of inhibitor and also can obtain many information about the mechanism of corrosion inhibition. For instance, an endothermic adsorption process (ΔH0ads >0) is related to chemisorptions; an exothermic adsorption process (ΔH0ads <0) may be attributed to physisorptions, chemisorptions, or a mixture of both. In other words, in the enthalpy for the physisorptions process, it is less than 40 kJ mol−1. Meanwhile, the enthalpy for the chemisorptions process is nearly 100 kJ mol−1 (Noor 2009; Bentiss et al. 2005). In this work, the value of ΔH0ads indicates chemisorption mechanism that is in agreement with the results obtained by the activation parameter (Ea) and, also, the value of ΔS0ads is larger than zero (ΔS0ads >0) which means that the process is a substitution process which can be related to the increase in water molecule desorption entropy (Solmaz et al. 2008; Li et al. 2009). It could also be interpreted with the increase of disorders due to the more water molecule which can be desorbed from the metal surface by one inhibitor molecule (Avci 2008; Solmaz et al. 2008).
Effect of hydrodynamic conditions
Potentiodynamic polarization study
Electrochemical potential polarization parameters for mild steel corrosion in the absence and presence of optimum concentration of inhibitor at different rotation speeds
Rotation speed (rpm)
Ecorr(mV vs. SCE)
In the hydrodynamic conditions, the flow can increase mass transport of inhibitor molecules that causes more inhibitor presence at the metal surface. This effect can improve the inhibitor performance.
Hydrodynamic conditions can increase mass transport of metal ions (Fe2+), produced during metal dissolution from electrode surface to the bulk of solution and hence lead to less [Fe2+-Inh] complex in the presence of the electrode; this is a harmful effect for inhibition performance.
The high shear stress resulted from high flow velocity can also separate inhibitor layer or adsorbed [Fe2+-Inh] complex and cause more desorption from the metal surface which acts as a negative factor on inhibition efficiency.
The balance of the abovementioned effects lead to changes in the surface coverage, θ, and inhibition efficiency (η%) with rotation speed.
Electrochemical impedance spectroscopy study
Electrochemical parameters of impedance for mild steel corrosion in absence and presence of optimum concentration of inhibitor at different rotation speeds
Rotation speed (rpm)
Corrosion attack morphology study
Quantum chemical study
Quantum chemical parameters of monomers calculated by DFT method
Copolymer acrylic (methyl methacrylate/butyl acrylate/acrylic acid) was synthesized, and it is a suitable inhibitor for mild steel in 1 M H2SO4 solution in static conditions, and increasing its concentration increases its inhibition efficiency.
Potentiodynamic polarization studies showed that the inhibitor act as a mixed-type inhibitor.
The results of EIS on mild steel in the solution containing the inhibitor in static conditions showed that the charge transfer resistance increases and the surface capacitance decreases. This is indeed an indication of inhibitor adsorption onto the mild steel surface.
The adsorption of the inhibitor obeys Langmuir's adsorption isotherm, and thermodynamic data extracted showed both anodic and cathodic adsorption.
The results of polarization and EIS measurements for mild steel in the inhibited solution in hydrodynamic conditions showed that maximum inhibition efficiency was at 1,500 rpm electrode rotation rate that could be attributed to enhance mass transfer of inhibitor species from the bulk solution to the metal surface. Beyond 2,000 rpm, the inhibition efficiency decreased that could be attributed to interference of the high shear stress which causes separation and dislodgement of the inhibitor protective layers from the metal surface.
Quantum chemical calculations of EHOMO and ELUMO showed that the EHOMO for butyl acrylate is higher than other monomers and ELUMO for methyl methacrylate is lower than other monomers. It also revealed that C = C double bond and unshared pair of electrons on O in C = O group and -OH group could be suitable centers for adsorption on the metal surface.
Optical microscopy obviously showed that there are severe corrosion and roughness on the surface of metal in the absence of inhibitor both in static and hydrodynamic conditions compared with those in the inhibited solutions.
The authors appreciate the Materials and Metallurgical Engineering Department, Faculty of Engineering, Khorasan Razavi, Neyshabur; Science and Research branch, Islamic Azad University, Neyshabur, Iran; Materials and Polymers Engineering Department, Hakim Sabzevari University; and Sabzevar University for providing the organic compound used in this research and laboratory facilities during this study.
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