HUNGARIAN JOURNAL OF INDUSTRY AND CHEMISTRY Vol. 49(1) pp. 59–69 (2021) hjic.mk.uni-pannon.hu DOI: 10.33927/hjic-2021-08 POLYVINYLPYRROLIDONE AS A CORROSION INHIBITOR FOR CARBON STEEL IN A PERCHLORIC ACID SOLUTION: EFFECT OF STRUCTURAL SIZE ABBES BENCHADLI1, TARIK ATTAR*1,2, BOULANOUAR MESSAOUDI1,2, AND ESMA CHOUKCHOU-BRAHAM1 1Laboratory of Toxicomed, University Abou Beker Belkaid Tlemcen, BP 119, 13000 Tlemcen, ALGERIA 2Higher School of Applied Sciences Tlemcen, BP 165 RP Bel horizon, 13000 Tlemcen, ALGERIA Polymers are materials composed of macromolecules characterized by duplicates of smaller molecules that are covalently bonded together to provide a set of properties. Corrosion inhibition by such compounds is usually attributed to their adsorption on the metal-solution interface. The inhibition effect of different sizes of polyvinylpyrrolidone (PVP) on the corrosion of carbon steel (C-steel) in solutions of perchloric acid was investigated. The inhibition efficiency increases as the size of the inhibitor and its concentration increases, but decreases as the temperature increases and can reach a value of 81.53 % and 5.0×10−3 mol L−1 (PVP: 58,000 g mol−1) at 30 ◦C. The most remarkable inhibition efficiency was confirmed by the presence of the film formed on the metal surface by scanning electron microscopy. The kinetic and thermodynamic parameters for the corrosion of C-steel and adsorption of the inhibitor were determined and discussed. The combination of PVP with potassium iodide produced a strong synergistic effect on the inhibition of C-steel corrosion leading to a significant improvement in the inhibition efficiency. Quantum chemical parameters were studied using density functional theory to determine the possible relationship between the inhibitor and its electronic properties. Keywords: Polyvinylpyrrolidone, Weight loss, Synergistic effect, SEM, DFT 1. Introduction Despite its relatively limited corrosion resistance, car- bon steel (C-steel) is widely used in chemical process- ing, nuclear and fossil fuel power plants, the oil and gas industries, construction and metal-processing equipment, mining, pipelines, transportation and marine applications. The corrosion of C-steel in acidic media is important be- cause of its high mechanical properties and low cost [1]. Levels of corrosion increase rapidly after the failure of the preventive barrier and are followed by a number of reactions that change the properties as well as composi- tion of the metal surface [2]. Generally speaking, a corrosion inhibitor is a com- pound that reduces or stops the rate of metal dissolution whenever introduced or added in small quantities to a corrosive environment by producing a preventive barrier film [3]. Moreover, a good corrosion inhibitor presents many characteristics such as high solubility, non-toxicity, biodegradability, stability, short- and long-term durabil- ity and low cost. The inhibition efficiency is often re- lated to different variables like the structural chemistry and amount of the inhibitor in the medium, the nature of the metal and aggressive electrolyte as well as its pH, *Correspondence: att_tarik@yahoo.fr temperature, and immersion time [4, 5]. Organic inhibitors mostly contain triple bonds, aro- matic rings, and heteroatoms such as O, N, and S which are active centers for the adsorption process on the metal surface [6]. These atoms are characterized by higher basicity and electron density, thus act as corro- sion inhibitors. Organic compounds behave as effective inhibitors due to their ability to be adsorbed onto the metal surface. Green corrosion inhibitors are drawing much attention in the field of corrosion because of their biodegradability, safety, renewability and ecological ac- ceptability [7]. The mechanism of adsorption can be ei- ther physical or chemical as a result of the protective layer or adsorbed blanketing on the metal surface [8]. Chemical adsorption implies charge transfer or charge sharing from the adsorbate to the atoms of the metal surface in order to form a coordinate bond. The free energy of chemisorp- tion is higher than that of physisorption. The latter type is due to electrostatic attraction between the inhibiting or- ganic ions or dipoles and the electrically charged surface of the metal. The use of polymers as corrosion inhibitors has at- tracted considerable attention recently because of their ability to form complexes with metal ions on the elec- trode surface. These complexes occupy a large surface, https://doi.org/10.33927/hjic-2021-08 mailto:att_tarik@yahoo.fr 60 BENCHADLI, ATTAR, MESSAOUDI, AND CHOUKCHOU-BRAHAM thus blanketing the metal surface and protecting it from corrosive agents present in the solution [9, 10]. The ap- plications of polyvinylpyrrolidone (PVP) are extensive in pharmaceutical, medicinal, cosmetics and industrial products [11]. It is used in food additives, adhesives, membranes, ceramics, paper, coatings and personal care products, e.g., toothpastes and shampoos, as well as in batteries, paints, environmental applications, etc. [12]. The PVP added to iodine (I2) forms a complex called povidone-iodine (PVP-I) that is widely used in various products like liquid soaps, ointments, surgical scrubs and pessaries due to its antiseptic properties or as an antimi- crobial and antibacterial agent in medical devices [13]. This complex is commonly known under the trade names of Wokadine, Pyodine, or even Betadine. Quantum chem- istry calculations have been extensively used to explain experimental phenomena. They have proven to be very effective in evaluating the corrosion inhibition efficiency [14]. The conceptual density functional theory (CDFT) has been used to analyze the molecular activity of in- hibitors [15]. The aim of the present work is to determine the effect of the size of polyvinylpyrrolidone compounds and their protection efficiencies on the corrosion of carbon steel in a solution of 1 M perchloric acid by weight loss methods. The synergistic corrosion inhibition effect between PVP and potassium iodide (KI) was also investigated and the steel surface examined by scanning electron microscopy (SEM). Afterwards, quantum chemical calculations using DFT were performed to elucidate the interaction between the main constituents of PVP and the carbon-steel sur- face. 2. Experimental 2.1 Material preparation Four forms of PVP polymers with molecular weights of 8,000, 29,000, 40,000 and 58,000 g mol−1 and the vinylpyrrolidone monomer were purchased from Sigma- Aldrich. The blank solution was 1 M perchloric acid prepared by diluting a concentrated solution of HClO4 (Sigma-Aldrich). The tested samples employed in this work and the chemical composition of C-steel are shown in Table 1. 2.2 Preparation of the test sample Before conducting any tests, the samples of C-steel were mechanically abraded using different grades of emery pa- per, namely 100, 400, 600, 800, 1000, and 1200, before being washed with distilled water, degreased with ace- tone, air-dried then dipped into the corrosive or/and anti- corrosive medium. These samples were used to study the loss in weight. 2.3 Weight loss technique Weight loss techniques provide more realistic results with regard to uniform corrosion than electrochemical tech- niques because the experimental conditions simulate real- life conditions. Weight loss measurements of the C-steel samples were calculated in the solution of perchloric acid with and without the addition of different concen- trations and molecular weights of polyvinylpyrrolidone. Loss in the weight of the specimen was resolved by mea- suring the difference in weight between the carbon-steel substrates before and after being placed in the acidic medium. Each sample was weighed by an electronic bal- ance (±0.0001 g) before being placed in the acid solution (50 mL). The immersion time was 2 h within the temper- ature range of 293 − 333 K. The experiments were each performed three times and the average weight loss noted [16]. The corrosion rate (CR) (mg cm−2 h−1) was deter- mined by [17] CR = (mb −ma)/tS, (1) where mb and ma denote the weight losses (mg) before and after being immersed in the acidic solution, respec- tively, S stands for the total surface area of the specimen (cm2), and t represents the immersion time (h). The inhi- bition efficiency (IE) (%) was calculated using [17] IE = 100 (CR′ − CR)/CR′ (2) The degree of surface coverage (θ) which represents the part of the metal surface covered in inhibitor molecules was calculated using [17] θ = 1 − CR/CR′, (3) where CR′ and CR denote the corrosion rates of C-steel samples in the absence and presence of polyvinylpyrroli- done, respectively. 2.4 Synergism between inhibitors and iodide ions One potential method to cost-effectively increase the cor- rosion inhibition efficiency is to use the concept of syn- ergism and employ a combination of inhibitors [18]. The synergism phenomenon in terms of corrosion inhibition results in an improvement in the capacity of the inhibitor Table 1: Chemical composition of C-steel (wt. %). C Mn Cu Cr Ni Si S Ti Co Fe 0.370 0.680 0.160 0.077 0.059 0.023 0.016 0.011 0.009 Bal Hungarian Journal of Industry and Chemistry POLYVINYLPYRROLIDONE AS A CORROSION INHIBITOR FOR CARBON STEEL 61 to resist corrosion in the presence of secondary com- pounds in the corrosive medium [19]. The presence of halide ions in acid media containing organic inhibitors has been found to stabilize the adsorption of organic cations, which leads to an increase in inhibition effi- ciency [20]. It has been shown that the synergistic ef- fect of halide ions increases in the following order: chlo- ride (Cl−) < bromide (Br−) < iodide (I) [21]. Iodide ex- hibited a higher synergistic effect compared to the other halide ions because of its large size and ease of polariza- tion [22]. Therefore, the purpose of this study is to in- crease the inhibition efficiency by using iodide ions in a similar way to the synergistic effect with inhibitors tested with regard to the corrosion of C-steel in a solution of 1 M perchloric acid. In order to determine the synergistic effect between inhibitors and iodide ions, the synergistic parameter (Sθ) was calculated from [23] Sθ = (1 − θPVP+KI)/(1 − θ′PVP+KI) (4) and θPVP+KI = (θPVP + θKI) − (θPVP × θKI), (5) where θPVP denotes the surface coverage of the PVP in- hibitor, θKI stands for the surface coverage of KI, and θ′PVP+KI represents the combined surface coverage of PVP and KI. If Sθ is less than unity, this indicates an antagonis- tic effect which may lead to competitive adsorption and when Sθ approaches unity, the inhibitor compounds stop interacting with each other, whereas when Sθ is greater than unity, a synergistic effect exists between the selected inhibitors [24]. 2.5 Scanning electron microscopy (SEM) The surface morphology of the specimens of carbon steel was studied by SEM using a Hitachi TM1000 Tabletop scanning electron microscope at a scale of 100 µm and magnification of ×1.0K. The samples were immersed in a solution of 1 M HClO4 and the effect of the inhibitors in- vestigated. After 24 h of immersion under optimum con- ditions, the SEM images of both the polished specimens of carbon steel and those immersed in perchloric acid in the presence and absence of inhibitors were captured. 2.6 Theoretical calculations The DFT method using the B3LYP/6-31G* approach was employed to test the quantum chemical calculations of the inhibitor using the Gaussian 09 program package. The values of the highest molecular orbitals (EHOMO) and the lowest occupied ones (ELUMO) were calculated. Other parameters such as electronegativity (χ), softness (σ), global hardness (η), electron affinity (A) and the ion- ization potential (I) were determined by Koopmans’ the- orem [25]. The HOMO energy is related to the ionization poten- tial (I), whereas the LUMO energy is dependent on the electron affinity (A), as is shown in [26] A = −ELUMO , I = −EHOMO. (6) The energy gap (∆E) and global electronic chemical po- tential (µ) are determined from ∆E = ELUMO − EHOMO (7) and µ = 1 2 (ELUMO + EHOMO), (8) respectively [27, 28]. Using the electron affinity (A) and ionization potential (I), the electronegativity (χ) and global hardness (η) can be calculated from χ = 1 2 (I +A) (9) and η = 1 2 (I −A) (10) respectively [29]. Global softness (σ) demonstrates the capacity of an atom or group of atoms to receive electrons and is estimated from [30] σ = 1/η. (11) The electrophilicity index (ω) is calculated from [31, 32] ω = µ2/2η. (12) The number of electrons transferred (∆N ) was calculated from [33] ∆N = (χFe − χInh)/2(ηFe + ηInh), (13) where χFe and χInh denote the absolute electronegativ- ities of iron and the inhibitor, while ηFe and ηInh stand for the absolute hardness of iron and the inhibitor, respec- tively. The theoretical values (χ = 7 eV mol−1 and η = 0 eV mol−1) for iron were taken from the literature [34]. 3. Results and discussion 3.1 Effect of the acid medium Acidic solutions have been widely applied in industry, e.g., in the removal of mill scale from metal surfaces, acid pickling, acid descaling, acidizing oil wells and in- dustrial acid cleaning. In the oil industry, the acidization of petroleum oil wells, an important stimulation tech- nique, is used to enhance production [35]. HCl, HClO4 and H2SO4 are the most applied acidic solutions because of their highly corrosive nature with regard to many met- als and alloys even at low concentrations [36], whereas HNO3 and H3PO4 are used occasionally. In this study, the inhibition efficiencies of polyvinylpyrrolidone are mainly applied in different acid media such as perchloric acid, 49(1) pp. 59–69 (2021) 62 BENCHADLI, ATTAR, MESSAOUDI, AND CHOUKCHOU-BRAHAM Figure 1: Variation in inhibition efficiency of carbon steel in different acid media (PVP: 58, 000 g mol−1 with 1.0×10−3 mol L−1 of acid medium for 2 h at 303 K). hydrochloric acid, sulfuric acid, phosphoric acid and ni- tric acid. According to Fig. 1, the best inhibition effi- ciency (80.55 %) is obtained in perchloric acid compared to hydrochloric acid (67.74 %), sulfuric acid (61.71 %), phosphoric acid (34.68 %) and nitric acid (18.01 %). This could be due to the beneficial adsorption of perchlorate ions on the C-steel surface. The inhibitor molecules strongly interact with the metal surface in the presence of perchlorate ions due to the low coordination capacity of perchlorate ions. How- ever, the coordination of chloride, sulfate, phosphate and nitrate ions with the metal surface render it less able to ad- sorb the inhibitor molecules. In this work, the study was conducted exclusively in perchloric acid because of the better adsorption of PVP in this acid compared to other acids that were tested. 3.2 Effect of concentration and molecular weight The inhibition effect of PVP on the corrosion of carbon steel at 303 K in 1 M HClO4 as the corrosive medium was evaluated by a weight loss method. The effect of different molecular weights, namely the monomer and polymers of 8,000, 29,000, 40,000, and 58,000 g mol−1, on the in- hibition efficiency (IE) was studied. The dependence of the inhibition efficiency on the concentration as well as on the different molecular weights of PVPs is presented in Fig. 2. The improvement in the inhibition efficiency compared to the polymer concentration and molecular weight of PVPs suggests that more molecules are re- quired to cover the metal surface when the concentration of the inhibitor is low. No further increase in IE beyond 10−3 mol L−1 of different molecular weights of PVP is attributed to the saturation of the adsorption process. The experimental results prove that the PVP with a molecular weight of 58,000 g mol−1 is more effective Figure 2: Variation in the inhibition efficiency against con- centration in a 1 M HClO4 solution, using various molec- ular weights of the inhibitor. in inhibiting the corrosion of C-steel in the aggressive medium than any other molecular weight of PVP at all concentrations. The monomer presents the least corrosion inhibition at all the concentrations studied and reaches a maximum efficiency of 21.81 % at 5×10−3 mol L−1. 3.3 Effect of immersion time The study of this parameter was conducted in order to avoid any confusion between the phenomenon of passi- vation which could occur if the dive time is very long and the inhibition rate of the inhibitors. The effect of the im- mersion time for all the inhibitors tested with regard to IE is shown in the results presented in Fig. 3. The inhibition efficiency is calculated from 1 to 24 hours in uninhibited HClO4 and in the presence of the optimum concentration of the monomer (10−3 mol L−1) and different molecu- lar weights of PVP at 303 K. It can be concluded that the inhibition efficiency increases as the immersion time lengthens due to the stability of the adsorbed layer on the surface of C-steel up until 2 hours of immersion at different concentrations and molecular weights of the in- hibitors. The inhibition efficiency decreases from 80.55 % to 68.25 % after immersion times of PVP58000 of 2 hours and 24 hours, respectively. This may be explained by the desorption of inhibitor molecules from the C-steel surface and the instability of the inhibitor film on the metal surface [37]. 3.4 Effect of temperature and activation en- ergy The effect of the temperature introduces many changes on the metal surface such as adsorption, desorption, re- arrangement or decomposition of the inhibitor. The ef- fect of the temperature and different molecular weights of PVP on the corrosion inhibition of C-steel in 1 M HClO4 Hungarian Journal of Industry and Chemistry POLYVINYLPYRROLIDONE AS A CORROSION INHIBITOR FOR CARBON STEEL 63 Figure 3: Variation in the inhibition efficiency against im- mersion time for carbon steel at various molecular weights of inhibition in a 1 M HClO4 solution. was studied between 293 and 333 K to determine the de- pendence of the inhibition efficiency on the temperature and corrosion behavior of C-steel in perchloric acid in the presence of inhibitors. According to the results presented in Table 2, at higher temperatures there is an appreciable decrease in the adsorption of the inhibitors on the metal surface leading to a rise in the corrosion rate. At a temperature of 60◦C, the polyvinylpyrrolidone with a molecular weight of 58,000 g mol−1 showed a maximum inhibition efficiency of 69.41 %, while that of the monomer was only 6.35 %. Those for the other molecular weights of PVP, namely 8000, 29000, and 40000 g mol−1, were equal to 56.55, 59.43 and 63.81 %, respectively, at the maximum tested concentration of 5×10−3mol L−1. An increase in the temperature of the solution reduces the inhibition effect by facilitating the counter-process of desorption [38]. The high rate of dis- solution of C-steel in a 1 M HClO4 solution observed at elevated temperatures could be attributed to an increase in the dissolution energy effect acquired by the corrosive agent within the aggressive medium. Moreover, the des- orption of the adsorbed inhibitor caused by enhanced ag- itation of the solution as a result of higher rates at which hydrogen gas evolves as the temperature rises is possible and can cause the capacity of the inhibitor to be adsorbed on the C-steel surface to be reduced [39]. For chemi- cal adsorption, the inhibition efficiency is expected to in- crease as the temperature rises, but for physical adsorp- tion, the inhibition efficiency is expected to decrease as the temperature increases [40]. The Arrhenius equation can be used to show the effect of the temperature on the inhibition performance of the studied compounds [41]: CR = A exp (−Eact/RT ) , (14) where CR denotes the corrosion rate of carbon steel, A stands for the pre-exponential factor of the Arrhenius equation, Eact (kJ mol−1) representsth e activation en- ergy, R refers to the gas constant (8.314 J mol−1 K−1) and T is the temperature (K). The activation enthalpy ∆Hact and entropy of activa- tion ∆Sact can be calculated by the following transition state equation [42]: log ( CR T ) = log ( R NAh ) + ∆Sact 2.3R − ∆Hact 2.3RT , (15) where R denotes the gas constant, NA stands for Avo- gadro’s number, and h represents Planck’s constant. A plot of log(CR/T ) against 1/T yields a straight line with a gradient of −∆Hact/2.3R and an intercept of (log(R/NAh) + ∆Sact/2.3R) from which the values of ∆Hact and ∆Sact can be calculated as is presented in Table 3. ∆Gact > 0, which means a non-spontaneous corro- sion reaction, the rate of which increases as the concen- tration of the inhibitor increases [42]. The entropies of activation were negative in the absence and presence of inhibitors implying that a decrease in disorder occurred as the reaction proceeded. The higher activation energy of the process in the presence of an inhibitor compared to in its absence is attributed to its physisorption, whereas the opposite is a result of its chemisorption [43]. The positive values of the activation enthalpy in the presence and ab- sence of different concentrations of the inhibitors reflect the endothermic nature of C-steel dissolution. This sim- ply means that the dissolution process of steel is difficult [44]. 3.5 Adsorption isotherm The adsorption isotherms provide information about the interaction between the inhibitor and C-steel surface [45]. The type of interaction is related to the adsorption of in- hibitor compounds as a result of chemisorption and ph- ysisorption [46]. It depends on many factors, e.g., the number of adsorption centers, mode of interactions with the metal surface, molecular weight and structure of the inhibitor. The data were used graphically using different isotherms such as Frumkin, Temkin and Langmuir. The Langmuir adsorption isotherm was found to best describe the adsorption. It can be seen that the results of the sur- face coverage in the presence of inhibitors are in good agreement with the weight loss technique (Eq. 3). The Langmuir adsorption isotherm model was employed us- ing [47] C θ = 1 Kads + C, (16) where θ denotes the surface coverage by the inhibitor molecules, C stands for the concentration, and Kads represents the equilibrium constant for the adsorption- desorption process. The value of the equilibrium constant obtained can be used to calculate the standard free energy by from [48] ∆Gads = −RT log(55.5Kads), (17) 49(1) pp. 59–69 (2021) 64 BENCHADLI, ATTAR, MESSAOUDI, AND CHOUKCHOU-BRAHAM Table 2: The percentage inhibition efficiency of C-steel in 1 M HClO4 using five inhibitors tested between 293 and 333 K. C (mol L−1) 293 K 303 K 313 K 323 K 333 K 5×10−5 6.25 4.22 3.1 2.25 1.50 7.5×10−5 8.33 8.05 5.63 4.55 3.42 Monomer 10−4 15.5 11.25 8.1 6.6 4.2 1×10−3 20.65 19.81 10.43 8.7 6.15 5×10−3 21.97 21.34 10.5 8.78 6.35 5×10−5 43.37 35.01 24.17 16.85 10.19 7.5×10−5 50.19 43.67 35.82 23.42 16.29 8,000 10−4 60.53 45.07 43.98 33.75 30.7 10−3 78.2 65.9 62.2 58.85 56.42 5×10−3 78.46 67.59 62.35 58.95 56.55 5×10−5 45.5 37.55 29.24 19.31 12.23 7.5×10−5 62.51 45.81 41.21 25.93 21.54 29,000 10−4 65.17 49.02 45.03 41.97 36.91 10−3 82.17 67.44 64.61 61.35 59.39 5×10−3 82.12 68.95 64.68 61.42 59.43 5×10−5 50.18 44.22 36.86 22.16 15.95 7.5×10−5 66.57 48.21 45.13 28.18 23.73 40,000 10−4 70.41 59.47 50.32 45.73 43.33 10−3 84.09 75.53 70.88 68.41 63.6 5×10−3 84.27 76.86 70.9 68.54 63.81 5×10−5 55.19 46.89 38.91 26.17 19.07 7.5×10−5 70.07 52.77 46.50 31.04 25.66 58,000 10−4 75.23 69.33 58.53 52.72 48.71 10−3 85.1 80.55 77.21 75.13 69.34 5×10−3 85.34 81.53 77.29 75.2 69.41 Table 3: Thermodynamic activation parameters of the dissolution of carbon steel in 1 M HClO4 in the absence and presence of different concentrations and molecular weights of the inhibitors by applying Arrhenius and transition state plots. Cinh Eact ∆Hact ∆Sact ∆Gact 303K (mol L−1) (kJ mol−1) (kJ mol−1) (J mol−1 K−1) (kJ mol−1) HClO4 1 58.79 56.2 −105.43 88.21 5×10−5 59.8 57.21 −102.48 88.26 7.5×10−5 59.81 57.22 −102.66 88.31 Monomer 10−4 61.08 58.48 −98.88 88.44 10−3 61.99 59.39 −96.27 88.55 5×10−3 62.05 59.46 −96.08 88.57 5×10−5 68.35 65.75 −77.44 89.21 7.5×10−5 69.69 67.1 −74.13 89.56 8,000 10−4 69.53 66.94 −75.84 89.91 10−3 71.74 69.14 −72.78 91.19 5×10−3 72.28 69.69 −72.16 91.55 5×10−5 68.61 66.01 −76.95 89.32 7.5×10−5 73.42 70.82 −63.04 89.92 29,000 10−4 69.61 67.02 −76.33 90.14 10−3 73.83 71.23 −66.83 91.48 5×10−3 74.07 71.47 −67.14 91.81 5×10−5 69.96 67.37 −73.26 89.57 7.5×10−5 74.95 72.38 −58.54 90.11 40,000 10−4 71.86 69.27 −70.24 90.55 10−3 74.45 71.86 −66.34 91.96 5×10−3 74.95 72.36 −64.86 92.01 5.0×10−5 71.07 68.47 −70.18 89.73 7.5×10−5 76.77 74.18 −53.27 90.32 58,000 10−4 74.21 71.61 −64.18 91.05 10−3 72.53 69.94 −74.07 92.38 5×10−3 73.15 70.55 −72.22 92.43 Hungarian Journal of Industry and Chemistry POLYVINYLPYRROLIDONE AS A CORROSION INHIBITOR FOR CARBON STEEL 65 Table 4: Thermodynamic parameters for the adsorption of five inhibitors on C-steel tested in a 1 M HClO4 solution at different temperatures. T R2 ∆Hads ∆Sads ∆Gads (K) (kJ mol−1) (J mol−1 K−1) (kJ mol−1) Monomer 293 0.999 28.76 −28.64 303 0.999 28 −28.7 313 0.999 −20.21 30.68 −29.82 323 0.999 29.93 −29.88 333 0.999 27.7 −29.44 293 0.999 −9.42 -34.39 303 0.999 −7.72 −34.81 8,000 313 0.999 −37.15 −8.79 −34.4 323 0.999 −8.42 −34.43 333 0.998 −9.13 −34.11 293 0.999 −4.44 −35.12 303 0.999 −3.99 −35.21 29,000 313 0.999 −36.42 −2.68 −35.58 323 0.999 −4.18 −35.07 333 0.998 −4.44 −34.95 293 0.999 1.09 −35.59 303 0.999 0.13 −35.31 40,000 313 0.999 −35.27 2.81 −36.15 323 0.999 0.15 −35.32 333 0.999 1.05 −35.62 293 0.999 −7.06 −36.2 303 0.999 −6.91 −36.18 58,000 313 0.999 −38.29 −6.17 −36.34 323 0.999 −7.74 −35.77 333 0.999 −6.67 −36.05 where R denotes the gas constant (8.314 J mol−1 K−1) and T stands for the absolute temperature (K). The con- stant value of 55.5 mol L−1 is the concentration of water in dilute aqueous solutions. The calculated changes in the enthalpy, entropy and free energy of adsorption are presented in Table 4. In this study, the negative values of the enthalpy of adsorption, which vary between −20.21 and −38.29 kJ mol−1, indi- cate that the adsorption of the inhibitors on the C-steel is an exothermic process. High values of ∆Gads show that the inhibitor is strongly adsorbed on the metal surface in the corrosive media. The negative values of the free en- ergy of adsorption indicate that the adsorption of the in- hibitors on the steel surface is spontaneous. The variation in ∆Gads between −29.06 and −36.34kJ mol−1 indi- cates that the adsorption of inhibitors on the carbon steel surface in a solution of 1 M perchloric acid at between 293 and 333 K is a mixture of chemical and physical ad- sorption. The determination of the free energy of adsorp- tion provides information about the strength of adsorption of the molecules. Generally speaking, values of ∆Gads between −40 and −20 kJ mol−1 are consistent with phys- ical adsorption, while those below −40 kJ mol−1 are in- dicative of chemical adsorption [49]. 3.6 Synergistic effect Synergistic inhibition is brought about by the combina- tion of PVP58000 and iodide ions for the corrosion of C-steel in 1M HClO4 as halide ions have a greater ten- dency to be adsorbed on the surface as a result of their attraction to organic cations. The relationship between the efficiency and concentration of each inhibitor added by potassium iodide at different temperatures is shown in Table 5. The obtained results in the present study clearly show that the inhibition efficiency was substantially en- hanced following the addition of KI and could occur be- cause of the existence of synergism between different concentrations of the tested inhibitor and iodide ions. The values of Sθ are higher than those for all con- centrations indicating that the interaction between PVP and KI is a synergistic effect. It is well known that the maximum efficiency reaches 97.18 % as a result of the synergistic effect, while for PVP in the absence of KI it is 85.34 %. The inhibition efficiency increased following the addition of potassium iodide ions due to the syner- gistic effect but decreased as the temperature rose. It has been concluded that more effective barrier films of in- hibitor molecules are produced on the surface of the sam- ple due to synergy, thus blocking the active sites on the steel surface to protect carbon steel from corrosion [50]. 49(1) pp. 59–69 (2021) 66 BENCHADLI, ATTAR, MESSAOUDI, AND CHOUKCHOU-BRAHAM Table 5: Synergism parameter (Sθ) for different concentrations of PVP58000 and in the presence of a solution of 5×10−6 M KI from weight loss measurements at 293, 313, and 333 K. C (mol/L) 293 K 313 K 333 K IE(PVP) IE(PVP+KI) Sθ IE(PVP) IE(PVP+KI) Sθ IE(PVP) IE(PVP+KI) Sθ 5×10−5 55.19 91.29 1.25 38.91 55.53 1.12 19.07 32.89 1.05 10−4 75.23 94.57 1.11 58.53 73.32 1.27 48.71 62.62 1.2 5×10−3 85.34 97.18 1.26 77.29 83.68 1.14 69.41 74.92 1.07 Figure 4: SEM images of carbon steel (a) before immersion, (b) after immersion in the acidic solution (1 M HClO4) in the absence of inhibitors, (c) after immersion in the acidic solution containing 5×10−3 M PVP58000, and (d): (c) + KI. Figure 5: (a) Optimized molecular structure, (b) HOMO and (c) LUMO of the protonated Polyvinylpyrrolidone molecule. 3.7 Scanning electron microscopy To confirm adsorption of the polyvinylpyrrolidone on the carbon-steel surface in the presence and absence of the inhibitor in 1 M HClO4, scanning electron microscopy experiments were carried out. Fig. 4a shows the surface morphology before corrosion testing. It is obvious that specimens immersed in the solution in the absence of PVP58000 and KI additives show significant degrada- tion of the surface and pitting corrosion is evident from the surface morphology as is shown in Fig. 4b. How- ever, in the presence of PVP, an obvious reduction in the degradation of the surface is shown. This amelioration can be attributed to the adsorption of PVP on the metal surface and the formation of a protective film, thereby isolating the carbon-steel surface (Fig. 4c). The addition of KI improves the degree of inhibition and the stability of the corrosion product on the steel surface as is shown in Fig. 4d. This indicates that the adsorption of iodide ions on carbon steel leads to the formation of more sta- ble films. The scanning electron microscopy (SEM) study confirmed that the corrosion inhibition of carbon steel and synergistic effect occur following the adsorption of inhibitor molecules on the metal surface. 3.8 Quantum chemical calculations The experimental study was completed by a theoretical study at the B3LYP/6-31G* level in order to correlate the results obtained experimentally with the molecular struc- ture and electrical properties of Polyvinylpyrrolidone as is presented in Fig. 5. The energies of the HOMO and LUMO, total energy (E), number of transferred electrons (∆N), softness (σ), electrophilicity index (ω) and global hardness (η) were calculated and are listed in Table 6: The energy gap between the HOMO and LUMO is another important descriptor that must be considered. In Table 6, it is shown that the PVP inhibitor has a higher EHOMO (−6.29 eV) energy and lower ELUMO (1.01 eV) energy as well as a small energy gap Ggap ∼ −7.3 eV between EHOMO and ELUMO. This strengthens its in- hibitory action on the C-steel surface. The electrophilic- ity index is another important parameter which shows the tendency of the molecule to accept electron(s). The charge transfer of 0.6 eV indicates that the PVP inhibitor Hungarian Journal of Industry and Chemistry POLYVINYLPYRROLIDONE AS A CORROSION INHIBITOR FOR CARBON STEEL 67 Table 6: HOMO and LUMO energies as well as the global reactivity indices µ, σ, ω, ∆N , and Gap for Polyvinylpyrrolidone compound at the B3LYP/6-31G* level of theory. EHOMO (eV) ELUMO(eV) µ (eV) η (eV) σ (eV−1) ω (eV) ∆N (eV) Gap (eV) −6.29 1.01 −2.65 3.65 0.27 0.96 0.6 7.3 is a strong electron donor [3]. Generally, if the fractions of electrons transferred are less than 3.6 eV, the inhibi- tion efficiency increases by increasing the electron donat- ing capacity on the metal surface [51]. Softness (σ) and chemical hardness (η) are important chemical properties to measure molecular reactivity and stability. Therefore, the chemical reactivity increases as the inhibition effi- ciency of adsorption rises. Actually, the molecule with the smallest chemical hardness should exhibit the great- est inhibition efficiency [52]. The PVP inhibitor exhibits a good degree of chemical reactivity on the metal surface due to the decrease in the chemical hardness (η = 3.65 eV) and increase in the softness (σ = 0.27 eV−1). 3.9 Inhibition mechanism Different techniques (Weight loss, SEM, and DFT) were used to determine the inhibitory effect of differ- ent molecular weights of Polyvinylpyrrolidone on the corrosion of C-steel in a solution of perchloric acid. PVP58000 yielded the highest corrosion inhibition, while the monomer exhibited the lowest. The results showed that inhibition efficiencies on carbon steel increase as the concentration of the inhibitors (PVPs) rises and were en- hanced following the addition of potassium iodide (KI) due to synergism. It is well known that iron exhibits co- ordinate affinity towards nitrogen-, sulfur- and oxygen- bearing ligands [53]. In this study, the electron pairs of oxygen and nitrogen atoms are responsible for chemical bonding to the C-steel surface. 4. Conclusion The obtained results showed that Polyvinylpyrrolidone is effective in the presence of perchloric acid. The studied compounds exhibited good corrosion inhibition perfor- mances of PVP (58,000 g mol−1) on C-steel in perchloric acid and its activity increased as the concentration of the inhibitor rose, while the efficacy decreased as the tem- perature increased. The thermodynamic adsorption pa- rameters show that the studied inhibitors are adsorbed on the C-steel surface following an exothermic, sponta- neous process. Inhibition is achieved by the adsorption of the molecules on the C-steel surface and follows the Langmuir isotherm. 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