Anticorrosive Effect of PVP 40000 Against AISI 1020 Carbon Steel in HCl

Polymers have received considerable attention for replacing synthetic inorganic and organic anti-corrosion inhibitors since they are not as toxic for the environment, besides being cheaper. Polyvinylpyrrolidone (PVP) has been studied due to its applicability, solubility, very low toxicity, cost and efficiency in corrosion inhibition against metals, such as zinc, aluminum, stainless steel, and others. Its anti-corrosion activity with 40000 g mol-1 was tested against AISI 1020 carbon steel in 0.5 mol L-1 HCl, and evaluated by electrochemical techniques: Potentiodynamic Polarization (PP), Linear Polarization Resistance (LPR), and Electrochemical Impedance Spectroscopy (EIS). The maximum efficiency was 90% and 89% in EIS and LPR, respectively, at concentration of 1.0 x 10-7 mol L-1. The adsorption mechanism was coherent with Langmuir isotherm model. The morphology of the corroding carbon steel surface, in the presence and absence of the PVP, was visualized using Scanning Electron Microscopy (SEM).


Introduction
Mild steel is widely used in a diversity of industries, such as chemical processing, petroleum production and refining, and marine applications, including the exposure in acidic media for pickling, and in saline media in the case of offshore operations.However, the deterioration of mild steel must be attenuated by various anticorrosive agents, including organic corrosion inhibitors.Organic compounds containing heteroatoms in their structure, such as nitrogen, oxygen and sulfur, are able to act as efficient inhibitors for some metals since they can form protective films on the surface 1-3 .The use of polymers in solution as corrosion inhibitors has been widely studied in the last years, due to its low cost and good inhibition efficiency [1][2][3][4][5][6] .
Recently, green corrosion inhibitors have been receiving special attention for the replacement of synthetic inorganic and organic substances with high environmental toxicity, at very expensive cost.Several polymers were investigated years ago, including the polyvinylpyrrolidone (PVP) due to its easy applicability, solubility, very low toxicity, cost and corrosion inhibition efficiency 3 .There are many studies involving the corrosion inhibition of several metals in solution, such as zinc, iron, carbon steel and stainless steel.
Corrosion inhibition of carbon steel was also studied by Umoren, 2011, who used PVP 44000 (1.0 x 10 -4 mol L -1 ) in 0.5 mol L -1 H 2 SO 4 , thus observing inhibition efficiency of 67% 10 .Al Juhaiman et al., 2012, reported the anticorrosion efficiency of PVP 8000 in 2.5 x 10 -4 mol L -1 against carbon steel in 0.1 mol L -1 NaCl; 44% was the maximum efficiency 2 .The most recent study about PVP 8000 as an anti-corrosion agent against carbon steel has been done by Al Juhaiman, 2016, in 2 mol L -1 HCl showing a maximum efficiency of 89.5% in 1.0 x 10 -3 mol L -14 .
Given the great interest in the study of PVP as an anticorrosion agent, and the lack of information on middle steel in HCl medium, this paper aims to describe the study of PVP 40000 against AISI 1020 carbon steel in 0.5 mol L -1 HCl, using electrochemical techniques, like Potentiodynamic Polarization (PP), Linear Polarization Resistance (LPR) and Electrochemical Impedance Spectroscopy (EIS).Moreover, the scanning electron microscopy (SEM) was used to investigate anti-corrosion activity.

Materials
Polyvinylpyrrolidone (PVP) was purchased from Sigma Aldrich Co., and its molecular weight was 40000 g.mol -1 .PVP chemical structure can be observed in Figure 1.Ethanol used as solvent was purchased from Vetec/Sigma-Aldrich (Rio de Janeiro, Brazil), and hydrochloric acid, from Proquimios Com.& Ind. Ltda.(Rio de Janeiro, Brazil).
Tests were carried out with a conventional electrochemical three-electrode cell, which consists of a platinum auxiliary electrode, a silver-silver chloride (Ag/AgCl, 3.0 mol L -1 KCl) reference electrode, and the AISI 1020 carbon steel used as a working electrode with composition (%wt): C: 0.17, P: 0.04, S: 0.05, Mn: 0.30, and the remainder Fe.The working electrode was prepared with epoxy resin that presented an exposed area of 0.8 cm 2 .Before each measurement, the working electrode was abrading using three emery papers with different grades: 400, 600 and 1200.After that, it was washed with water and acetone and dried 11 .

Electrochemical procedure
All experiments were carried out in 40 mL solutions of naturally aerated electrolyte at 25 °C, and all electrochemical measurements were performed using an Autolab Potentiostat/ Galvanostat model PGSTAT 302N, and analyzed using NOVA 1.9 software.The electrochemical measurements were conducted after 1h immersion in experimental solution to attain a stable open circuit potential (OCP).After that, three electrochemical techniques were applied: Electrochemical Impedance Spectroscopy (EIS), Linear Polarization Resistance (LPR), and Potentiodynamic Polarization (PP).

Electrochemical Impedance Spectroscopy (EIS)
Impedance measurements were performed over a frequency range of 10 kHz -100 mHz with a 10 mV peak-to-peak amplitude using AC signal.Inhibition efficiency (IE) was calculated using the following equation 12 : (1) where R ct is the charge transfer resistance in the presence of the inhibitor, and R 0 ct is the charge transfer resistance in its absence.These values were obtained from the electrochemical impedance diagrams.

Linear Polarization Resistance (LPR)
Linear Polarization Resistance experiments were performed using a scan rate of 1 mV s -1 in the potential range of ± 10 mV, around the open circuit potential (E ocp ).Inhibition efficiency was calculated using the following equation [12][13][14][15][16] : (2) where R p and R 0 p are the Polarization Resistance in the presence and absence of the inhibitor, respectively.The Polarization Resistance was obtained by the following equation: These values were determined by the graph of the current (i) vs the potential (E), where R p is the slope of the line.

Potentiodynamic Polarization (PP)
Potentiodynamic anodic and cathodic polarization curves were performed using a scan rate equal to 1 mV s -1 , from -200 mV up to +200 mV, around the open circuit potential.Inhibition efficiency was calculated using the following equation [12][13][14][15][16] : where i corr,0 and i corr are the Polarization Resistance in the absence and presence of the inhibitor, respectively.

Scanning Electron Microscopy (SEM)
Carbon steel samples, dimension 4 cm x 4 cm x 0.1 cm, were abraded with 280, 600 and 1200 grade emery paper; washed with water and ethanol; dried and immersed in 0.5 mol L -1 HCl in the absence (Blank) and presence of PVP 40000 for 3 h at room temperature; and, naturally aerated.The specimens were removed, washed with water, ethanol, dried and weighted in analytical balance (SHIMADZU, model ATX224).The measurements were taken using HITACHI TM 3000 Tabletop Microscope 16 .

Open Circuit Potential
The OCP plots can be observed in Fig 2, the OCP values were stabilized after 40 minutes, but OCP was measured until 60 minutes.After the OCP stabilized, the EIS, LPR and PP tests were carreid out against carbon steel AISI 1020 in HCl 0.5 mol L -1 in the presence and absence of PVP.

Potentiodynamic Polarization (PP)
Figure 3 presents the Potentiodynamic Polarization curves of carbon steel in 0.5 mol L -1 HCl solutions, with and without PVP 40000, at five concentrations; they determined the values to plot the graphs of log i versus E. At that stage, the Potentiodynamic Polarization technique was used so that the behaviour of the current density, in the presence and absence of the inhibitor, could be observed and the inhibitor type could be characterized.
As observed in Figure 3, there is a decrease in the current density (i) when PVP concentration increases in the cathodic region, and more remarkably, in the anodic one; with a great E corr shift in the anodic direction.These results indicate that PVP 40000 acted as a mixed-type inhibitor, which could slow down both the metal dissolution and the cathodic process 16 .The electrochemical parameters are shown in Table 1.

Linear polarization resistance (LPR)
Linear Polarization Resistance to carbon steel in HCl 0.5 mol L -1 were evaluated in the presence and in the absence of PVP 40000.Equation 3shows the graph of values obtained for the current density (i) versus applied potential (E), which provided the slope of the lines corresponding to R p values by linear regression.Table 2 shows the values of R p and of

Electrochemical impedance spectroscopy (EIS)
Figure 4 shows the Nyquist plots related to the measurements made on carbon steel in acid media 0.5 mol L -1 HCl, in the presence and the absence of PVP at different concentrations The fitted curves obtained by the [R(RQ)] circuit can also be observed in the Figure 4.As shown in Figure 4, the semicircle increases along with the concentration, thus resulting in the rise of the inhibition efficiency.This indicates that the corrosion of carbon steel in acidic medium is controlled by a charge-transfer process 8 .
The intersection of the semicircle and the real axis at high frequencies provides the solution resistance (R s ) value, whereas at low frequencies, a charge-transfer resistance (R ct ) was found from the difference in impedance at lower and higher frequencies.
According to the above results, the EIS data were analyzed using the [R(RQ)] equivalent circuit (Fig. 5) that includes the solution resistance (R s ), polarization resistance (R p ) and constant phase element (CPE), which can be represented in equation 5 13,14,15 : (5)   where Y 0 is the magnitude of the CPE, n is the phase shift, j is the imaginary unit and w is the angular frequency.The constant phase element, CPE, was introduced in the circuit instead of a pure double layer capacitor to give a more accurate fit [16][17][18][19] .
The double layer capacitances,C dl , for a circuit including a CPE were calculated from the following equation 6 [17][18][19][20] : (6)   where Y 0 is the magnitude of the CPE, f max is the frequency at which the imaginary component of the impedance is maximal [17][18][19][20] .
The increase of R ct and the decrease of C dl indicates that PVP inhibits the corrosion rate of carbon steel by adsorption mechanisms 19 .The equivalent circuit fit (Figure 5) was obtained by EIS spectra.
Bode plots for carbon steel in acidic medium with and without PVP are shown in Figure 6.At low frequencies,

Z Y jw CPE
The negative value of free energy of adsorption revealed the spontaneity of its process; also, the higher the adsorption equilibrium constant, the more stable the adsorbed layer on the steel surface.The Langmuir isotherm describes that each inhibitor molecule is adsorbed on a distinct steel surface site; therefore, an adsorbed water molecule is replaced by an organic molecule on the steel surface.The adsorption of organic molecules occurs as a monolayer over the metallic surface without any interaction with other adsorbed molecules 18,19 .

Scanning Electron Microscopy (SEM)
Results obtained using the Scanning Electron Microscopy for carbon steel samples after immersion of 3 h in 0.5 mol L -1 HCl, in the presence and the absence (Blank) of PVP 40000 (at the concentration of 1.0x10 -7 mol L -1 ), can be observed in Figure 8.
The formation of pitting corrosion was clearly observed on the carbon steel surface in both cases, probably due to the presence of chloride ions 12 .However, pitting corrosion was attenuated in the presence of PVP, which might be related to the adsorption of the organic inhibitor on the surface that blocks the metal sites, thus avoiding chloride adsorption, and consequently, inhibiting pitting corrosion formation 16,24 .
Table 3. Electrochemical Impedance data for carbon steel in 0.5 mol L -1 HCl, at different PVP concentrations.

Inhibitors
Cinh (mol Z mod is a metric that can be used to compare the corrosion resistance at different concentrations of inhibitor, and its increase contributes to a better inhibitory performance 16,19 . Table 3 shows the values of corrosion inhibition efficiency (IE EIS ), resistance of charge-transfer (R ct ), inhibitor surface coverage (Ө) and double layer capacitance (C dl ).
In Table 3, it is noteworthy that the efficiency increases when the PVP concentration rises.An efficiency of 90% was observed at the concentration of 1.0 x 10 -7 mol L -1 by EIS.The lowest C dl value is observed at the maximum concentration of PVP; this can be associated with a better adsorption of the inhibitor in metallic surface, thus leading to the decrease of the electric double layer.
It is important to highlight that n LPR values obtained from LPR presented correlation with the values obtained by the EIS measurement (IE EIS ), in which the efficiency increase was observed along with the increase of PVP concentration, 89% and 90% at 1.0x10 -7 mol L -1 for IE EIS and IE LPR , respectively.

Adsorption isotherm
PVP adsorption mechanism over a metal surface was investigated, and the most frequent one was modelled by Langmuir equation ( 7) 11,16,19 .Figure 7 shows the plot for Langmuir isotherm model.(7)   where C inh is the inhibitor concentration, θ is the degree of surface coverage and K ads is the adsorption equilibrium constant.The standard free energy of adsorption (Δ G( 0 ads 0 ads ) was calculated by equation 8 11 .(8)   where R is the universal gas constant (8.31 J K -1 mol -1 ), T is the temperature (K), and 55.55 is the molar concentration (mol L -1 ) of water in the solution.
The thermodynamic parameters obtained in this study indicated K ads = 2.32 x 10 +9 L mol -1 and ∆G 0 ads = -59.18kJ mol -1 .The slope (1.09) and the linear correlation coefficient (0.9993) were close to one, thus characterizing the Langmuir isotherm model 11,16,[21][22][23][24] .The maximum inhibition efficiency value obtained in this work was 90%, by EIS, using 1.0x10 -7 mol L -1 PVP 40000 in 0.5 mol L -1 HCl.No studies with PVP 40000 had been found in literature before; however, it is noticeable that at small concentration, PVP 40000 shows higher inhibition efficiency when compared with the ones reported in literature even though they are under distinct conditions 2,4,8,10 .

Conclusion
PVP 40000 acted as a good corrosion inhibitor of AISI 1020 carbon steel in HCl 0.5 mol L -1 , showing maximum efficiencies of 90% and 89% at low concentration (1.0x10 -7 mol L -1 ), measured by LPR and EIS, respectively.A semicircle increase was observed in Nyquist plots when the inhibitor concentration rises.PP curves showed that PVP 40000 can be classified as a mixed inhibitor shifting to the anodic region.The adsorption on metal surface was in accordance with the Langmuir adsorption isotherm.

Comparison with literature data
The results of PVP 40000 inhibition efficiency obtained in this study were confronted with data-related literature to verify their relevance.Literature data were compiled and can be seen in Table 4.

Figure 2 .
Figure 2. OCP plots of carbon steel obtained in HCl 0.5 mol L -1 in the presence and absence of PVP.

Figure 3 .
Figure 3. Polarization curves of carbon steel in 0.5 mol L -1 HCl in the presence and absence of PVP.

Figure 4 .
Figure 4. Nyquist plots of carbon steel obtained in HCl 0.5 mol L -1 , in the blank and in the presence of PVP 40000.

Figure 5 .
Figure 5. Equivalent circuit model used to fit EIS experimental data, in the presence and the absence of PVP.

Figure 6 .
Figure 6.Bode plots of carbon steel obtained in 0.5 mol L -1 HCl, in the presence and the absence of PVP.

Figure 7 .
Figure 7. Langmuir isotherm plot for the adsorption of PVP on carbon steel surface in HCl 0.5 mol L -1 .
1. Al Juhaiman LA, Mustafa AA, Mekhamer WK.Polyvinyl Pyrrolidone as a Green Corrosion Inhibitor of Carbon Steel in Neutral Solutions Containing NaCl: Electrochemical and Thermodynamic Study.International Journal of Electrochemical Science.2012;7:8578-8596.2. Ban C, He Y, Shao X, Wang Z. Effects of polymer corrosion inhibitor on widening etch tunnels of aluminium foil for capacitor.Corrosion Science.2014;78:7-12.

Table 2 .
Electrochemical parameters obtained from LPR measurements for carbon steel, in the presence and the absence of different PVP 40000 concentrations, in 0.5 mol L -1 HCl.

Table 4 .
. Experimental data of inhibition efficiency, corrosive medium, concentration, metal-type and technique reported in literature about the PVP polymer.