Inhibition Effect of Pteris vittata Extractive on the Corrosion of Steel in Hydrochloric Acid Solution

Wen Chen

（School of Resources，Environment and Chemistry，Chuxiong Normal University，Chuxiong 675000）

Abstract：Pteris vittata extractive（PVE）achieved by immersion was investigated as a corrosion inhibitor for Q235 steel in 1 mol·L-1 HCl solution by means of the electrochemical methods.Polarization curves results indicated that PVE behaved as an effective mixed-type inhibitor in control of steel corrosion，and the inhibition performance of PVE on Q235 steel was found to be enhanced with the increasing PVE concentration.The analysis of electrochemical impedance spectroscopy implied that the increase of charge transfer resistance of corrosion reaction and the change of the structure of the electric double layer，due to the adsorption of PVE molecules on steel surface.Adsorption behavior studies have shown that the adsorption of PVE molecules conforms to the Langmuir monolayer adsorption model.On average，the adsorption of each extractive molecule will cause more than 3 water molecules to desorb from the steel surface，and there is a repulsive effect among the adsorbed molecules.

Key words：Q235 steel;hydrochloric acid;Pteris vittata extractive;adsorption;inhibition

1 Introduction

In the process of industrial pickling，acid leaching，and oil well acidification，corrosion inhibitors are often added to reduce the corrosion of metal substrates［1］.Organic corrosion inhibitors with excellent performance are mostly compounds containing polar groups such as O，N，S，P，etc.These polar groups act as active centers to adsorb and form films on the metal surface，thereby reducing metal corrosion［2］.Compared with synthetic routes，extracting active ingredients from natural plants as corrosion inhibitors has environmental advantages such as wide sources，low cost and biodegradability，and has regained the attention of researchers in the field of corrosion［3，4］.Yunnan Province is rich in plant resources and known as the“kingdom of ferns”［5］.Pteris vittatabelongs to the family pteridaceae.The chemical composition of stems and leaves contains flavonoids，sterols，tannins，organic acids and glycosides［6，7］.These organic compounds have hydroxyl，carbonyl and heteroatoms，etc，which can be used as potential corrosion inhibitor adsorption centers.In this paper，the effective components were extracted from the stems and leaves ofPteris vittataby leaching method，and the corrosion inhibition effect of plant extracts on carbon steel in 1 mol·L-1HCl solution was studied by electrochemical method，and the adsorption behavior of corrosion inhibitor molecules was investigated.These researches are expected to provide some experimental basis for its application in the field of corrosion.

2 Experiment

2.1 MaterialsThe experimental material was Q235 steel，chemical composition（mass fraction）：C 0.12，Mn 0.35，Si 0.14，P 0.019，S 0.010，Fe balance.Hydrochloric acid and acetone were all analytical reagents.Pteris vittatawas picked in the Zixi Mountain，Chuxiong，Yunnan Province，washed and dried in the sun.The stem and leaf were crushed，passed through a 300-mesh sieve，and stored in a desiccator for later use.The preparation process ofPteris vittataextractive（PVE）was as follows：30 g of plant powder was immersed in 300 mL of 75%（volume）ethanol aqueous solution in the ultrasound at 30℃for 0.5 h of ultrasound，and then the extractive was cooled and filtered.For purpose of exhaustive extraction，the same operation was repeated twice.Two filtrates are mixed and rotary-evaporated to obtain a concentrated solution.The difference between the quality of the powder sample and the residue was determined as the total amount of the corrosion inhibitor，and the content of the extractive in the concentrate was calculated based on this amount.

2.2 Electrochemical measurementsAll the electrochemical measurements were carried out on an electrochemical workstation（CHI 604，Shanghai Chenhua，China）in a three-electrodes system at room temperature.A Q235 steel electrode acted as the working electrode.A large platinum plate（2 cm2）and a saturated calomel electrode（SCE）with a Luggin capillary were used as the counter electrode and the reference electrode，respectively.The working electrode was immersed into the test solution for 1 h to attain a quasi-stable state，where open circuit potential（OCP）can be obtained，before any electrochemical measurements were done.The experimental temperature was 20°C.Polarization curves were carried out in a potential window of-0.75 V to-0.15 V at a scan rate of 1 mV·s-1，and the related parameters were obtained by using Tafel extrapolation.Electrochemical impedance spectroscopy（EIS）was measured at OCP with a voltage perturbation of 10 mV in the frequency range from 104Hz to 10-2Hz，and the raw data were fitted and analyzed by Zsimpwin 3.10 software based upon the suitable equivalent circuit.

3 Results and discussion

3.1 Polarization curves（PC）The potentiodynamic polarization curves of Q235 steel in 1 mol·L-1HCl solution with various concentrations of PVE at 20℃are presented in Figure 1.The relevant electrochemical kinetic parameters，such as corrosion potential（Ecorr），corrosion current density（Jcorr），cathode Tafel slope（bc），anode Tafel slope（ba），and corrosion inhibition ef f iciency（ηJ%）as function of PVE concentration are listed in Table 1.Values of the corrosion inhibition ef f iciency（ηJ%）were calculated from polarization measurements according to the equation given below:

Table 1 Polarization data and inhibition efficiency for Q235 steel in 1 mol·L-1 HCl solution in the absence and presence of PVE

Fig.1 Potentiodynamic polarization curves for Q235 steel in 1 mol·L-1 HCl solution without and with different concentrations of PVE

whereandJ corrare the corrosion current densities obtained in inhibited and uninhibited solutions，respectively.

It can be seen from Figure 1 and Table 1，that the corrosion potential of Q235 steel shifts slightly towards negative direction after adding the PVE，and with the increase of the PVE concentration，the anionic and anodic polarization curves both shift to the low current direction.The corrosion current density gradually decreased，indicating that PVE inhibited both anodic dissolution and cathodic hydrogen evolution in the corrosion reaction of Q235 steel.In Figure 1，the almost parallel and large negativeshift cathodic curve and the cathodic Tafel slope with little change（also see Table 1）indicate that the effect corrosion inhibition of PVE on Q235 steel is mainly by inhibiting the hydrogen evolution process.The adsorption of corrosion inhibitor molecules in PVE reduces the active sites of cathodic reaction，and decreases the corrosion rate.Compared with the cathode curve，the anode branch is greatly affected by the electrode surface potential after adding PVE.Due to the adsorption of the PVE on the surface of Q235 steel，the polarization current in the weakly polarized region is smaller.The polarization current increases rapidly in the strongly polarized region，which is consistent with the smaller anodic Tafel slope in Table 1，indicating that the adsorption of inhibitor molecules in PVE on Q235 steel depends on the electrochemical properties of the metal surface.The increase of polarization potential increases the dissolution rate of Q235 steel，resulting in the higher surface desorption rate compared with its adsorption rate［8］.After adding PVE to hydrochloric acid，the corrosion potential of carbon steel was shifted within 85 mV relative to the blank solution，and the anodic and anodic reactions of Q235 steel corrosion were significantly inhibited.Therefore，the PVE is considered as a mixed corrosion inhibitor mainly based on cathodic inhibition in this corrosion system［9，10］.In addition，the slope of Tafel at the cathode and anode did not change significantly compared with the blank solution，which indicated that the corrosion mechanism of the steel did not change after adding the corrosion inhibitor，and the corrosion inhibition mechanism of the PVE should be“Geometric Covering Effect”［11］.When the concentration of PVE increased to 480 mg·L-1，the corrosion inhibition rate（ηJ）reached 90.2%，which indicated that PVE displays excellent corrosion inhibition performance for Q235 steel in HCl solution.

Table 2 Impedance parameters and the percent of protection eff iciency for Q235 steel in 1 mol·L-1 HCl solution without and with different concentrations of PVE

Fig.2 Nyquist plots for Q235 steel in 1 mol·L-1 HCl solution without and with different concentrations of PVE.

3.2 Electrochemical impedance spectroscopy（EIS）Figure 2 shows the typical Nyquist plots of Q235 steel in 1 mol·L-1HCl solution in the absence and presence of various concentrations of PVE.In these spectra，depressed semicircles were observed in the whole frequency domain.It can be speculated that the corrosion process is controlled by one time constant，that is，the charge transfer process is the main process.The capacitive reactance reflects the relaxation process of the interfacial electric double layer and the electron transfer resistance.It is a semi-circular arc with the center below the real axis of the map.This is due to the heterogeneity and roughness of the electrode interface during the corrosion process of carbon steel caused by the diffusion effect［12，13］.With the increase of the PVE concentration，the radius of the capacitive arc in the Nyquist diagram increases gradually，indicating that the PVE molecules adsorbed on the steel surface gradually increase and the formed film layer is more compact and complete，and the corrosion inhibition effect is enhanced.Taking into account the spectroscopic characteristics and real corrosion system environment，the fitting of the impedance data adopts the equivalent circuit in Figure 3.In the equivalent circuitRsis the solution resistance，Rctis the charge transfer resistance，CPE is a constant phase element.CPE was used instead of double layer capacitance in the equivalent circuit models，which can more accurately f it the experimental data.The impedance of CPE is def ined as follows:

whereYoandnis the proportionality coefficient and diffusion index of the CPE，ωis the angular frequency（ω=2πf），wherefis the AC frequency，jhere is the imaginary unit（j2=-1）.

The values of the CPE-parametersYoand n were only determined through a fitting procedure and the capacitance（Cdl）could then be derived from the following equation［15］。

The inhibition eff iciency（ηct%）was calculated using the following equation：

whereandR ctare the charge transfer resistance values with and without adding the PVE，respectively.

Fig.3 Electrochemical equivalent circuit

3.3 Adsorption behaviour analysisThe f itted data of impedance plots are given in Table 2.It can be seen that with the increase of PVE concentration，the value of charge transfer resistance（Rct）increases significantly，which indicates that the inhibitory effect of PVE molecules on steel corrosion is gradually enhanced，and the charge transfer process is more difficult for the carbon steel.The gradual decrease of the electric double layer capacitance（Cdl）value indicates the adsorption of PVE organic molecules with a smaller dielectric constant，resulting in the desorption of a large number of water molecules from the carbon steel surface.Correspondingly，the carbon steel/solution interface electric double layer structure is also changed.When the concentration of PVE reaches 480 mg·L-1and the corrosion inhibition rate increases to 93.1%，its change rule is consistent with the polarization curve results.From the electrochemical results，it can be seen that the corrosion inhibition mechanism of PVE should be“eometric coverage effect”，and the corrosion inhibition of organic corrosion inhibitors to metals is formed by the adsorption of metal/solution interface.Assuming that the adsorption of PVE molecules on the surface of Q235 steel conforms to the common Langmuir adsorption isotherm［16］，the relationship between the concentration of PVE molecules（cPVE）and the film coverage（θ）is:

Herein the values ofθwere evaluated via the relevant parameters from polarization curves［θ=ηI/100］and electrochemical impendence measurements［θ=ηct/100］.cis the concentration of PVE as a corrosion inhibitor（mg·L-1）；Kadsis the adsorption equilibrium constant（L·mg-1），and the thermodynamic meaning of its value can be expressed as［17］:

wherecsolventis the concentration of solvent water（1×106mg·L-1），andΔG0adsis the standard Gibbs free energy change value of the adsorption process（kJ·mol-1）.The fitting result is shown in Figure 4.It can be observed from the figure thatc/θhas a good linear relationship withc，and the coefficient of determinationR2（Table 3）and the slope are both close to 1，indicating that the PVE molecule on the surface of carbon steel conforms to the Langmuir monolayer adsorption model.However its slope value is slight deviation from 1，which indicates that there may be an interaction force between the adsorbed molecules or that the surface adsorption is not uniform.

Fig.4 The relationship between c/θand c of PVE

Fig.5 The relationship between c andθof PVE

Fig.6 The relationship between ln c andθ

The adsorption process of corrosion inhibitor molecules on the surface of carbon steel is actually a process of replacing the water molecules adsorbed on the interface［18］:

Among them，lnh(sol)and lnh(ads)are the corrosion inhibitor molecules in solution and adsorbed on the metal surface，respectively.H2O(ads)andH2O(sol)are the water molecules adsorbed and desorbed on the metal surface，respectively.xis the number of water molecules replaced by organic molecules.In order to study the substitution process of inhibitor molecules and water molecules in PVE，the Dhar-Flory-Huggins［19］and Temkin［20］isotherm adsorption equations were used:

whereθis the force factor for the adsorption of inhibitor molecules on the steel surface.The results after data processing are shown in Figure 5 and Figure 6，and the fitting results and calculated values are listed in Table 3.According to the three adsorption models the calculated values are all negative，indicating that the adsorption of organic corrosion inhibitor molecules in PVE on the surface of carbon steel belongs to spontaneous adsorption.Thexvalues obtained by fitting the Dhar-Flory-Huggins adsorption model are all negative.It is greater than 1 and is between 3 and 5，indicating that the adsorption of each inhibitor molecule will lead to the desorption of more than 3 water molecules from the carbon steel surface［19］.Theαvalues obtained by fitting the Temkin adsorption model are all negative value，which means that there is a mutual repulsive force between the PVE adsorbed molecules［21］.

Table 3 Fitting parameters of three adsorption models：Langmiur，Dhar-Flory-Huggins and Temkin

3.4 Surface morphological observationFigure 6 shows the surface SEM images of the Q235 carbon steel test piece after immersion in blank（a）and 1 mol·L-1HCl solution supplemented with 480 mg·L-1PVE（b）for 12 hours.In the experiment，it can be observed that a large number of bubbles gatheron the surface of the steel sheet in the blank HCl solution，and the surface color gradually becomes dark and black.It also can be seen from Figure 7（a）that the overall corrosion of the steel sheet in the blank hydrochloric acid solution is severe，and there are pits formed by corrosion on the surface.However，after adding PVE，the corrosion of the steel sheet（Figure 7（b））is significantly inhibited，and the surface is relatively smooth.These observations confirm that the plant extractive can effectively retard the corrosion of Q235 steel in hydrochloric acid.

Fig.7 SEM image of the surface of Q235 steel test piece after immersion in hydrochloric acid for 12 hours

4 Conclusion

Pteris vittataextractive（PVE）can effectively inhibit the corrosion of Q235 steel in 1 mol·L-1HCl solution，and the efficiency of corrosion inhibition increases with the increase of extract concentration.

Polarization curve results show that PVE is a mixed corrosion inhibitor，and the corrosion inhibition mechanism is“geometric coverage effect”.Electrochemical impedance spectroscopy shows that the corrosion reaction of steel is controlled by the interfacial charge transfer process，and the adsorption of PVE on the steel surface changes the interfacial electric double layer structure，which increases the charge transfer resistance of the corrosion reaction.

The PVE molecules obey the Langmiur monolayer adsorption on the steel surface.The fitting data of the Dhar-Flory-Huggins and Temkin adsorption equations show that the PVE molecules have interactive repulsion during the adsorption process，and the adsorption of each extract molecule will cause more than 3 water molecules to desorb from the steel surface.