Experimental and quantum chemical studies of carboxylates as corrosion inhibitors for AM50 alloy in pH neutral NaCl solution

Junjie Yng ,Pingli Jing ,Youmin Qiu ,Chih-Yu Jo ,Crsten Blwert ,Svitln Lmk ,Aniss Bouli,Xiopeng Lu,Mikhil L.Zhelukevich,e,Wei Li

a Institute of Advanced Wear &Corrosion Resistant and Functional Materials,Jinan University,Guangzhou 510632,PR China

b Helmholtz-Zentrum Hereon,Institute of Surface Science,Geesthacht 21502,Germany

c School of Materials Science and Engineering,Zhengzhou University,Zhengzhou 450001,PR China

d Shenyang National Laboratory for Materials Science,Northeastern University,3-11 Wenhua Road,Shenyang 110819,PR China

e Faculty of Engineering,University of Kiel,Kaiserstra?e 2,Kiel 24143,Germany

Abstract Sodium salts of mono-and di-carboxylic acids (glycolic,fumaric and benzoic acid) were studied as corrosion inhibitors for AM50 alloy in pH neutral aqueous NaCl environment.Hydrogen evolution,electrochemical and surface characterization techniques were employed to reveal their corrosion inhibition mechanism,whilst the molecular features of inhibitors were investigated by quantum chemical calculation.All inhibitors reduced anodic dissolution of AM50 and their efficiency generally increased with time and concentration from 5 mM to 100 mM.The inhibition mechanism can be described as physisorption of inhibitive molecules on the surface of the intrinsic oxide layer followed by chemisorption with Mg2+ and Al3+,and the difference in inhibition action among these inhibitors was explained on the molecular scale.

Keywords: Mg-Al magnesium alloy;Neutral NaCl solution;Corrosion inhibitors;Electrochemistry;Density functional theory.

1.Introduction

Magnesium-aluminum (Mg-Al) alloys,containing 2-10 wt.% Al and minor manganese (Mn),are the most commercially used Mg alloys,especially in automotive and aerospace industries where a weight reduction can significantly minimize the fuel consumption and improve the performance of the vehicles and aircrafts [1].Apart from the enhancement in mechanical properties (e.g.tensile strength and hardness),castability and weldability,the corrosion resistance of Mg is significantly improved by adding proper amount of Al and Mn[2].Explanation for such corrosion performance of AM series alloys can be ascribed to two factors.Firstly,Mn reduces the harmful effect imposed by transition metallic impurities(e.g.iron,copper and nickel) through forming intermetallic compound that settles to the melt bottom or encapsulating impurities that remains within the metal [3].Secondly,the dissolution of Al-richα-Mg matrix favors Al enrichment on the metallic surface which improves corrosion resistance of the primary passivation layer,whereasβ-phase (Mg-Al) network precipitated along the grain boundary can act as a barrier against the progression of corrosion attack [4-7].In spite of these advantages,anticorrosion actions are still indispensable for Mg-Al alloys for industrial application.

Corrosion inhibitors are either organic or inorganic compounds that can effectively reduce the degradation rate or even retard the corrosion process of metallic-based materials at a relatively low concentration [8].Compared with other corrosion protection techniques,reducing the degradation rate of metallic materials by using corrosion inhibitors is a more easy-going and promising strategy in practice.Corrosion inhibitors can be classified into anodic,cathodic and mixed type depending on their impact is mainly in retarding the anodic,cathodic of corrosion process or both of them [8].In principle,anodic inhibitors that hinder the anodic dissolution of metals are expected to be the best candidates for effective corrosion inhibition,since they can straightforwardly passivate,precipitate or adsorb on the surface by forming protective layer(s) on the surface.The inhibition efficiency of precipitating inorganic anodic inhibitors is closely related with the solubility product constant (Ksp) of compounds formed between the inhibitive anions and Mg2+,which determines the stability of the corrosion products layer.For instance,the superior corrosion inhibition performance of phosphate can be ascribed to the lowKspof Mg3(PO4)2(1.0×10-25,where[Mg2+]=4.9×10-6M),which is about two orders lower than that of Mg(OH)2(1.8×10-11,where[Mg2+]=1.6×10-4M)[9,10].Besides,dissolvable carbonates (6.8×10-6) [10] and fluoride (5.2×10-11) [11] were also reported as corrosion inhibitors for AZ31 Mg alloy.However,their inhibition efficiencies are not high enough especially at relative low concentrations,since the stability of their Mg containing compounds is close to that of Mg(OH)2,which can hardly change the conditions of corrosion products layer.On another hand,organic inhibitors prevent corrosion mainly through their adsorption on the metal or hydroxide surface via electrostatic interaction (physisorption) or forming adsorptive products with metallic cations released during corrosion (chemisorption).However,the out most molecular orbital (3 s) of Mg is fully occupied by a pair of spinning electrons,which implies that the adsorption of organic inhibitors on Mg surface through donating or accepting their electrons to a vacant orbital is more difficult compared to many other metallic materials.Albeit,a few exceptions e.g.2-hydroxy-4-methoxy-acetophenone(paeonol)[12],tetraphenylporphyrin(TPP) [13] and 8-hydroxyquinoline (8-HQ) [14] can complex with Mg2+via multi substitutes forming insoluble precipitates within the defective areas of the porous Mg(OH)2film.

As a logical and promising alternative strategy in corrosion inhibition of Mg alloys,cathodic inhibitors decrease the H2O and O2reduction rate (cathodic reaction:2H2O+O2+4e-→4OH-),which in turn limit the anodic dissolution of Mg [15].The cathodic sites within Mg alloys are those electrochemically nobler components including intermetallics and impurities.Anionic surfactants (e.g.sodium dodecyl sulfate [16],sodium salt of N-lauroyl-Nmethyltaurine and dodecylbenzensulphonic acid[17])are well studied inhibitors,since they have negatively charged hydrophilic substitutes selectively adsorb on the surface of micro cathodes with leaving their long hydrophobic tails suspending towards the solution.Thus,it is understandable that a satisfactory corrosion inhibition efficiency of anionic surfactants can only be achieved at relatively high concentration when the surface of cathodic sites is fully covered by inhibitive molecules.For instance,selective adsorption of sodium dodecyl sulfate (SDS) on AlMn intermetallic of AZ91 is confirmed by Lu et al.employing electron probe micro-analyzer(EPMA) [16].They suggested that adsorption of SDS was realized via interaction between the unshared paired electrons of S-O or S=O bonds and the vacant orbital of Mn,and the inhibition efficiencyIEpreached 88.8% when 0.05 M of the inhibitor is added in 3.5 wt.% NaCl solution.Except for the intermetallics,metallic impurities (e.g.nickel (Ni),copper (Cu) and iron (Fe)) introduced during the alloy preparation process may also serve as micro cathodes and promote the corrosion of Mg alloys [18,19].A reasonable explanation about the corrosion promotion mechanism of trace Fe particles was proposed by H?che et al.[20].They claimed that Fe inclusions undergo “self-corrosion” after their loss of direct contact with bulk Mg,and the resultant Fe2+/3+cations or Fe oxide(s) would re-plated back on the Mg surface,which promote the corrosion of the Mg substrate.In view of the role of Fe impurities during Mg corrosion,Lamaka et al.elucidated that Fe2+/3+complexing agents (e.g.cyanide,salicylate,oxalate,methylsalicylate and thiocyanate) can also serve as inhibitors for Mg alloys.The inhibition efficiency of the latter complexing agents was in line with their capability of forming complexes with Fe3+[21].Unlike anodic and cathodic inhibitors,mixed type inhibitors (e.g.2,5-pyridine dicarboxylate and fumarate [22,23]) were characterized by their adsorption or formation of coordination precipitation on the surface of MgO/Mg(OH)2via carboxyl groups,which enhances the compactness of the corrosion products layer and reduces both anodic and cathodic reaction rates.

Among the known inhibitors,carboxylates are widely accepted as effective inhibitors for Mg alloys and their inhibition mechanism was generally ascribed to the interaction between the negatively charged carboxyl groups and positively charged metallic ions on the surface [24].Nevertheless,the inhibitions efficiency of carboxylates significantly varies with respect to the composition of alloy as well as the type and position of functional groups within the inhibitor molecule[25-27].As reported by Lamaka et al.[27],the inhibition efficiency of 2,5-pyridine dicarboxylate (2,5-PDCA) on pure Mg,regardless of the content of Fe,was more than 80%,which was almost twice that of ZE41.Moreover,the inhibition efficiency for AM50 increased from minus 97% (for salicylic acid) to 29%,41% and 66% when -CH3substituted at para-(5-methylsalicylic acid),meta-(4-methylsalicylic acid)and ortho-(3-methylsalicylic acid) positions relative to -OH of salicylic acid.In light of this fact,it is of great importance to start the study from inhibitors with simple molecular structure,which can provide (1) the most basic understanding of the influence of functional groups and their position on the corrosion inhibition performance,and (2) fundamental information for machine learning approaches to design new inhibitors or predict the efficiency of untested compounds[28,29].In the present work,three characteristic compounds containing mono-or bi-carboxyl group were used as corrosion inhibitors for Mg-Al-Mn (AM50) alloy.The influence of inhibitor concentration and presence of secondary phases on the corrosion inhibition actions of each inhibitor werestudied through hydrogen evolution,electrochemical measurement and subsequent surface analysis.As follows,quantum chemical calculation based upon density functional theory(DFT) was performed to disclose the relationship between the inhibitive actions of the inhibitors and their molecular configuration which contributes to the setup of corresponding inhibition mechanism.

Fig.1.Molecular structure of (a) glycolic acid,(b) fumaric acid and (c)benzoic acid.

2.Experimental

2.1.Materials and reagents

Cast AM50 alloy was used as substrate material in the present study.Spark optical emission spectroscopy (Spark OES,Spark analyzer M9,Spectro Ametek,Germany) was employed to determine the elemental composition of the AM50 alloy,and the results are shown in Table 1.For hydrogen evolution test,the AM50 ingot was machined into chips with a larger surface area (area-to-mass ratio of 56.86±16 cm2/g) using a turning lathe equipped with a diamond cutting tool.Details about the preparation process can be found in previous work [24].Rectangular coupons with dimension of 15×15×4 mm3sliced from the ingot were used for electrochemical corrosion testing and surface characterization.Prior to the tests and characterizations,all specimens were ground using silicon carbide (SiC) papers successively up to 1200 grit.For optical microscopy observation,the specimens were further polished with a mixture of alumina (Al2O3) slurry and 1 μm non-aqueous diamond suspension.Eventually,the specimens were rinsed using deionized water and ethanol,and then dried in a pressured air flow at room temperature.

Table 1Elemental composition of AM50 alloy (in wt.%).

All chemicals used in present study were in analytical reagent (AR) grade and no further purification was conducted before application.Three corrosion inhibitors(Fig.1),including glycolic acid (HOCH2COOH,CAS:79-14-1,Sigma-Aldrich Chemie GmbH,Germany),fumaric acid (HOOCCH=CHCOOH,CAS:110-17-8,Alfa Aesar,Germany) and benzoic acid (C6H5COOH,CAS:65-85-0,Sigma-Aldrich Chemie GmbH,Germany),were dissolved in 0.5 wt.% NaCl solution to obtain different concentrations (i.e.5 mM,10 mM,50 mM and 100 mM).3 M NaOH was used to adjust the pH of the inhibitor-added solutions until the neutral range (pH:6.8-7.2).The pH value of the prepared solutions was monitored by a pH meter (Metrohm 691) during preparation.

2.2.Hydrogen evolution

Hydrogen evolution was recorded during immersion of 0.5 g AM50 chips (with specific surface area to mass ratio of 56.86 cm2/g) in 0.5 wt.% NaCl solution with and without addition of inhibitors using eudiometers (NS45/27,Neubert-Glas,Germany).A series of different concentrations of inhibitor solutions were prepared to investigate the influence of the concentration on inhibition efficiency.Magnetic stirrers were used to avoid aggregation of chips on the bottom of the bottle,and the speed was 200 rpm.All measurements were conducted for 24 h at room temperature,and the air resided in the eudiometer (less than 10 cm3) was not purged before running the experiments.The inhibition efficiency (IEH) derived from hydrogen evolution results was calculated as follows:

2.3.Electrochemical measurements

Interface 1010B potentiostat (Gamry,USA) was employed for potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements.A conventional three-electrode cell was used.In this cell,bulk AM50 specimens with 1 cm2exposed area served as the working electrode,a platinum (Pt) wire as the counter electrode and a saturated Ag/AgCl electrode as a reference electrode.All the electrochemical measurements were carried out in 0.5 wt.%NaCl solution with and without 50 mM each inhibitor.Each electrochemical measurement was repeated at least three times and all measurements were performed at room temperature.

Prior to the polarization measurement,the working electrode (AM50 alloy) was immersed in the solution for 10 min to obtain a relatively stable open circuit potential (OCP).After that,the potential was scanned from OCP to -300 mV and 300 mV (vs.Ag/AgCl) respectively with a scan rate of 0.5 mV/s.The EIS tests were performed in the frequency range from 105Hz to 10-2Hz with applying an amplitude of 10 mV rms sinusoidal perturbation with respect to OCP.Commercial software Zview was employed for fittin the experimental impedance spectra.The inhibition efficiency derived from the EIS results can be calculated through the equation as follow:

Fig.2.Metallographic microstructure (a),phase composition (b) and SEM microstructure (c and d) of as-cast AM50.

2.4.AFM/SKPFM analysis

The Volta potential measurement of AM50 alloy was carried out using atomic force microscopy(AFM,JPK NanoWizard) equipped with scanning Kelvin probe force microscopy(SKPFM) in AC mode.Cr/Pt coated silicon (Si) probe was employed for AFM/SKPFM scanning,and the resonance frequency and force constant of the probe are 75 kHz and 3 N/m,respectively.Polished samples were immediately used for SKPFM measurement to avoid oxidation.Measurement was conducted in open air at room temperature.A conductive paste was placed between the sample and the AFM stage to achieve electrical connection.The lift height of the probe was set as 100 nm above the sample surface.Scanning was performed on an area of 40×40 μm with a resolution of 512×512 pixels.The data was analyzed using JPKSPM Data Processing software.

2.5.Morphology and composition characterization

An optical microscope (OM,DM2500M,Leica,Germany)was used to observe the phases on the well-polished sample surface,whereas a scanning electron microscope (SEM,Tescan Vega3) equipped with an energy dispersive X-ray spectrometer (EDS) was used for further analyzing the surface morphology and elemental distribution before and after corrosion.XRD measurements were carried out using a D8 Advance AXS diffractometer (Bruker,Germany).X-ray diffraction patterns (2θ) were recorded from 10° to 80° using Cu Kα(λ=1.54,056 °A)radiation at room temperature.The tube voltage and tube current were 40 kV and 40 mA respectively,whereas the scanning speed was 0.1°/s.

2.6.Quantum chemical calculation

Geometrical optimization of deprotonated glycolic acid,fumaric acid and benzoic acid was conducted using Gaussian 16 program package at the B3LYP/6-31+G??level of density functional theory (DFT).Effects of water solvation are included by using a self-consistent reaction field (SCRF) with polarizable continuum model (PCM).

3.Results

3.1.Microstructure,phase composition and AFM/SKPFM analysis

The investigation of metallographic (Fig.2a) and SEM(Fig.2c and d) microstructure and phase composition(Fig.2b) of AM50 was conducted on the as-polished AM50 alloy.As shown in the optical microscopy (OM) image,the investigated AM50 alloy exhibits a typical metallographic microstructure featured by an average grain size of 1.44±0.41 μm determined according to ASTM E112-88.In addition to the primaryα-Mg phase,two different types of intermetallic precipitates (β-Mg17Al12and Al8Mn5) are identified in the XRD patterns,which can be clearly observed by different contrast in backscattered electron mode since the electron scattering ability of these phases is different.According to the EDS analysis (Fig.2c inset),the darker phases contain Al and Mg,which can be correlated withβ-Mg17Al12peaks in the XRD pattern.Meanwhile,the light color phases contain Al and Mn,which are preferentially referred to Al8Mn5phase considering the characteristic XRD pattern.In addition,the large partially divorced eutecticβ-Mg17Al12exhibits an island-like morphology containing lamellar eutectic Mg sphere residues,whilst the Al8Mn5precipitate is commonly in polygonal shape.

Fig.3.Topography (a) and VPD maps (b) acquired upon polished AM50 substrate employing AFM/SKPFM.

Fig.3 illustrates topography and Volta potential difference(VPD) measured on a specific area (40×40 μm) that contains bothβ-Mg17Al12and Al8Mn5phases.As shown in the topography map (Fig.3a),obvious contrast can be observed betweenβ-Mg17Al12and Al8Mn5phases implying significant height difference between the two phases.The peak heights of Al8Mn5are 209.5 nm and 305.6 nm respectively,whereas the maximum height value ofβ-Mg17Al12is only 61.5 nm.Such a height difference can be mainly resulted from the hardness differences of these phases.β-Mg17Al12is much softer compared to Al8Mn5intermetallic but still harder than theα-Mg matrix.Hence,the hard Al8Mn5intermetallic is less removed by grinding during the preparation process leaving such a comet-like trajectory along the AFM/SKPFM scanning direction [30].Following topography measurement,the VPD measurement is performed over the same area.As shown in Fig.3b,the VPD illustrates a trend similar to the topography results.The VPD values of P1 and P2 (Al8Mn5intermetallic)are 165.6 mV and 131.2 mV respectively,whilst theβ-phase(P3) shows an average of 52.5 mV in the VPD map.Also,it is important to notice that the largeβ-Mg17Al12particles are not uniform in VPD containing regions featuring higher values than the average.This is attributed to the casting process that solidifies close to equilibrium and generates largerβ-Mg17Al12grains [31].As a result,Al partially dissolves into theβ-Mg17Al12intermetallic.Based on the VPD results,microgalvanic corrosion is expected,especially between Al-Mn intermetallic andα-Mg,which is the dominant corrosion mechanism for AM50 alloy [32].

3.2.Hydrogen evolution analysis

Fig.4 shows the variation of H2volume as function of immersion time of 0.5 g AM50 chips(the overall exposed area is estimated to be 28.43 cm2)in 0.5 wt.%NaCl solution without and with addition of different concentrations (5 mM,10 mM,50 mM and 100 mM) of the three inhibitors.Hydrogen evolution was used as a basic technique for screening inhibitors and metallic chips with increased the surface area were employed with the aim of obtaining results in a relatively short period,as shown in Fig.4a [33,34].Fig.4b-d show the H2evolution curves gained from immersing 0.5 g AM50 chips in pH neutral NaCl solution (0.5 wt.%) containing various concentrations of inhibitors.Similar H2release trend can be seen for glycolate (Fig.4b) and benzoate (Fig.4d),where the hydrogen evolution rate (HER) generally decreases with increasing concentration of the inhibitor except for 100 mM glycolate addition,which promotes H2evolution.This unexpected H2volume increase witnessed upon the 100 mM glycolate addition specimen may be attributed to the further deprotonation of hydroxyl induced by the pH increase due to alloy corrosion.In contrast,the corrosion of AM50 chips is instantly inhibited by the addition of fumarate into the solution,and increasing the concentration from 5 mM to 100 mM does not affect the amount of evolved H2to a large extent.The calculated corrosion inhibition efficiencies at 4 h and 15 h immersion with the different inhibitors’ addition is given in Fig.5.As expected,the efficiency values obtained at 15 h are higher than those at 4 h for all inhibitors,and fumarate reveals the highest inhibition efficiencies among three tested inhibitors.It has been shown by multiple examples before that the absolute value of inhibiting efficiency is often dependent on immersion time [35].The inhibition efficiency of glycolate,fumarate and benzoate after stabilization is 57.5%(50 mM),81.5% (100 mM) and 59% (100 mM),respectively.

3.3.Potentiodynamic polarization

DC potentiodynamic polarization was conducted in two separate sweeps from OCP to -300 mV and from OCP to 300 mV (vs.Ag/AgCl) for the bulk AM50 specimens immersed in NaCl solutions (0.5 wt.%) in absence and presence of 50 mM inhibitors (Fig.6).Cathodic Tafel extrapolation,as described in [36],is adopted to estimate the corrosion current density (icorr) from the polarization curves,and the critical parameters derived from the polarization curves are listed in Table 2.Since only one representative curve was selected from three or more repetitive measurements,slight difference can be identified between the plots in Fig.6 and averaged values (with errors) in Table 2.As shown in Fig.6a,theEcorrof reference group measured in blank NaCl solution without inhibitor addition shifts gradually from -1.54 V after 10 min immersion to -1.49 V after 24 h,whilst the corrosion current density stabilizes around 10 μA/cm2after 3 h immersion.Moreover,both anodic and cathodic slopes of the polarization curves are slightly suppressed during immersion,which suggest that the intrinsic hydroxide layer on the surface of Mg alloy would impede both anodic Mg dissolution and cathodic hydrogen evolution during the corrosion of AM50 alloy.With addition of carboxylates in the blank NaCl solution,obvious corrosion inhibition mainly occurs on the anodic branches.As for glycolate and benzoate,the initial(10 min)polarization curves shift towards nobler potential and lower current density direction comparing to that of the control specimen,which indicate suppression of corrosion by both inhibitors.More importantly,pseudo-passive behaviors,characterized by moderate increase of current density with potential shift,appear on the anodic branches of glycolate and benzoate specimens after 6 h and 3 h,respectively,which indicate the barrier property of hydr(oxide) is strengthened by inhibitors.However,breakdown of the inhibitor incorporated layers occurs at specific potentials (Ebd),which suggests such an enhancement is limited.Up to 24 h,benzoate registers lowericorr(2.5±3.0 μA/cm2) than that (4.6±3.5 μA/cm2)of glycolate,which suggests better corrosion inhibition ability.Similar potentiodynamic behaviors are also observed upon the specimen with 50 mM fumarate addition,whose polarization curves shift towards positive potential and lower current density with achieving anicorrof 2.5±3.0 μA/cm2by 24 h.Besides,more significant passivation behavior can be define upon the anodic branches of fumarate specimen since 3 h,and even theEbdis not shown since 6 h immersion.Such a remarkable suppression of anodic behavior indicates improved barrier property of the fumarate modified hydr(oxide) layer,which would result in better corrosion performance.

Table 2Critical polarization parameters for AM50 immersed in NaCl solution (0.5 wt.%) without and with addition of 50 mM inhibitors.

Fig.4.Released hydrogen volume in function of time of AM50 in NaCl solution (pH=6.8-7.2) without and with addition of inhibitors.(a) Comparison of hydrogen volume between bulk and chipped AM50,(b) glycolate,(c) fumarate and (d) benzoate.

Fig.5.Corrosion inhibition efficiency (%) of three tested inhibitors (glycolate,fumarate and benzoate) in the concentration range from 5 mM to 100 mM after 4 h and 15 h immersion.

Fig.6.Potentiodynamic polarization curves of AM50 in NaCl solution (0.5 wt.%) without and with addition of 50 mM inhibitors.(a) Blank NaCl solution,(b) glycolate,(c) fumarate and (d) benzoate.

3.4.Electrochemical impedance spectroscopy (EIS)

Electrochemical impedance spectroscopy (EIS) was employed to study the corrosion process and interface conditions in NaCl solution without and with addition of 50 mM inhibitors.Nyquist and Bode plots recorded after consecutive immersion periods (10 min,1 h,3 h,6 h,12 h and 24 h)are shown in Fig.7 at the same scale.It can be observed from the Nyquist that all inhibitor protected specimens reveal larger semi-circles than the control one,which indicate superior corrosion performance.In the blank NaCl solution(Fig.7a and b),a double semi-circle pattern is shown in the Nyquist spectrum shortly after 10 min immersion,which then merge into a larger one after 1 h immersion.In general,the low-frequency (LF:10°-10-2Hz) and high-frequency (HF:103-10° Hz) loops can be associated to the electrochemical processes occurring at the metal/electrolyte interface and hydr(oxide)layer,respectively.Hence,the combination of two loops might be attributed to the loss of the barrier property of the hydr(oxide) layer.Based on this thought,the contribution of inhibitors on the corrosion performance of AM50 alloy is mainly resulted from their adsorptive modifi cation of the hydr(oxide) layer.Similar evolution trend of EIS curves can be found on the glycolate and fumarate specimens(Fig.7c-f).The diameters of the double semi-circles in the Nyquist plots of both specimens demonstrate a sharp increase by 1 h immersion with achieving their highest impedance of 5 kohm·cm2and 4.9 kohm·cm2at 10-2Hz,respectively.Since then,the impedance decreases but fluctuate indicating the glycolate and fumarate modified hydr(oxide) cannot completely retard Cl-from penetrating this layer and the substrate is still vulnerable to corrosion attack.In contrast,50 mM benzoate added specimen demonstrates more effective corrosion inhibition action as manifested by the consecutive shift of the middle frequency (MF:103-10° Hz) phase towards lower frequency and growth of overall impedance from 2.8 kohm·cm2(10 min) to 6.9 kohm·cm2(24 h)(Fig.7h).These results suggest that the barrier property is enhanced with time,which may result from the increase of compactness of the hydr(oxide) layer due to the adsorption of inhibitive molecules.

Fig.7.EIS curves of AM50 alloy immersed in blank NaCl solution without and with 50 mM inhibitors within 24 h.(a)(b) Blank NaCl solution,(c)(d)Glycolate,(e)(f) Fumarate and (g)(h) Benzoate.

Fig.8.Equivalent circuit employed to fi the experimental EIS data in blank NaCl solution without and with addition of 50 mM inhibitors.

To quantify the physical-chemical processes involved within different corrosion cells,a typical equivalent circuit(Fig.8) comprising of two time-constants is used to fi the experimental EIS results.In this circuit,Rs stands for the resistance of NaCl solution with and without 50 mM inhibitors,CPEfandRfare associated with the capacitance and resistance of the protective layer on the top of AM50,andCPEdlandRctrepresent the double layer capacitance and charge transfer resistance occurred at the metal/electrolyte interface.Considering the imperfect surface or interface condition caused by the non-uniform formation of corrosion products,distribution of charge and adsorption of inhibitive molecules [26],constant phase element,rather than pure capacitor,was used for the simulation.In addition,effective capacitance (Cf) was calculated depending on the following equation:

whereωmaxis the frequency at which the imaginary impedance achieves the maximum value for the respective constant [37].The evolution of film resistance (Rf),film effective capacitance (Cf),charge transfer resistance (Rct) and corrosion inhibition efficiency (IEE) in function of time is depicted in Fig.9.As shown in Fig.9a,the blank and inhibitor protected specimens register quite similarRfvalues (approx.1.3 kohm·cm2) at the beginning of immersion,which implies that the electrochemical reactions and property of hydroxide layer are not significantly affected by the inhibitors.But after 1 h,theRfof the control one remains almost unchanged,whereas the inhibitor incorporated groups reveal significant increase (to 2.5 kohm·cm2(glycolate),2.8 kohm·cm2(fumarate) and 3.7 kohm·cm2(benzoate) respectively followed by slight fluctuations In the meantime,all inhibitor protected specimens demonstrate constant increase ofCfwith time achieving 16.0±2.2 μF/cm2,23.2±2.6 μF/cm2and 15.0±0.7 μF/cm2by 24 h (Fig.9b).These consistent trend inCfmay result from hydrolytically degradation of the alloy and oxide [38] or growth and compactness of the hydr(oxide)layer.Thus,the faster adsorption of fumarate on the corroding surface would result in thinner and/or more compact hybrid hydr(oxide) layer.Furthermore,influence by the resistance of the hydr(oxide)layers,a similar variation tendency is found upon theRct(Fig.9c).TheRctof the control specimen meets a consecutive reduction from 0.8 kohm·cm2(10 min)to 0.2 kohm·cm2(24 h).Simultaneously,glycolate and fumarate,having lower initialRctvalues (0.3 kohm·cm2and 0.6 kohm·cm2),exhibit a fast increase to 1.4 kohm·cm2and 1.3 kohm·cm2at 1 h,and then stabilize until 24 h.Moreover,the addition of 50 mM benzoate results in a higherRct(1.0 kohm·cm2) since the beginning,which is followed by a moderate increase to 3.4 kohm·cm2by the end of immersion.Fig.9d illustrates the corrosion inhibition efficiency(IEE)calculated from comparing the overall impedance difference between the specimens with and without inhibitorto that of the former partAs shown,the efficiency values surge significantly from-43%(glycolate),6%(fumarate)and 28% (benzoate) at 10 min to 51%,60% and 72% (1 h),and then remain slight increase tendency 66%,65% and 88%till 24 h.The initial negative value in efficiency for glycolate indicates corrosion acceleration,which may be induced by the reduced rate of hydroxide thickness and/or formation of soluble complex with Mg2+[22,23].

3.5.Morphology of corroded surface

In order to investigate the inhibition mechanism,the surface and cross-section morphology of corroded specimens obtained after 24 h immersion in NaCl electrolyte without and with addition of 50 mM inhibitors are shown in Fig.10 As can be seen in Fig.10a,AM50 alloy illustrates a conventional corrosion morphology,which is characterized by a relatively thick layer and inhomogeneous skeletal surface morphology.Someβ-Mg17Al12phases can still be vaguely observed along the grain boundaries,whilst the smaller AlMn intermetallics are covered by corrosion products and are hard to be seen at current resolution.Taking a closer look at the surface,one can realize the corroded surface is not compact.Instead,islands of large loose areas decorated with scattered domes can be distinguished on the surface,which might be a result of dehydration of Mg(OH)2in atmosphere and/or H2evolution during immersion [39].Much more information about the mentioned features can be obtained from the cross-section view in Fig.10b and c.It is obvious that theα-Mg phase around the AlMn intermetallics(Fig.10b) is more preferentially corroded as a result of microgalvanic coupling caused by the high potential difference.As a result,the corrosion product of Mg(OH)2predominately forms in these areas even completely burying the secondary phase particles.However,as clearly indicated in the left inset of Fig.10b,the layer of Mg(OH)2precipitate reveals a porous and laminated microstructure that can inevitably be penetrated by the electrolyte containing Cl-species.Hence,it can be expected that severer localized damage is about to occur (right inset of Fig.10b) with increasing immersion period.It is interesting to notice that Mg17Al12phases may drop out of Mg substrate being surrounded by corrosion products on the surface.In contrast,corrosion products are rarely observable on the top of Mg17Al12phases,which indicates the galvanic coupling betweenβ-Mg17Al12andα-Mg may be not fully activated in the relatively diluted NaCl solution.

Fig.9.Evolution of film resistance Rf (a),film capacitance (b),charge transfer resistance Rct (c) and corrosion inhibition efficiency IEI (d) during immersion in 0.5 wt.% NaCl solution without and with addition of 50 mM inhibitors.

The corrosion inhibition of AM50 alloy due to addition of 50 mM inhibitors in NaCl solution are also manifested by a corroded surface morphology (Fig.10d,g and j).Primarily,paralleled scratches induced during the polishing process can still be identified on the surfaces,which indicates the anodic dissolution of Mg is retarded by the inhibitors and less Mg(OH)2is formed.Cracks appear on the dark areas,which highlights the places where severe galvanic corrosion occurs.Both intermetallicsβ-Mg17Al12and AlMn are observable on the corroded surface in the case of fumarate and benzoate addition,but they are hard to see on the glycolate protected specimen.These surface observations are further confirmed by their cross-section views.The local areas containing AlMn inclusion illustrate absence of galvanic corrosion attack for fumarate and benzoate specimens (Fig.10e and h),whereas glycolate still reveals slightα-Mg dissolution caused by its galvanic coupling with AlMn (Fig.10k).Moreover,it can be expected that the corrosion behaviors ofβ-Mg17Al12phase is not much influence by the inhibitors (Fig.10f,i and l),since almost no corrosion can be distinguished in theβ-Mg17Al12included regions and this situation is the same as in the control group.Considering the corroded surface morphology analysis,the corrosion inhibition functions of present three inhibitors can be ascribed to their adsorption ability on the oxide surface and specifically on the AlMn intermetallics also impeding their galvanic coupling withα-Mg.

4.Discussion

4.1.Effect of carboxylates on the corrosion of AM50 in NaCl solution

Combining the metallographic observation (Fig.2),phase VPD determination (Fig.3) and corroded surface analysis(Fig.10a-c),it is not hard to figure out that the microgalvanic coupling betweenβ-Mg17Al12phase/Al8Mn5intermetallic andα-Mg is the dominant factor that triggers the corrosion of AM alloys in pH neutral Cl-containing solution.Within these galvanic couples,theα-Mg is continuously corroding during the aqueous exposure with leaving cathodicβ-Mg17Al12phase and Al8Mn5intermetallic unattacked.As corrosion proceeds,Mg(OH)2precipitates on the surface with a less compact and inhomogeneous surface microstructure.Both H2release and pH increase caused by the cathodic reaction (2H2O+2e-→H2+2OH-)may contribute to these morphology features.Al enriches in the skeletal network areas left after the dissolution ofα-Mg and in the primary corrosion product layer,which acts not only as boundaries that impedes the propagation of corrosion among the surface but also improves the corrosion resistance of the corrosion product(s) layer [40].

Fig.10.Corroded surface (a,d,g and j) and cross-section (b,c,e,f,h,I,k and l) observation of AM50 after immersion in NaCl solution without (a,b and c) and with addition of 50 mM of glycolate (d,e and f),fumarate (g,h and i) and benzoate (j,k and l).

Despite the fact that all three inhibitors can effectively strengthen the resistive capability of AM50 against corrosion attack in NaCl solution,their adsorption behaviors vary significantly .As shown in Fig.4b-d,the initial (within 0 h-5 h) H2evolution tendency of glycolate and benzoate reveal slight difference comparing to that of the control group,whereas fumarate reveals significant suppression even at the lowest concentration.Such a difference can be ascribed to the presence of double carboxyl moieties within fumarate molecule that benefit its physisorption at the metal (negative charge)/electrolyte (positive charged species) interface.Further,the corrosion inhibition efficiency generally increases consistently with increasing concentration and immersion duration (Figs.4 and 5).Given an exception that the inhibition efficiency of glycolate dramatically decreases from 50.8% to 1.2% along with the concentration increase from 50 mM to 100 mM at 4 h may result from a further deprotonation of-OH group,which is stimulated by the increasing pH as a result of the cathodic reaction (2H2O+2e-→H2+2OHand/or 2H2O+O2+4e-→4OH-).Another plausible explanation might be formation of soluble Mg-glycolate complex at higher concentration of glycolate that results in corrosion acceleration.However,the corrosion rate gradually reduces when chemisorption of glycolate on the hydroxide occurs,which modified the microstructure of the hydroxide layer and improves its compactness [41].

The difference in corrosion inhibition behavior of these studied carboxylates can be further understood from the potentiodynamic polarization results in Fig.6.All three investigated compounds behave as anodic inhibitors judging from the consecutive shift ofEcorr,which implies that the anodic dissolution ofα-Mg is restricted.Besides,both fumarate and benzoate incorporated specimens reveal a “pseudo-passive”behavior in the anodic branch of polarization curve shortly after 10 min and 24 h exposure respectively to the inhibitor containing NaCl solution,which can hardly be define upon the glycolate one.These difference in anodic branch of polarization curves indicate that the barrier property of hydroxide layer is gradually reinforced by fumarate and benzoate whilst such a strengthening effect on hydroxide is much weak for glycolate.

Fig.11.Mullikin charge (a,d and g) and the frontier molecule orbital density distributions (HOMO (b,e and h) and LUMO (c,f and i)) of deprotonated glycolic,fumaric and benzoic acid.

This hydroxide enhancement effect can be also elucidated by the EIS results,in which the diameters of HF semi-circle in Nyquist spectra of benzoate group are not only much larger than those of glycolate and fumarate but also keep constant increase with immersion time.However,discrepancy in the inhibition efficiency evaluation can be distinguished between the results obtained from electrochemical measurements and hydrogen evolution.In details,the efficiency of glycolate,fumarate and benzoate (all in 50 mM,@15 h) from hydrogen evolution is 57.7%,79.1% and 50.6% respectively,whereas the efficiency values interpreted from EIS are 65.4%,65.0%and 80.1% (@12 h).These inconsistency in efficiency estimation can be mainly ascribed to the state of material used in measurement.For hydrogen evolution test,Mg chips characterized with much higher surface area (56.86 cm2/g) are immersed in the testing solutions and kept stirring,which benefit the physisorption of organic molecules on the surface and reduction of the degradation rate of the alloy.However,due to the constant mechanical stirring evolved during hydrogen evolution,formation of hydroxide on the surface of Mg chips and subsequent chemisorption of inhibitor on the hydr(oxide) may be impaired,which in turn induces relatively lower corrosion inhibition rate than that obtained from EIS (i.e.glycolate and benzoate).From this point of view,inhibition efficiency calculated from EIS is much closer to the real atmospheric corrosion condition since only a sinusoidal 10 mV amplitude potential perturbation is exerted during the measurement,which is believed to cause much weak impact on the adsorption behaviors of inhibitors.As a result,evaluating the performance of inhibitor employing electrochemical approaches conducted on specific surface area is more reliable especially when the investigated parts serve under free corrosion conditions.

4.2.Corrosion inhibition mechanism of studied carboxylate inhibitors

To further explain the molecular difference on the corrosion inhibition performance of carboxylates,molecular analysis was conducted employing quantum chemical calculation method.Fig.11 shows the optimized molecular structure and distribution of HOMO and LUMO of glycolate,fumarate and benzoate,respectively.It is worth to mention that all inhibitors are calculated in their deprotonated state and the solvent influence (i.e.,water) is considered.It is not surprising that the most negative charge can be found upon theOatoms within the carboxyl and hydroxyl moieties of the inhibitors,which denotes the binding sites that most likely adsorb on the metal surface through electrostatic attraction.TheEHOMOandELUMOelectron clouds distribution within each inhibitor define the occurrence probability of electrons,which determines if electrons may be donated or accepted.As shown,the electron donating sites within present inhibitive molecules are conformably define around their carboxyl groups,whereas their electron accepting locations can be found in the vicinity of hydroxyl (glycolate),C=C bond (fumarate) and benzene ring (benzoate).

Fig.12.Corrosion inhibition mechanisms of carboxylates on AM50 in NaCl solution.(a) Adsorption on passivation layer and (b) Formation of complex during hydroxide precipitation.

Considering the experimental and DFT study on corrosion inhibition behaviors and molecular features of glycolate,fumarate and benzoate,the feasible inhibition mechanism can be described as shown in Fig.12.As supported by the electrochemical results(Figs.6 and 7),the firs scenario of corrosion inhibition action of these inhibitors can be descripted as their simultaneous adsorption on intrinsic passivation film with a few nanometers thickness,which increases the stability of this barrier film However,such an adsorption behavior varies with respect to the charge state of molecule.As illustrated in the hydrogen evolution results in Fig.4,fumarate with double equal deprotonated carboxyl group can more easily absorb on the oxide surface in comparison with glycolate and benzoate employing only one carboxyl group.Nevertheless,it is reasonable to suggest that fumarate incorporated film registers higher thickness or compactness than that of the other candidates judging from the evolution of film capacitance(Fig.9b).However,the highest thickness cannot be equivalent to the best inhibition performance in current situation since the resistance of adsorptive inhibitor layer against corrosion attack can be also affected by molecular structure and properties of inhibitor.Based on this thought,benzoate employing lower thickness exhibits higher and stable impedance since it tends to be deposited in plane with the surface due to the uniform distribution ofπbonds in benzene ring may also form bonds with hydroxides,which demonstrates higher surface coverage than the other two inhibitors in the same concentration.Despite of the adsorption of inhibitors on the oxide surface,hydrolytical degradation of the oxide would also occur with time.Since then,the inhibitors tend to be involved during the formation of hydroxide products.Chemisorption of fumarate was found as a layer in the middle of corrosion product(s) via in situ Raman by Maltseva et al.[22] (but was not confirmed by in situ ATR-FTIR characterization [23]),who suggested fumarate improved the corrosion performance by forming bridging bidentate carboxylate coordination complex that enhances the barrier property of the corrosion product(s)layer.Moreover,function of inhibitors may be assigned during the hydroxide formation through considering their ion complexing capability Mg2+and Al3+(from the dissolution of Al enrichedα-Mg),as shown in Fig.12b.The stability constant logKfor Mg2+-glycolate complex is 0.92 (ML/M.L@ 25 °C,0.1 ionic strength),whilst that for Mg2+-benzoate complex is 0.1 (ML/M.L @ 30 °C,0.4) and Al3+-benzoate complex is 12.09(MOHL/M.OH.L@25°C,0.5)[42],which indicate the formation of Men+-Inhibitor complex(s) may also contribute the enhancement of corrosion.Unfortunately,this assumption cannot experimentally be envisaged in the present study due to the limit in skill to characterize the trace amount of complex in aqueous solution,which deserves a further investigation.

5.Conclusions

In the present work three characteristic carboxylates were studied as corrosion inhibitors for reducing the corrosion rate of AM50 Mg alloy in neutral NaCl solution.The optimum concentrations of inhibitors were preselected via hydrogen evolution measurements.Experimental (including electrochemical methods and surface analysis techniques) and theoretical calculations (DFT) were employed to understand their inhibition mechanism.Based on the obtained results,three critical conclusions can be drawn:

1.The sodium salts of glycolate,fumarate and benzoate can effectively reduce the corrosion rate of AM50 in 0.5 wt.%NaCl solution and their inhibition efficiency generally increases with concentration and immersion time.

2.All investigated carboxylates reveal anodic corrosion inhibition action,and their inhibition behaviors are ascribed to initial physisorption on the passivation and hydr(oxide)surface,followed by chemisorption with Mg2+and Al3+.

3.Adsorption of carboxylate inhibitors on the surface of corroding AM50 alloy mainly occurs at the carboxyl (electron donating) and hydroxyl group,C=C bond and benzene ring (electron accepting).Increasing the number of equally deprotonated carboxyl would facilitate the adsorption of carboxylate inhibitors,whilst the inhibition efficiency is mainly determined by the stability of complex formed between inhibitor and ions formed during corrosion.

Declaration of Competing Interest

The authors declare that they have no conflict of interest to this work.We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgment

Junjie Yang appreciates the financial support by Guangdong Basic and Applied Basic Research Founding (Grand No.2020A1515110754),MOE Key Lab of Disaster Forest and Control in Engineering,Jinan University (Grand No.20200904008),Educational Commission of Guangdong Province (Grand No.2020KTSCX012),the Fundamental Research Funds for Central Universities (Grand No.21620342)and the High Performance Public Computing Service Platform of Jinan University.Xiaopeng Lu acknowledges the financial support from National Natural Science Foundation of China(Grand No.52071067) and the Fundamental Research Funds for the Central Universities (Grand No.N2002009).

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.