Study on Surface Adsorption and Inhibition Behavior of Corrosion Inhibitors Contained in Copper Foil Rolling Oil

Xiong Sang; Sun Jianlin; Jiang Wei; Xu Yang; Zeng Yingfeng; Xia Lei

(1. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083; 2. Research Institute of Petroleum Processing, Sinopec Corp., Beijing 100083)

Study on Surface Adsorption and Inhibition Behavior of Corrosion Inhibitors Contained in Copper Foil Rolling Oil

Xiong Sang1; Sun Jianlin1; Jiang Wei1; Xu Yang1; Zeng Yingfeng2; Xia Lei1

(1. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083; 2. Research Institute of Petroleum Processing, Sinopec Corp., Beijing 100083)

Adsorption and inhibition behavior of 2,5-bis(ethyldisulfanyl)-1,3,4-thiadiazole (DMTDA) and N-((6-methyl-1H -benzo[d][1,2,3]triazol-1-yl)methyl)-N-octyloctan-1-amine (EAMBA) as corrosion inhibitors contained in copper foil rolling oil have been investigated using gravimetric and electrochemical techniques. Meanwhile, scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) have been employed to observe the surface topography and analyze the components on copper foil. The results show that the rolling oil containing DMTDA and EAMBA can significantly decrease the dissolution rate and increase the inhibition efficiency of samples, especially in the case of best compounded rolling oil system. The SEM and EDS investigations also confirmed that the protection of the copper foil surface is achieved by strong adsorption of the molecules which can prevent copper from being corroded easily. Reactivity descriptors of the corrosion inhibitors have been calculated by the density functional theory (DFT) and the reactivity has been analyzed through the molecular orbital and Fukui indices. Active sites of inhibitor are mainly concentrated on the ring and the polar functional groups, and in the meanwhile, the distribution is helpful to form coordination and backbonding among molecules and then to form stable adsorption on the metal surface. And this work provides theoretical evidence for the selection of corrosion inhibitors contained in copper foil rolling oil.

copper foil surface; corrosion inhibitor; electrochemistry; DFT; reactivity; adsorption

1 Introduction

Rolled copper foil is widely used in aerospace equipment, transportation facilities, automobile satellite direction positioning device, liquid crystal TV, laptop computers and others electronic products related with the production of flexible printed circuit board[1-2]. High frequency and high speed transmission of electric signal for rolled copper foil are attributed to its high strength, good toughness and high density. However, the surface oxidation and discoloration problem of rolled copper foil cannot be effectively prevented during its soft state product storage period[3], because the surfactants must be added to rolling oil to improve the lubricating property[4-5]. Even these substances will induce oxidation and corrosion[6-7]. In order to satisfy the high surface quality requirements for rolled copper foil products, it is necessary to take measures to reduce its surface oxidation. Using a suitable inhibitor in rolling oil is one of the effective methods for protection against corrosion[8]. Therefore, corrosion of copper and its inhibition in air have been attracting the attention from a number of investigators[7,9].

Organic compounds containing polar groups including nitrogen, oxygen, and sulfur[10-11], and heterocyclic compounds with conjugated double bonds and polar functional groups[12]used as corrosion inhibitors have been reported in the field of copper corrosion inhibition. The inhibiting action of these organic compounds is usually attributed to their interactions with the copper surface via their adsorption[13-14]. Polar functional groups are regarded as the reaction center that stabilizes the adsorption process[15-16]. Several researchers have studied the inhibitory effects of some ring-substituted benzotriazole under different conditions[17-19]. Some other heterocycliccompounds such as imidazole derivatives have been arousing the interest to be noted as corrosion inhibitors for copper metals and alloys[20]. Many heterocyclic compounds containing a mercapto group also have been used as copper corrosion inhibitors for different industrial applications[21-22]. These complexes are generally believed to be polymeric in nature and form an adherent protective film on the copper surface[23-24]. And benzotriazole (BTA) can form protective films by a strong triazolyl metallocyclic substrate bond on copper surface[25]. The introduction of an aliphatic chain in the position 5 of the BTA molecule can improve their performance by promoting the formation of thicker, more hydrophobic and less defective films[26]. The corrosion inhibition efficiency of BTA is attributed to the film structure of Cu/Cu2O/Cu+·BTA on copper surface[27], which mostly consists of Cu+·BTA species and some are oxidized to Cu2+·BTAH, so as to protect copper foil surface and effectively inhibit the corrosion[28]. In fact, it just gives a ranking of the temporary anticorrosive performance of the formed films[29-30].

Chen[17]and other authors[28]investigated 2,5-dimercapto-1,3,4-thiadiazole (DMTD) as a corrosion inhibitor of copper in acid solution using electrochemical techniques, and DMTD has been found to be a heterocyclic compound containing N, S, and thiadiazole ring which could possibly serve as the active sites for the adsorption process. Their experimental results showed that the high inhibition efficiency of DMTD was derived from the adsorption of three S atoms on copper[31]. However, a study on the adsorption activity and inhibition effect of DMTD and its derivative on the corrosion of copper foil in rolling oil has not been reported.

The present work focuses on the assessment of 2,5-bis(ethyldisulfanyl)-1,3,4 -thiadiazole (DMTDA) and N-((6-methyl-1H -benzo[d][1,2,3]triazol-1-yl)methyl)-N-octyloctan-1-amine (EAMBA) as corrosion inhibitors contained in copper foil rolling oil. The anticorrosion study has been carried out using gravimetric and electrochemical techniques coupled with scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) investigations. The reactivity of the inhibitor molecules and Fukui indices have been investigated by means of the density functional theory (DFT) in order to give a further insight into the experimental results.

2 Experimental

2.1 Materials

The copper foil sample has had a purity of 99.99%. Since the corrosion inhibitors have had a certain toxicity, therefore, very small amounts of inhibitors were used in the process. 2,5-Bis (ethyldisulfanyl)-1,3,4-thiadiazole (DMTDA), N-((6-methyl-1H-benzo[d] [1,2,3] triazol-1-yl)methyl)-N-octyloctan-1-amine (EAMBA) and the compounds mixed at different proportions were used. The blank copper foil rolling oil was used in the comparison test (denoted as ‘BLANK’). The specific parameters of base oil are given in Table 1. Compositions of copper foil rolling oil are shown in Table 2.

Table 1 Performance parameters of base oil

Table 2 Composition of copper foil rolling oilm%

2.2 Mass loss measurements

The mass loss experiments were carried out using copper foil samples having a total area of 50.36 cm2(S) apiece. The sample surface was cleaned with absolute ethanol (C2H5OH, Fisher, 97%) in order to wipe out the adsorbed surface oil and bent into certain degree to be dried before the copper foil was weighed (M0). And the samples with freshly prepared surfaces were then fully immersed in 100 mL of the rolling oil with and without desired inhibitors at different exposure periods of 3—15 h placed in a thermostatic drying oven at 100 ℃. After the designated exposure to the copper foil rolling oil, the sample waswithdrawn with its surface oil blotted with a filter paper, rinsed with absolute ethanol, immersed in an ultrasonic bath for 15 minutes in acetone to remove the surface adsorbate, and then was dried between two tissue papers and weighed again (M1) using an analytical electronic balance accurate to 0.1 mg. The mean corrosion rates (R), and the percentages of inhibition efficiencies (IE) over the exposure period were calculated according to the following equations[7]:

where T is the immersion duration (in h), D is the density of the copper foil (in kg/m3), and R0and R are the corrosion rates (in mm/a) of rolling oil in the absence and in the presence of the inhibitor, respectively.

2.3 Electrochemical experiments

Electrochemical experiments were performed using a Versa STAT MC electrochemical workstation provided with a potentiostat-galvanostat. For potentiodynamic polarization experiments the potential was scanned from -800 mV to 400 mV at a rate of 1 mV/s. Before the potentiodynamic polarization measurements, the electrode was preconditioned at -800 mV versus KOH for 2 min in the test electrolyte. For surface analysis of the copper foil electrode, a German model EVO 18 scanning electron microscope (SEM) attached with an energy dispersive spectrometer (EDS) analyzer was used. All solutions were prepared using re-distilled water and ethanol and all measurements were carried out at room temperature.

2.4 Theoretical computations

The equilibrium geometries of the DMTDA and EAMBA molecules were optimized by the first principles method using the density functional theory (DFT). Semi-empirical molecular orbital calculations were performed using the PM3[11]method (MOPAC Version 6) to calculate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of thiadiazole at ground state. The MOPAC program was built as a 32 bit DOS extender version and calculations were performed on an IBM PC (486DX 66MHz). Initially, molecular structures were fully optimized, and then the HOMO and LUMO energy levels of those structures were calculated. In order to obtain molecular local reactivity, the Fukui indices were used to study the electrophilic and nucleophilic reactivity of organic compounds and it was also an effective method to determine the active sites of the molecules. The Fukui function is de fined as the number of electrons N of an electron density ρ(r) in the first order partial derivatives. The chemical reactivity of the different sites of the molecules was evaluated by the Fukui indices, which are de fined in the literature[31-33].

With respect to the Fukui indices

With respect to the nucleophilic attack:

With respect to the electrophilic attack:

where qkis the first k atoms in the molecule with static electricity, and qk(N), qk(N-1) and qk(N+1) denote electronic populations of the atom k in neutral, cationic and anionic systems, respectively. These quantities were calculated using the natural population analysis[33].

3 Results and Discussion

3.1 Mass loss measurements

The mass losses (ΔM) versus time for the copper foil samples in rolling oil in the absence and in the presence of inhibitors are shown in Figure 1. It is shown that the maximum mass loss in rolling oil increases linearly with time without using the inhibitor. This occurs because of the continuous dissolution of copper ions as a result of additive attacks. It has been reported that the reaction of copper is attributed to dissolution of Cu species to Cu+ions and transformation of Cu+ions to Cu2+ions in the solution[10].

Under this condition Cu+ions produced thereby reacts very fast with R-radicals to form the complex in an acidic solution of [HR]<1 mol/L, which shows poor adhesion to the copper surface and is transformed to the soluble cuprous complex[11,18,20].

This radical in turn is dissolved by further reaction when the cuprous complex is adsorbed on the surface[34-35].

Figure 1 Variation of the mass losses (Δm) with time for copper foil samples immersed in rolling oil in the absence and in the presence of inhibitors■—BLANK;●—DMTDA added;▲—EAMBA added;◆—3DMTDA: EAMBA=1:4 added;▼—DMTDA: EAMBA=1:3 added;—DMTDA: EAMBA=1:2 added;—4DMTDA: EAMBA=1:1 added

According to Sherif and Park[10], the anodic dissolution of copper in the acidic solution is controlled by both diffusion of CuR2-to the solution bulk and electrodissolution of copper; thus, the overall reaction for copper corrosion can be represented by,

However, cuprous hydroxide complex absorbed on Cu surface stemming from the interaction between water and Cu electrode in alkaline solution[36-37]has poor stability and is transformed to the stable cupric hydroxide complex[38].

These complexes are dissolved by oxygen reduction reaction, and then copper oxides are produced[39-40],

According to Norkus[41], the corrosion of Cu in alkaline solution can generate CuOH and Cu(OH)2, and then CuOH and Cu(OH)2can decompose into CuO and Cu2O. Hence the main product is CuO. The corrosion reaction is a part of the reversible reaction and a greater effect of the activation energy is produced by changing the electrode potential.

The mass loss of copper foil immersed in rolling oil containing the corrosion inhibitors are smaller than the BLANK, which might be ascribed to the inhibition film adsorbed on the surface by mechanical and chemical reaction[35]. Addition of inhibitor molecules decreases the corrosion rate of copper foil samples significantly. Perhaps, the adsorption of inhibitor molecules onto the copper foil surface (vide SEM/EDS results) plays an important role in preventing copper from being corroded easily. This is also confirmed by plotting the variation of the inhibition efficiency (IE) of the copper specimens by inhibitors with time, which is calculated from the loss in weight by Eq. (2), and is shown in Figure 2. It is seen that IE% increases with an increase in exposure time which is applicable to all rolling oil containing the inhibitors. The inhibition efficiency is approximately 73% and 74% after 15 h of exposure to rolling oil when DMTDA and EAMBA are used, respectively. It is obvious that the compounds lead to lower corrosion velocity and higher inhibition efficiency than a single inhibitor system or a system without inhibitors. A best corrosion inhibition efficiency of 89% can be achieved when the weight percentage of EAMBA is twice the content of DMTDA. Eventually, no corrosion products are visible after the mass loss experiments on the surface were completed regardless of whether the experiments have been run with or without using the inhibitor.

Figure 2 Variation of the inhibition ef ficiency IE with time for copper foil samples immersed in rolling oil in the absence or in the presence of inhibitors■—DMTDA added;●—EAMBA added;▲—DMTDA: EAMBA=1:4 added;▼—DMTDA: EAMBA=1:3 added;◆—DMTDA: EAMBA=1:2 added;—DMTDA: EAMBA=1:1 added

3.2Potentiodynamic polarization measurements

The copper foil samples have been immersed in rolling oil samples without inhibitor or with addition of EAMBA, DMTDA, and the optimized compound, the potentiodynamic polarization curves of the copper foil electrode immersed in the 1.0 M KOH solution are shown in Figure 3. Anodic currents in the 1.0 M KOH in the absence of inhibitor (Figure 3) display three distinct regions：a Tafel region at lower over-potentials extending to the potential of the peak current density (Ipeak) due to the dissolution of Cu (0) to Cu+ions; a region of decreasing currents until a minimum (Imin) is reached due to the formation of Cu2+ions; and a region of sudden increase in current density leading to a limiting value (Ilim) as a result of Cu+ions formation, which is responsible for copper foil corrosion due to its dissolution into the solution[21].

Addition of inhibitor decreases the cathodic corrosion current (ICorr), Ipeak, Imin, and anodic current, as shown in Figure 3b-d ; also, the ECorrvalues show slight changes in negative directions, while both the anodic (βa) and cathodic (βc) Tafel slopes are shown to increase. The optimized compound of these effects is significant. The values of Ipeak, Imin, βc, βa, RCorr, polarization resistance (Rp), degree of surface coverage (θ) and IE% are obtained from Figure 3 and have been calculated[35]as listed in Table 3, from which it is clearly seen that the ICorr, Ipeak, Imin,Rpand RCorrvalues decrease with slight negative shifts in ECorrvalues, while βc, βa, θ, and IE increase in the presence of the optimum molecules. The decreases in ICorr, Ipeak, Iminand RCorrvalues are mainly ascribed to the decrease in the extreme pressure agents attack on the copper foil surface, which causes the decrease in Cu dissolution by adsorption of DMTDA and EAMBA molecules (vide infra: SEM/ EDS).

Figure 3 Potentiostatic polarization curves for the copper electrode at 100 mV immersed in 1.0 M KOH in the absence (a) and in the presence of DMTDA (b), EAMBA (c), and in the presence of the optimized compound (d)

Table 3 Corrosion parameters obtained from potentiodynamic polarization curves

The negative shift in ECorrand the increase in βcwith the optimum corrosion inhibitors are mainly attributed to the adsorbed compound inhibitors layer. Furthermore, the increase in βavalues is related to the decrease in the anodic current, which in turn limits the electrodissolution of copper foil. The inhibition efficiencies (IE) listed in Table 3 have been calculated from the polarization data according to the equation[35]:

where I0corrand Icorrare the corrosion current density in the absence and in the presence of inhibitor molecules, respectively. These IE values are in good agreement with those obtained by mass loss measurements and are shown in Figure 2. This confirms again that DMTDA, EAMBA and the optimized compound can inhibit the corrosion process on the copper foil surface.

3.3 SEM and EDS investigations

Figure 4 (a) SEM images of the copper foil surface immersed in different solutions and (b) the corresponding EDS profile analyses of the area shown in the image

In order to check up whether the inhibitor molecules are adsorbed on the copper foil surface or not, SEM and EDS analyses have been carried out. The sample has been washed briefly with water and then was thoroughly dried. The SEM micrograph of the copper foil surface with inhibitors immersed in the 1.0 M KOH solution at 100 mV for 120 min during upward-stepping of the potential is shown in Figure 4 (a) and the corresponding EDS profile analysis is shown in Figure 4 (b). It can be seen that thesurface texture of rolled copper foil is clear after adding corrosion inhibitor and the results show that inhibitors can effectively prevent the surface corrosion. A certain amount of C, N and O elements are detected on the surface of the copper foil by EDS profile analyses. The existence of O element is ascribed to the partial surface oxidation of copper foil. Meanwhile, the existence of C and N elements is assigned to surface adsorption, which shows the existence of certain amount of hydrocarbons and corrosion inhibitor on the copper foil. In addition to the C, N and O elements on the copper foil surface, it also contains some S element after addition of DMTDA. Since the S atom reveals its strong adsorption on copper[10]and DMTDA is the only sulfur-containing additive, it is confirmed that the corrosion rate of rolling oil on the surface of copper foil is reduced due to the surface adsorption of DMTDA. The sulfur element is detected on the copper surface, which indicates that the molecules are likely adsorbed on the copper foil surface forming Cu-EAMBA bonds and Cu-DMTDA bonds that can prevent the surface from being corroded. Many reports have demonstrated that the nitrogen-containing and sulfur-containing compounds can self-assemble on the surface of metals such as copper, silver, and gold by forming strong covalent bonds between the sulfur atoms and the metal surface[42-44]. This indicates that the presence of molecules greatly decreases the possibility of forming copper oxide complexes and limits the copper corrosion as evidenced by the presence of very low oxide content on the copper surface in Figure 4 (b).

3.4 Theoretical calculations

The above results allow for the clarification of some details about corrosion mechanism of the copper foil and the mechanism of corrosion protection by benzotriazole and thiadiazole derivatives. It can be seen from abovementioned data that the inhibition efficiency of EAMBA is higher than that of DMTDA. The inhibition efficiency depends on many factors including the number of adsorption centers, the mode of interaction with metal surface, and the molecular size and its structure.

In general the organic corrosion inhibitor has excellent corrosion inhibition performance, because its molecules of N, O, P and other atoms contain the lone pair electrons which tend to form coordination bonds with d orbitals of metals, or form antiligand bonds with heteroatoms and electrons from metal, and then form a protective film with metal. The calculated quantum chemical indices such as EHOMO, ELUMO, the energy gap ΔE(ΔE=ELUMO-EHOMO), the global hardness η (η=ΔE/2) and total charges Z are important indices of molecular reaction activity. The higher the EHOMOof organic molecules, the easier is it to offer electrons to unoccupied d orbitals of metals, and the higher the inhibition efficiency would be. The lower the ELUMOof organic molecules, the easier the acceptance of electrons of d orbitals of metals, and the higher the inhibition eff iciency would be[47]. Also, the less negative EHOMOand the smaller energy gap can reflect the stronger chemisorbed bonds and greater inhibitor efficiency as indicated by the front orbital theory[35]. The energy gap between EHOMOand ELUMOshowed that the greater inhibition effect could be related with the lower energy difference and global hardness, i. e. the molecules that could be more readily excited to undergo a charge transfer interaction with the metal surface[45]. Molecular properties of corrosion inhibitors by the density functional theory calculation are given in Table 4.

Table 4 Calculated molecular properties of corrosion inhibitors by the density functional theory

According to the reported theoretical data[46-47], the inhibition efficiency decreased with a decreasing EHOMOlevel[48]. The DMTDA molecule has a lower EHOMOvalue, and a lower inhibition efficiency could be expected. The molecular structures of optimized compound, HOMO and LUMO of corrosion inhibitors are given in Figure 5. It can be seen that HOMO and LUMO of corrosion inhibitors are mainly concentrated on benzene rings, heterocyclic rings and heteroatoms (such as N and S). Between them, HOMO of EAMBA is mainly located on C10 and N11 outside the benzene ring, and LUMO is concentrated on the benzene ring and heterocyclic groups. However,HOMO and LUMO of DMTDA are all concentrated on heterocyclic groups and impurity atoms (such as, S6 and S8). This suggests that active sites of inhibitor molecules in electrophilic reactions and nucleophilic reactions are mainly located on the benzene rings, heterocyclic groups and hetero atoms.

Figure 5 Optimized compound (I), HOMO (II) and LUMO (III) related molecular structures of corrosion inhibitors (Ball and stick model.)

In order to give a further insight into the experimental results, the Fukui indices are used for predicting the preferential sites of electrophilic attack on DMTDA and EAMBA. The Fukui indices are widely used as descriptors of site selectivity for the soft-soft reactions[49]. According to Gece[20], the favorite reactive site is one which possesses a high value of Fukui indices. Hence, the relationship between electronic structure and efficiency of DMTDA and EAMBA can be deduced from the Fukui indices calculations. In this way, we have calculated the electrophilic Fukui indices fk+, defined by Eq. (4), for heteroatoms in the three systems. The Fukui exponential distribution of EAMBA and DMTDA molecules are shown in Figure 6 with the results given in Table 5. It turns out that the local point of EAMBA is mainly focused on the reactivity of N11 atom, and the local reactive points of DMTDA are concentrated in S6, S7, S8 and N1 atoms.

Figure 6 Active sites of molecules in free radical (I), electrophilic (II) and nucleophilic reaction (III)

Table 5 Calculated Mulliken atomic charge and Fukui indices

Activity sites of DMTDA and EAMBA are mainly focused on N and S atoms, which have multiple points susceptible to adsorption. The molecules should be adsorbed on metal surface when the corrosion inhibitor molecules interact with the metal surface, since the effective corrosion inhibitor has at least one kind of polar functionalgroup adsorbed on metal surface according to the mechanism of corrosion inhibitor adsorption. Partially non-polar functional groups, which are on the outer side of protective film, can prevent the corrosive particles from entering the metal surface, in order to achieve the role of anticorrosion. Upon comparing the Fukui indices of DMTDA, it can be seen that fk+>fk-, which suggests that the atoms in DMTDA with nucleophilic property are easier to get electrons during the nucleophilic reaction. It also turns out that the sulfur atom in DMTDA possesses a larger value of Fukui indices (0.161, and 0.152 for fk+and fk-, respectively). Thus, the sulfur atom in DMTDA is more reactive for nucleophilic attack than nitrogen atoms in the system. However, EAMBA shows its strong electrophilic ability, in which fk+

Indeed, the nitrogen atoms, in both DMTDA and EAMBA systems have very small values of nucleophilic Fukui indices (less than 0.067). These results seem to indicate that the N atom of EAMBA has a greater electrophilic character, which is involved in the chemical reactivity of this molecule with the metal surface. Therefore, EAMBA has a stronger interaction and higher corrosion inhibition efficiency with copper foil. The nitrogen-containing heterocyclic compounds are considered to be effective corrosion inhibitors.

Typically, organic molecules are usually attached to metal surfaces by physical adsorption, chemical adsorption or electrochemical adsorption. In order to study the adsorption behavior of organic molecules on copper surface, the adsorption energy of corrosion inhibitors adsorbed on copper crystal (110) plane can be calculated by using the molecular dynamics simulation method. The interaction energy between copper crystal and corrosion inhibitor molecules can be determined using Eq. (17) and Eq. (18) as shown below:

where EComplexis the total energy between copper crystal and inhibitor molecules adsorbed on copper surface, ECuand Einhibitorare the total energy of the copper crystal and corrosion inhibitor molecules, respectively. Ebindingis the adsorption energy of inhibitor molecule adsorbed on copper surface, and it is the negative of its interaction energy. ECu-inhibitorand Ebindingfor the molecules on copper (110) are presented in Table 6.

Table 6 ECu-inhibitor and Ebindingfor molecules on copper (110)

Upon comparing the calculated values, it can be seen that Ebindingis positive in value and Ebindingof EAMBA is higher than that of DMTDA. Through theoretical study it can be seen that the corrosion inhibition efficiency of EAMBA is higher, because it has higher adsorption energy and it can be more readily adsorbed on copper surface, which is consistent with the test results performed previously.

The models of corrosion inhibitors molecules adsorbed on copper crystal (110) plane are presented in Figure 7. It is shown that the molecules are readily adsorbed onto the bare copper foil surface by aromatic and heterocyclic rings or impurity atoms to form a layered protective film on the surface, which can prevent the copper foil from being corroded. Chemisorption of organic molecules occurs due to chelation on the metal surface. Organic molecules may offer electrons to the unoccupied d orbitals of metals and accept the electrons in the d orbitals of metals by using antibonding orbital, to form backbonding and then a polymeric film composed of the (Cu+EAMBA orCu+DMDTA) complex. The optimum corrosion inhibition performance can be obtained, when the two molecular compounds are mixed at a proper ratio.

Figure 7 Corrosion inhibitors are absorbed on (110) crystal plane of the copper foil with the surface adsorption model

4 Conclusions

1) Corrosion inhibitors effectively eliminate the undesirable destructive effect and prevent metal dissolution due to adsorption of the inhibitor molecules on the copper surface to block its active sites. The inhibition efficiencies obtained by weight-loss, dynamic polarization, and theoretical calculations are in reasonably good agreement with each other. The inhibition of the compound inhibitor is better due to their synergistic interaction as compared to the single inhibitor system.

2) Mass losses of copper foil samples in rolling oil indicate that the addition of DMTDA and EAMBA significantly decreases the dissolution rate and the optimized compound can achieve a best corrosion inhibition efficiency which can reach 89%. Results of potentiodynamic polarization experiments, potentiostatic current-time measurements, SEM and EDS investigations also confirmed that the protection of the copper surface is achieved by strong adsorption of inhibitor molecules. By adding inhibitors into rolling oil the dissolution currents in both cathode and anode are decreased.

3) By calculating the Fukui indices of atoms in two molecules, DMTDA and EAMBA molecules show their strong nucleophilic properties and electrophilic ability, respectively. Active sites of inhibitor are mainly focused on the aromatic and heterocyclic rings and the polar functional groups. Surface adsorption model of inhibitors on (110) crystal plane of the copper foil is established, and it provides theoretical evidence for the selection of corrosion inhibitors to be used in copper foil rolling oil.

Acknowledgments:The authors gratefully acknowledge the financial assistance provided by the National Natural Science Foundation of China (No. 51474025) and the Cooperation Program between USTB and SINOPEC (No.112116). Thanks are also extended to all individuals associated with the project.

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NC310 Catalyst for Methanol Synthesis Developed by SINOPEC Research Institute of Nanjing Chemical Company

The NC310 type catalyst for methanol synthesis developed by the SINOPEC Research Institute of Nanjing Chemical Company has passed the appraisal of research achievements organized by the Science and Technology Division of the Sinopec Corp. The group of specialists attending the appraisal meeting has recognized that this catalyst has reached the internationally advanced level in terms of its overall catalytic performance. Till now this catalyst has been operating for two years in the commercial scale on the 150 kt/a methanol unit. The test results have revealed that this catalyst features good catalytic activity and stability, as evidenced by its high methanol output, low specific materials consumption and high grade of byproduct steam to offer effective economic benefits. Currently this research institution has applied for eight Chinese invention patents, among which three patents have been granted.

date: 2014-04-09; Accepted date: 2014-11-29.

Prof. Sun Jianlin, Telephone: +86-10-62333768; E-mail: sjl@ustb.edu.cn.